Hadron and Nuclear Physics with Electromagnetic Probes
Hadron and Nuclear Physics with Electromagnetic Probes
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Hadron and Nuclear
Physics with Electromagnetic
Probes Proceedings of the Second KEK-Tanashi International Symposium Tanashi,Tokyo, October 25-27, 1999
edited by
K. Maruyama Center for Nuclear Study, University ofTokyo Tokyo, Japan
H. Okuno Institute for Nuclear and Particle Physics KEK Tokyo, japan
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Preface The second KEK-Tanashi International Symposium on "Hadron and Nuclear Physics with Electromagnetic Probes" was held on the Tanashi campus of KEK, High-Energy Accelerator Research Organization from Oct. 25 to Oct. 27, 1999. 'The new KEK was established in April 1997 by merging three laboratories including Institute for Nuclear Study (INS): the University of Tokyo. INS has a long successful history of hosting international symposia and schools on particle and nuclear physics. The new KEK intended to continue its activity by organizing a series of international symposia. This is the second of the new series. The aim of this Symposium was to discuss recent experimental and theoretical developnients of hadron and nuclear physics, where emphases were placed on the hadron and nucleus studies wit,h electron and photon beams. At KEK-Tanashi, the 1.3-GeV Electron Synchrotron had long been operated for these purposes. In rccent years, the main research areas were photonuclear reactions and meson productions by using the first highduty tagged-photon beam and the TAGX spectrorrieter. Although this field is developing quite rapidly, the synchrotron was closed in 1999 after 37-year operation, and these activities will be continued at new facilities. n'e think it is a good time to discuss the present status and future directions of this field at this occasion. The Symposiuni was attended by 85 physicists and 35 talks were presc11tr:d. This book contains the papers presented in thc scientific program of the Symposium. We would like to thank all the speakers and the participants for their stirnulating talks and active discussions. We are also grateful t,o the members of the international advisory coninlittee and the organizing committee for polishing up the program. We appreciate all the staff members, in particular Mrs. I. Yarnamoto and Dr. K. Niki, for smooth organization. Finally, we would like to thank the Inoue Foundation for Science and the Foundation for Accelerator Science for their financial support.
KEK-Tanashi, March 2000 Editors: Koichi Maruyama and Hideki Okuno
ORGANIZATION International Advisory Committee: L.S. Cardman (Jefferson Laboratory, USA) H. Ejiri (Osaka University, Japan) R. Redwine (MIT, USA) B. Schoch (University of Bonn, Germany), T. Walcher (University of Mainz, Germany) J. Wambach (Julich, Germany) W. Weise (Miinchen, Germany) S . Yamada (KEK, Japan) K. Yazalu (University of Tokyo, Japan) President: S. Sugimoto (KEK-Tanashi) Organizing Committee: Y. Akaishi (KEK-Tanashi) M. Asakawa (Nagoya University) I. Endo (Hiroshima University) H. En'yo (Kyoto University) 0. Hashmoto (Tohoku University) J. Kasagi (Tohoku University) T. Kishimoto (Osaka University) K. Maeda (Tohoku University) K. Maruyama (CNS, University of Tokyo, Co-Chair) T. Motoba (Osaka University of Electric-Communication) H. Okuno (KEK-Tanashi, Co-Chair) T. Oshima (Nagoya University) T.-A. Shibata (Tokyo IT) Y. Sumi (Hiroshima International University) K. Tokushuku (KEK-Tanashi)
Host Institute Institute of Particle and Nuclear Studies, KEK
Sponsors Inoue Foundation Foundation for High-Energy Accelerator Science
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CONTENTS Preface Opening address S. Sugimoto I. INTRODUCTION TO THE SYMPOSIUM Hadrons and nuclear physics with EM probes - from QCD point of view T. Hatsuda 11. MESONS IN NUCLEAR MEDIUM Vector mesons in medium and dileptons in heavy-ion collisions R. Rapp and J. Wambach p0 mesons in the nucleus
K. Maruyama Nuclear production of 4 meson at KEK H. En'yo, J. Chiba, H. Funahashi, H. Hamagaki, M. leiri, M. Ishino, S. Mihara, T. Mipashita, T. Mumkami, R. Muto, M. Naruki, M. Nomachi, K. Ozawa, O. Sasaki, M. Sekimoto, H. D. Sato, T. Tabaru, K. H. Tanaka, S. Yamuda, S. Kokkaichi and Y . Yoshimura (KEK-PSE325 Collaboration) Lepton pair spectroscopy with HADES at GSI J. Friese (HADES Collaboration) 111. NUCLEON RESONANCES IN NUCLEI AND RELATED TOPICS 61 Sll (1535) resonance in nuclei H. Yamazaki, T. Yorita, T. Kznoshita, T. Okuda, H. Matsui, T. Maruyama, J. Kasagi, T. Suda, K. Itoh, T. Miyakawa, H. Okuno, H. Shimizu, H. Y. Yoshida and T. Kinashi Delta in nuclei excited by hadronic processes J. Chiba The physics of AA exitation G. M. Huber
The neutral pion photoproduction on the proton near threshold Il-T. Cheon and M. T. Jeong Photoneutron cross section measurement on 'Be by means of inverse Compton scattering of laser photons H. Utsunomiya, Y. Yonezau~a,H. Akimune, T. Yamagata, M. Ohta, M. Fujishiro, H. Toyokawa and H. Ohgaki Nuclear disintegration induced by virtual photons at heavy-ion colliders I. A. Pshenichnov
IV. STRANGENESS PHYSICS Kaon photoproduction on nuclei K. Maeda Subthreshold and near threshold K+ meson photoproduction on nuclei E. Ya. Paryev Phenomenological aspects of kaon photoproduction on the nucleon T . Mart, S. Sumowidagdo, C. Bennhold and H. Haberzettl Hyperon polarization in Kaon photoproduction from the deuteron H. Yamamura, K. Miyagawa, T. Mart, C. Bennhold, H. Habeerzettl and W. Glockle Physics of associated strangeness production at ELSA E. Paul Retrospect and prospect of hypernuclear physics 0. Hashimoto Spin-orbit splitting of 13*C H. Kohri, S. Ajirnura, R. E. Chrien, P. M. Eugenio, G. Fkanklin, J. Franz, T. Fukuda, L. Gun, H. Hayakawa, P. Khaustov, T. Kzshzmoto, K . Matsuoka, M. May, S. Minami, Y . Miyake, T. Mori, K. Morikubo, J. Nakano, H. Noumi, H. Outa, K. Paschke, P. Pile, B. Quinn, A. Rusek, E. Saji, A. Sakayuchi, R . Sawafta, Y. Shimizu, M. Sumihama, R . Sutter, T . Tamagawa, H. Tamura, K. Tanida, L. Tang and L. Yuan
V. N-N CORRELATIONS AND FEW-BODY PHYSICS Photodisintegration reactions of 3He and *He at TAGX T . Suda
Photonuclear cross sections of three-nuclcon systcms and the role of threenucleon forces G. Orlun,dini, W. Leidernann, V. D. Efros and E. L. Tomsiak
169
Quasi-deuteron picture for %e and 4He photodisintegration S. Hirenzaki, Y . Umemoto and K . Kume Two-nucleon emission experiments at Mainz P. Grabmayr Quark substructure and isobar effects on deuteron form-factors E. Lomon
VI. NUCLEON STRUCTURE STUDIED BY HIGH-ENERGY ELECTRONS The proton and the photon, who is probing whom in electroproduction? 20 1 A . Levy High-Q2 neutral- and charged-current reactions at HERA K. Nagano Spin structure of the nucleon studied by HERMES Y. Sakemi First double polarization measurements on the way to test the GerasimovDrell-Hearn sum rule W. Meyer (GDH- and A2-Collaboration)
239
VII. NEW FACILITIES Few-body physics at Jefferson Laboratory F. W. Hersman (Hall A and C L A S Collaboration) Experimental test of the K-A relative parity - Use of polarized photon beams at high energies Y. Yamaguchz Nuclear physics experiments with 1.2-GeV STB ring at LNS-Tohoku J. Kasagi
265
Laser electron photon facility at Spring-8 T. Hotta, J. K . Ahn, H.Akimune, Y. Asano, W. C. Chang, S. Date, M. Fujiwara, K. Hicks, K. Imai, T. Iwata, T. Ishikawa, H. Kawai, 2. Y. Kim, T. Kzshimoto, N. Kumagai, S. Makino, T . Matsumura, N. Matsuoka, T. Mibe, M. Miyabe, Y. Miyachi, T . Nakano, M. Nomachi, Y. Ohashi, T. Ooba, H.Ookuma, M. Ooshima, C. Rangacharyulu, A. Sakaguchi, T. Sasaki, D. Seki, H. Shimizu, Y. Sugaya, M. Sumihama,
27 1
T. Tooyarna, H. Toyokawa, A. Wakai, C. W. Wang, S. C. Wang, K Yonehara, T. Yorita and M. Yosoi MUSES project at FUKEN RI beam factory T. Katayama, K. Maruyarna and M. Wakasugi Summary of the symposium H. OAuno Symposium program List of participants
Opening Address
Ladies and Gentlemen: On behalf of the organizers, I would like to express our hearty welcome to all of the participants of the 2nd KEK-Tanashi International Symposium, especially to those from abroad. We are very glad to have you all here, in this Tanashi Campus. Tl-le series of Tanashi Symposium was established in 1998 after the merger of three laboratories including Institute for Nuclear Study of the University of Tokyo, which was commonly known as INS-Tokyo. The INS has a long successful history of hosting a medium-sized international symposium every year on a topics concerning nuclear and particle physics. The new Research Organization, KEK-Tanashi also intends to continue such an activity in organizing a scries of intcmational symposia for the research field. This symposiunl is the 2nd of the new series and the 27th of the previous one. As you know, about 44 years ago Tanashi campus was opened for the first interuniversity research institute for nuclear and particle physics. At, the beginning our Electron Synchrotron was built here as the first high-energy accelerator in Japan. The ES played an essential role in creating high energy physics in this country and provided very important steps toward the birth of KEK-PS and Synchrotron Radiation facilities. Since we are going to move to the Tsukuba site within several months to expand our research activities, we have just closed the ES this summer after 37-year successful operation. The wide research field we have developed a.t the ES will bc taken over by new electron facilities of domestic and foreign laboratories. We think it is a good time to discuss the present status and future directions of t,his field at this occasion. So, the topics of this synlposiunl is focused on Hadron and Nuclear Physics with Electromagnetic Probes.
I hope that this symposium will contribute to further development of this field through active and fruitful discussions. In conclusion, I wish all participants a pleasant stay in Tokyo. Thank you for your attention.
Shojiro Sugimoto President of the 2nd KEK-Tanashi International Symposium
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I. INTRODUCTION TO THE SYMPOSIUM
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Hadron and Nuclear Physics with ElectromagneticProbes K. Maruyama and H. Okuno (Editors) 2000 Elsevier Science B.V. All rights reserved.
'
Hadrons and nuclear physics with EM probes - from QCD point of view "Phys. Dept., Kyoto Univ., Kyoto 606, Japan After a brief introduction of the current problems in QCD, selected topics on the physics of in-medium hadrons are discussed. They are in-medium QCD sum rules, spectral functions in lattice QCD, and critical fluctuation of the chiral order parameter in dense medium. Special emphasis is put on the QCD spectral function which is the key observable in both theoretical and experimental studies. 1. INTRODUCTION
Quantum Chromodynamics (QCD) is the "theory of everything" in the physics of strong interaction. The QCD action can be written as [l]:
where m being the quark mass, G:, is the field strength tensor of the gluon field A:, and D p is the covariant derivative. This action has been well-established by high energy experiments such as the deepinelastic lepton-hadron scattering (DIS) and e+e- annihilation into hadrons. A key ingredient here is the asymptotic freedom of QCD 121 . . which leads to the logarithmic decrease of the QCD running coupling constant CX,(K) as K (renormalization point the typical scale of a given process) increases:
--
where N, and Nfare the number of color and flavor respectively, and AQcD is the QCD scale parameter which takes a value around 200 MeV. Due to the asymptotic freedom, the interaction among quarks and gluons become weak at high energies and the systematic perturbative analyses with appropriate factorization of the hard and the soft part can be ) high chemical potential carried out. Also, at very high temperature ( K -- T >> A Q c ~and (K p >> AQcD), the small running coupling constant as well as the Debye screening allow us to treat the system as non-interacting gas of quarks and gluons in the lowest order [3]. (Even at extremely high T a,nd p, however, the system still may show some interesting phenomena such as the spatial confinement [4]and the color superconductivity 151.1
-
At low energy scale, non-perturbative effects emerge as a result of the strong non-linear gluon self-interactions. Typical examples are the confinement of quarks and gluons and the dynamical breaking of chiral symmetry, both of which are related to the non-trivial QCD vacuum structure. Although it is still difficult to show analytically how these effects appear from the QCD action eq.(l), they are fundamental phenomena for the existence of mesons and baryons, and also of the atomic nuclei. The current physics issues in QCD may be classified into the following three categories: (1) QCD at extreme conditions such as at high temperature or at high baryon density. This is relevant to the physics of ultra-relativistic heavy-ion collisions at CERN and BNL, and also to the physics of neutron star and quark star [6]. (2) Interplay between the hard physics and soft physics in high energy QCD processes. Any physical process is always a combination of the hard scattering part and the soft part: how to factorize them and how to extract information of the non-perturbative effect are theoretical and experimental challenge.
(3) The properties of hadrons in the hotldense medium. Recent experimental developments with hadron-nucleus and nucleus-nucleus collisions give us a new insight into the properties of hadrons inside hot and/or dense matter. It is also a challenging many-body problem in QCD and could give us information on how nuclear matter undergoes a phase transition t o the quark gluon plasma. Since QCD is the theory of everything, we should always start with eq.(l) to attach the above problems. However, there are many ways to represent the same dynamics in different field choices. This is because the quantum amplitude is obtained after the integration over the field variables. For example, consider the full partition function in QCD: =
J
[dQdqdA]exp [iSQco(qlY, A)] .
This can be equivalently rewritten by integrating out the high frequency quarks and gluons above an arbitrary cutoff A, which gives the eq.(5) below. One can then make change of variables from q, Q, A to the hadronic fields (e.g., pions and nucleons), and obtain eq.(6) below.
..I
-
[ d ~ d N . exp L
A
(a,N, . . .; A)] .
Here sZf is the effective action written in terms of the low-energy quarks and gluons, while Sef is the effective action written in terms of the low-energy hadrons. Both effective actions must have explicit A dependence which should be cancelled by the A dependence from the field integration so that the full partition function is independent of A and gives a unique prediction: namely dZ/dA = 0.
-
So far, the argument is exact, but is not practical at all. In the actual application, one sometimes sets A 47r f, = 1.2 GeV (the chiral symmetry breaking scale) and construct a most general S,Hffallowed by symmetry and then determine the parameters in S,Hff by experiments. The chiral perturbation theory is a typical example of this sort [7]. If one has a big parallel computer, one may carry out the integration [dqdqdA] in eq.(5) numerically by taking A large ( E l / a with a being the lattice spacing). This is the lattice QCD numerical simulation which has considerable progress in recent years [8]. QCD sum rules which have great success in reproducing hadronic properties [9] are based on the Wilson's operator product expansion; in a way it makes use of the appropriate window of energy scale where both hadronic description and the quark-gluon description are valid (quark-hadron duality). After this introductory remarks, we will focus on in-medium hadrons in the following and review some recent theoretical developments. 2. IN-MEDIUM HADRONS
The idea of the in-medium hadrons has started in early 80's. In 1982, Pisarski discussed hadron modifications at finite temperature using phenomenological models and pointed out a possibility to observe the mass shift of the pmeson in dilepton measurement [lo]. In 1985, Kunihiro and I have studied the change of the scalar-isoscalar meson "a" near the critical point of chiral phase transition as a typical example of the dynamical critical phenomena in QCD [Ill. Brown and Rho has generalized these ideas and proposed a universal scaling hypothesis of hadron masses, which is, however, not yet proved. Heavy mesons such as J / Q have been also studied in relation to the deconfinement transition at finite T. In 1986, Hashimoto et al. has discussed the mass shift of J / Q and 7, just below the deconfinement transition [13],while Matsui and Satz proposed the disappearance of J / Q as a signature of the formation of the quark-gluon plasma [14]. In go's, it was widely recognized that the spectral functions of hadrons (instead of the ambiguous quantities such as "mass-shift" and "width-broadening") are the relevant quantity to be studied both theoretically and experimentally, although it was already implicit in ref.[11]. New theoretical tools were also developed in 90's such as the in-medium QCD sum rules [15],meson-baryon effective lagrangians [16],quark-meson coupling model [17]and so on. Since the number of references are enormous, I will refer to the Proceedings of the recent Hirschegg meeting on hadrons in dense matter [18]. In the next section, we will first show a direct connection of the spectral function in the vector channel with various experimental observables to illustrate its importance. 3. SPECTRAL FUNCTIONS FOR EM CURRENT
Among the various two point functions in QCD, the time-ordered correlation function of the electro-magnetic current is the most important one from the experimental point of view. We define the imaginary part of this correlation (the spectral function) as
where /target) is a given many-body state and J,, is the electromagnetic current in QCD:
The physical meaning of the spectral function Im(TJJ)ta,,et becomes clear by inserting the complete set between J's. Then the spectral function is a sum of contributions from all the states having non-vanishing matrix element with the state Jltarget). Namely, it probes the excited state of the many-body system which is obtained by acting the external current J onto the target. Now, let us show some example of the direct connection of Im(TJJ)tar,et with observable~. 1. For e+e- annihilation into hadrons (e+e- -+ y* 4 X ) , the cross section is directly proportional to the vacuum matrix element,
where s = q;. > 0 and the "target" is the vacuum. Therefore one is probing the structure of the vacuum through the time-like photon in this experiment. 2. For the deep-inelastic process with proton target (y* is written as
+ N -+ X), the cross section
is the Bjorken variable. Here the "target" is the where Q2 = -q:, < 0 and nucleon. Therefore, we are probing the nucleon structure through the space-like virtual photon.
+
3. For the emission of dileptons from the hot plasma (plasma -+ y* X ) , the emission rate per unit phase-space volume is
with q;, = u2 - q2 > 0. Here the "target" is the thermal distribution of hadrons or quarks and gluons. Thus we are probing the hot plasma through the emission of the time-like photon which eventually decays into dileptons. 4. If one can carry out the following hypothetical experiment ?*(time like) with A being a nucleus, the cross section is written as
which gives an information on the vector meson propagation in matter.
+A
4
X
5. In hadron-hadron collisions associated with a production of time-like photon ( A B + y* X) such as the Drell-Yan process, the cross section is written as
+
+
from which one can probe the the internal structure of the projectile and target and also the production mechanism of the time-like photon. 4. THEORETICAL TOOLS
Now, what kind of theoretical tools can be used to study the spectral functions in QCD? I will pick up two of them in the following; QCD sum rules and lattice QCD. 4.1. QCD sum rules By using the dispersion relation for the two-point function and the operator product expansion at short distance, one can derive a set of sum rules for the spectral function in the medium at finite density and at finite temperature [15]. They have the following general form of the energy weighted sum;
where Cnis the known Wilson coefficients, 0,is the gauge invariant local operators such as
Im(TJJ)pQcDis the known spectral function calculated perturbatively and thus does not depend on the target structure. The left hand side of eq.(14) can be estimated in the medium by the low energy theorem and low temperatureldensity theorem or possibly by the direct lattice QCD simulations. Therefore, one can make some constraints on the exact spectral function Im(TJJ)t,r,,t in the medium through the sum rules eq.(14). This QCD sum rule constraints have been useful for making some predictions of the spectral shift as well as for checking the validity of the effective field theory calculations ~91. 4.2. Lattice QCD The lattice QCD simulations have remarkable progress in recent years for calculating the properties of hadrons as well as the properties of QCD phase transition. In particular, the quenched QCD simulation on the masses of light mesons and baryons agree within 5-10 % with the experimental spectra [20]. However, the lattice QCD had difficulties in accessing the dynamical quantities in the Minkowski space, because measurements on the lattice can only be carried out for discrete points in imaginary time. The analytic continuation from the imaginary time to the real time using the noisy lattice data is highly non-trivial and is even classified as an ill-posed problem. Recently a first attempt to extract spectral functions (SPFs) of hadrons from lattice QCD data by using the maximum entropy method (MEM) has been reported [21]. MEM
is a method which has been successfully applied for similar problems in quantum Monte Carlo simulations in condensed matter physics, and image reconstruction in crystallography and astrophysics [22]. 4.2.1. Basic idea of MEM The Euclidean correlation function D ( r ) of an operator O(T,2)and its spectral decomposition at zero three-momentum read
D(T) =
J (ot(r,Z)O(O,
6))d3x=
1"
K ( r , w)A(w)dw,
(17) where T > 0, w is a real frequency, and A(w) is SPF (or sometimes called the image), which is positive semi-definite. The kernel K ( r , w) is proportional to the Fourier transform of a free boson propagator with mass w: At zero temperature (T = 0) in the continuum limit, K ( r , w) = exp(-TW).
(18)
Monte Carlo simulation provides D(ri) on the discrete set of temporal points 0
< ri/a
, j
+d
7-
K+
+ A(C) + N are expressed by the operator Tias
i, j = Ah[, EN,
= Figure 1. Inclusive d ( y ,K + ) cross section as a function of lab momentum p ~ for + 0" and the photon lab energy E, = 1.3 GeV. The two thresholds K f A N and K + C N are indicated by the arrows. The results around the K + C N threshold are enlarged in (b).
where the operator t y i describes the elementary processes y+ N + K++A(C), and j9,j > represents the deuteron state which is generated by the Nijmegen 93 N N interaction [2]. The operator Uijcorresponds to the Y N final state interaction processes, and is represented as
where V,, is Y N interaction including AN - C N coupling. We ignore the K + meson interaction with the nucleon and hyperon in the final states. From Eqs.(l) and (2), one can deduce the coupled set of integral equations for Ti,
We solve this set (3) after partial-wave decomposition in momentum space. The three elementary process y + p -+ K+ + A(Co) and y + n --+ K + + C- are properly incorporated in the driving term in Eq.(3). Equation (3) is solved on isospin bases A N and E N , but the resulting amplitudes are transformed into those on the particle bases An, Con and C - p by which the inclusive d(y, K + ) , exclusive d(y, K + Y ) cross sections and hyperon polarizations are calculated. For details, we refer the reader to ref.[3]. 3. RESULTS
At present, we calculate the observables only for the K+ meson scattered to 0 degree = 0"). The Nijmegen soft-core Y N interactions NSC97f[4] and NSC89[5] and a
(OK+
+
+
+
recently updated production operator[6] for the y N -+ K+ A(C) N processes are used. Figure l ( a ) shows the inclusive cross sections which sum up the contributions of the K'An, K + C O nand K+C-p final states. The solid and dashed lines are the predictions of the NSC97f and NSC89 Y N interactions, respectively. The dotted line shows the results of the plane wave impulse approximation (PWIA). The arrows indicate the two thresholds K + A N (pK = 977.30 MeV/c) and K + C N (pK = 869.14 MeV/c). The two pronounced peaks around p~ =945 and 809 MeV/c are due to the quasi-free processes of A and C, where one of the nucleon in the deuteron is spectator and has zero momentum in the laboratory system. Significant FSI effects are found around the K + A N and K f C N thresholds. The cross section is increased up to 86% by FSI near the K + A N threshold. Around the K f C N threshold, as shown Fig.l(b), the strengths and shapes of the enhancements by the NSC97f and NSC89 are quite different.
Figure 2. (a) Exclusive d ( y , K+A) cross section and (b) A recoil polarization with incoming polarized photon as a function of the A scattering angle in the An c.m. system. The photon lab energy is E, = 1.3 GeV. The outgoing kaon lab momentum and angle are p ~ =+ 870 MeV/c and 8 ~ =+ 0" respectively.
Figure 2(a) illustrates the exclusive d(y, K f A ) cross sections just below the K f C N threshold ( p K = 870 MeV/c). The FSI effects are seen both at very forward and at large angles. The PWIA cross sections are basically zero at backward angles, while the FSI calculations still show some strength. Figure 2(b) demonstrates the A recoil polarizations with incoming polarized photon. The A recoil polarizations in PWIA are almost one. This is because the incoming photon is polarized along the z axis, but the target deuteron is unpolarized and the outgoing K + meson carries no spin and angular momentum in this case (OK = 0"). However, the final state interactions cause the large deviations from one, and the prediction by NSC97f is quite different from that of NSC89.
- --
0.08 -
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,
's
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-- NSC89
8
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,
- - - PWIA - NSC97f '
-
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'
[degl
Figure 3. (a) Exclusive d(y. K+C-) cross section and (b) C- recoil polarization with incoming polarized photon as a function of the C- scattering angle in the C-p c.m. system. The photon lab energy is E, = 1.3 GeV. The outgoing kaon lab momentum and angle are PK+= 865 MeV/c and Q K + = 0' respectively.
Figure 4. Inclusive d(y, K + ) cross section in the PWIA as a function of lab momentum p ~ + for Q K + = 0' and photon lab energy E, = 1.3 GeV. The solid curve shows the prediction by the present version[6] of the production operator for y N -t K+ + A @ ) , while the dashed line corresponds to the prediction by an old version[7].
+
The exclusive results to the K+C-p final states just above this threshold (pK =865 MeV/c) are shown in Fig.3. The prominent FSI effects are seen both in the cross sections and in the double polarization observable. Finally, we briefly discuss the production operator for 7 N -+ K+ A(C) processes. In Fig.4, the inclusive cross sections in PWIA in which an old version[7] of the operator is used are compared to those with the present version[6]. The latter has been improved in the fitting to the data of y + N -+ K+ + A(C) including the new SAPHIR data. The difference between the predictions by the two versions are quite large as in Fig.4, which suggests this reaction d(y, K + ) Y N is another promising candidate for investigating the operator.
+
+
REFERENCES 1. K. Miyagawa, H. Yamamura, Phys. Rev. C60 (1999) 024003; nucl-th/9904002. 2. V. G. J. Stoks, R. A. M. Klomp, C. P. F . Terheggen, and J. J. de Swart, Phys. Rev. C49 (1994) 2950. 3. H. Yamamura, K. Miyagawa, T . Mart, C. Bennhold, W. Glockle, Phys. Rev. C61 (1999) 014001; nucl-th/9907029. 4. Th. A. Rijken, V. G. J. Stoks, and Y. Yamamoto, Phys. Rev. C59 (1999) 21. 5. P. M. M. Maessen, T h . A. Rijken, and J . J. de Swart: Phys. Rev. C40 (1989) 2226. 6. C. Bennhold, T. Mart, A. Waluyo, H. Haberzettl, G. Penner, T . Feuster, and U. Mosel, in Proceedings of the Workshop on Electron-Nucleus Scattering, Elba, Italy, 1998, edited by 0.Benhar, A. Fabrocini, and R. Schiavilla (Edizioni ETS, Pisa, 1999), p. 149; nucl-th/9901066. 7. C. Bennhold, T . Mart, and D. Kusno, in Proceedings of the CEBAF/INT Workshop on N* Physics, Seattle, USA, 1996, edited by T.-S. H. Lee and W. Roberts (World Scientific, Singapore,1997), p.166
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Hadron and Nuclear Physics with ElectromagneticProbes K. Maruyama and H. Okuno (Editors) 2000 Elsevier Science B.V. All rights reserved.
'
Physics of associated strangeness production at ELSA Ewald Paul Physikalisches Institut, Universitat Bonn, Germany 1. INTRODUCTION
Associated strangeness production has been measured by the SAPHIR Collaboration [I] with the SAPHIR detector [2] at the Electron Stretcher and Accelerator ELSA in Bonn. Results will be reported from about 6% of the data on tape [3,4]. 2. THE STRANGE PARTICLE PROGRAMME AT SAPHIR
The SAPHIR experiment is devoted to measuring photoproduction with a beam of real photons at energies following a bremsstrahlung spectrum from electrons up to 3 GeV from ELSA. The strange particle reactions measured are essentially those with two-body final states:
Results from these reactions [3,4] are presented and discussed in this talk. Data analyses are in progress for the reactions:
The measurements with SAPHIR allow events to be reconstructed and identified over the whole angular space of produced charged particles. The results consist of total and differential cross sections, and transverse polarization of the hyperons. They are compared to models in three distinct parts. In the first part we consider isobar models. They describe the data at the level of hadronic degrees of freedom by fitting a superposition of Born terms and resonant contributions to the data. Parameters which are common with other reactions are determined in a coupled-channel approach. In the second part, the question whether hyperon polarizations also have a more general origin than stemming from interference of specific amplitudes is discussed. This is suggested since A and C states are polarized in similar manner for various beams and over
orders of magnitude in energy [5]. Moreover hyperons indicate the polarization of the s quark within the framework of the static quark model. If the s-quark carries polarization this implies that the polarizations of A and C states are related to each other. This is what is observed in general. The third part is concerned with chiral model calculations based on an effective chiral Lagrangian and on chiral perturbation theory. Such perturbative calculations describe pion photoproduction processes near threshold. When going to kaon-hyperon final states the mass of the kaon, which is considerably larger than the mass of the pion, implies new questions concerning the perturbative calculations directly as well as the handling of chiral symmetry breaking in general.
,
\;:,
drift chambers ,
:, ,
P
,
Figure 1. Topological and kinematical reconstruction of an event of the type yp
+
K°C'
3. THE EXPERIMENT
The ELSA accelerator provides electron beams in the energy range 0.5 to 3.5 GeV with nearly 100% duty cycle. Such a beam was used to produce bremsstrahlung photons which were tagged one-by-one in a tagging system for SAPHIR. The SAPHIR detector comprises a large magnet with a target inside filled with liquid hydrogen or deuterium and surrounded by a cylindrical drift chamber. Downstream and to the sides are planar drift chambers and large scintillator hodoscopes. Further downstream follows an electromagnetic calorimeter, which was not used for the strange particle data analyses. Details of the experimental setup are given elsewhere [2]. The SAPHIR drift chamber system is well suited to measure tracks of charged particles in the full angular range. Pions, kaons and protons are identified by time-of-flight measurements in the scintillator hodoscopes.
The measurements of the primary photon (in the tagging system) and of the charged particles in the central drift chamber are sufficient to reconstruct and identify complete events [6]. As an example we consider a measured event of reaction (3) which has been reconstructed and identified successfully (fig. 1). Three tracks were measured: two are the decay pions from K: -, T + T - decay; the other is the proton from C+ -+ p r o decay. The measurements of the pions yield the reconstruction of the K: at the decay point, the 3-momentum and line of flight in space. The 4-momenta of primary photon and Kfwere used to calculate (in a fit procedure) the 4-momentum of the C+. Then primary vertex and C+ decay vertex were determined simultaneously in an iterative procedure, where the spatial event topology is tested by moving the primary vertex along the K: line of flight. Finally the event kinematics was tested in fits at all vertices and overall. With this procedure the total acceptance of reactions (3) was close to 10%. The contamination by misidentified events from other reactions was negligible.
4 s [GeVI A: SAPHIR 1.5
0:ABBHHM
0.5
0
I
1
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---..,
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1.4
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Figure 2. Total cross sections.
4. EXPERIMENTAL RESULTS
Total reaction cross sections were measured over the photon energy range from threshold to 2 GeV for reactions (1) and (2) and 1.5 GeV for reaction (3), respectively. They are shown in fig. 2 in comparison with the only previous measurement in the energy range carried out with a bubble chamber 30 years ago [7]. The most striking observation is the difference in the rise at threshold: it is very steep for the K A reaction (1)and moderate for the KC reactions (2) and (3). The cusp effect where the KC reactions are opened is clearly visible. At larger energies the cross sections are similar for K+A and K+CO.The cross section for the K°C+ reaction (3) is about 112 of those of the other reactions. Differential cross sections are considered as a function of the production angle of K+ in yp center-of-mass system. They are determined in various energy bins for the reactions (1) and (2), as shown in fig. 3. The full line corresponds to a fit to Legendre polynomials for angular momenta up to 3 according to [3]:
The coefficients a. to a3 are shown in fig. 4. The K+A reaction (1) has a large s-wave contribution near threshold. In addition a p-wave and an s-p interference term give a substantial contribution. The KC cusp effect is visible. In fits to isobar models (shown below) the rise of the s-wave is caused by a significant resonance production of 5 1 (1650). The p-wave contribution is related to a superposition of the two resonances P1~(1710)and P13 (1720). The coefficients of the K+COreaction (2) show large s-, p- and d-wave contributions which peak around 1.4 GeV. Isobar model calculations identify S31(1900) and P31(1910) in this region. When extending the Legendre fits to higher waves (not shown) the data indicate also some f-wave contribution which may originate from known resonances [3]. The differential cross sections of reaction (3) are compared to reaction (2) in some wider energy bins in fig. 5. The main observation is that the distributions differ in the resonance region of reaction (2). Measurements of the transverse polarizations of A, C0 and C+ are shown in fig. 6. The polarization parameters are given as a function of the K+ production angle and for the reactions (1) and (2) in three energy bins. Two observations are made: 1. The angular shapes of A and C0 are nearly independent of energy and described well by a fit allowing angular momenta up to 1 (which is consistent with the Legendre fits shown above).
2. The polarizations of A and C0 have in general opposite signs. The polarization of C+ in reaction (3) was measured for the first time by SAPHIR. However, statistical errors are still large meaning conclusions about shape and sign cannot be drawn.
Differential cross sections: y p
-+ K + h
Fig. 3: Differential cross sections for y p fit explained in the text.
Differential cross sections: y p
-+ K f h
and y p
-+
K+c'.
-+ K+CO
The full line corresponds to a
-
w
-0 2
-02 E, IGeVI
E, [GeVI
E, IGeVl
E, IGeVI
15
15 2 E, [GeVI
E, [GeVI
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-0 2 2
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Figure 4. Coefficients of Legendre Polynomials.
cos(0.)
:0 4 m
203
A: 7 + p + K 0 + T '
1 250
L E,
ro interact via hadron exchange potentials, as given by known hadron masses and coupling constants fixed by independent experiments or symmetry conditions. The separation radius, ro, must be within the range of asymptotic freedom (5 0.85 of the equilibrium radius of the interior bag model) because of the sensitivity of p& to the derivatives at ro of the internal wave function. Eq. (1) also requires that ro satisfy $ 2 ( r 0 , Wz(rO)) = 0. Using the low energy data, the latter condition fixes the value of ro with some precision, giving a value consistent with the first condition for the Cloudy bag model, but not for the MIT bag mode1[4,5], ruling out the latter for the multi-hadron domain. The Cloudy bag model determines ro = 1.05 fm. The model then gives a good detailed fit to NN data for TLab5 0.8 GeV[9] and also is consistent with some evidence for
the lowest I = 1 exotic resonance, J P = 0+[10] at 2.70 GeV. These resonances, produced correspond to multi-quark configurations other than the minimal near the f-poles at Wi qq and q3 configurations. The lowest in the NN system is the I = 0, JP = 1+ at 2.63 GeV. 3. DEUTERON PREDICTIONS 3.1. Interior wave function The first f-pole in the EN system, corresponding to the [q(ls;)I6 configuration, is in the I = 0, J P = 1+ state, 0.76 GeV above the deuteron mass. As the width of the exotic resonance is only 0.03 GeV, the fap are nearly constant. It has been shown[ll] that the interior wave function vanishes for constant f , and the actual probability of being in the interior has been estimated to be 5 0.004[6]. This implies a large "hole" in the deuteron wave function, which has long been noted as a property of the edff and is also embodied in the effective "hard core" of NN scattering. In our model this apparent repulsion arises from the rapid change in the effective d.0.f. at ro. However at the energy W, there is "matching" of the interior and exterior wave functions, resulting in a substantial interior probability and a resonance. The fact that ro is 2-3 times larger than the cores of repulsive core models is compensated by the discontinuous increase of wave functions at ro, so that the experimental effective range parameters can be produced by both types of core effect. However the models differ at higher NN scattering energies for large q edff. 3.2. Exterior wave function In the I = 0, JP = 1+ system, the NN (3S1)and NN (3D1) states are coupled to each other and to isobar channels by meson exchange potentials as well as the f-matrix. AA(3Dl) Because of their low threshold mass and strong tensor coupling, the and AA(3D1) channels are most important and are included in all our models. NN (3Sl) and AA (3S1)states have nonvanishing 6 with the [q(ls2)l6quark configuration, channel also has a low threshold, contributing to the f-pole residue. The NN*(1440)(~s~) and with its large width is of next importance to the deuteron. But over the energy range which includes the first exotics. other channels are also of some importance and the NSll(1535) ( l PI),NSll (1650)( l Pl) and (1620)(3 PI) are included in our recent work. These channels modify the best choice of f$ for the lower threshold channels, but have negligible components in the deuteron. 3.3. Determining the f 0 for three models The f i p are sharply restricted by fitting the N N scattering data, which in the deuteron 6 and ~ 7 ( ~ D 1 ) sector requires a fit to the np(3S1 -3 Dl) phase parameters 6 and ~7(~S1), and €1 for TLab5 0.8 GeV. For the models C1 and D1[12],which have only N N and AA channels, the lower energy behavior of the 6's and €1 determine the NN sector fap, while the energy dependence for 0.4 GeV < TLab< 0.8 GeV fixes those coupling the NN and LIA(~S~)sectors, and the NN to the sum of AA(3D1) and AA('D1) sectors. The last two have the same threshold behavior, affecting the elastic scattering in the same way. Only detailed A-production data could separate them, so the ratio is free to adjust to the edff. The magnet,ic form factor q-dependence is very sensitive to the ratio because of the opposite spin and convection currents of these states[6].
The model C' did not consider quark configurations[l3]. Without f-poles the best choice of separation radius was ro = 0.74 fm. Model D' included the lowest f-pole with ro = 1.05 fm as discussed above. As shown previously[l2], C' and D' give equivalent fits to the 6's while case D' is a better fit to €1. 12.5
-4
5
10.0
M
7.5
*
5.0
IW
2.5
-5
0.0
--
0
20
-10
8
-
B
m
z
When the other isobar channels are included, the NN*(1440), because of its larger width, modi-3 fies the energy dependence and in6 creases the inelasticity. This reduces the required coupling to the AA channels. This model E results in a better fit to 51,6(3D1)and to , the 7j's (Fig. 1).
0
-20
w
-20
Fw
-30 1 00
1.00
0 98
0.98
Figure 1. The phase parameters for
3 0 96
0.96
2 I = 0, J~ = 1+, n p scattering. Model E (solid curues); SAID SPOO
V]
D
60.94
0.94
0.92
0.92
0 90
o
250
500
TL ( M e V )
750
o
250
500
750
O
phase parameters (dashed curves); Bugg 1990-1991 phase parameters (squares).
TL ( M e V )
3.4. The EDFF predictions In previous work[l2], the edff for models C' and D' were calculated with the nonrelativistic, coupled channel impulse approximation (IA) and the meson-exchange current (MEC) terms of i7, p, and w , "pair" corrections and the p r y term to first relativistic order. For the IA the isobar form factors are assumed proportional to the nucleon electromagnetic form-factors. In all cases the MEC corrections to the isobar channels are neglected. Both Hohler et a1.[14] (HO) and Gari-Kriimpelmann[l5] (GK) nucleon form factors were used. The results (Figs. 3-6 of [12]) are seen to be a good to A(q2) for the HO choice and to B(q2) for the GK choice. This is not inconsistent as A(q2) is dominated by the nucleon electric form-factor and B(q2) by the nucleon magnetic form-factor. The t20(q2) predictions were consistent with the very low q experimental results available at the time. Here we present, versus the extended data range of the edff, the results of models C', D' and E where the first order relativistic correction has been added to the impulse approximation and the second order relativistic corrections have been included in the MEC[16,17]. For model E the ratio of AA(7D1) to AA(3D1) coupling to the N N sector was guided by the C' and D' model ratios. It has not yet been optimized to the data.
Figure 2. A ( q 2 ) : Data points are de- Figure 3. t z o ( q ) :Data points described in scribed in Ref.[l]. Model C' ( H O ) (solid Ref. [3]. Model C' (solid line); model D' line); model (2' ( G K ) (dash-dash);model (dash-dot); model E (long dashes). The D' ( H O ) (dash-dot);rmdel D' ( G K ) (dot- dependence on nucleon emff ( H O or G K ) dot); model E ( H O ) (long dashes); model is negligible. E ( G K ) (dash-dot-dot). Also the balance of AA and N N * ( 1 4 4 0 ) coupling to N N and the value of the N*(1440) magnetic moment, unknown from independent data, have not been varied. Table 1 shows the results of model E for the static properties of the deuteron. For models C' and D' the results are as stated in [12]except for a small relativistic change in Qdeut
.
Table 1 Static deuteron properties of Model E B E ( M e V ) PD (%) PAS(%) Pas(%) PA,(%) Model E 2.2247 5.21 .006 3.24 1.79 EX^ e 2.2246
PN*(%)a Q(f m2) 0.71 .273 .286
PD (p?)
360 .857
"The higher mass channels have neglibible probability.
A ( q 2 ) is shown in Fig. 2. It is seen that model E with either choice of nucleon form) ~ , is only large enough for factors fits the data reasonably well for q2 < 2.5 ( G ~ V / C but larger q2 when the G K choice is made. Models C' and D' on the other hand are better with the HO choice for q2 < 2.5 ( G e V / c 2 ) ,but at 6 ( G e V / c 2 )only model C with G K is large enough. t20(q2) is shown in Fig. 3. For the momentum transfer range there is negligible sensitivity to the choice of nucleon form-factors, as these cancel in the ratio of quadrupole to
monopole electric amplitudes which dominates t z o The result is however very sensitive to the model used. The simple constant f-matrix model, C', gives a good fit over the whole range of q. Model D' puts the maximum of tzo at much too small a momentum transfer. This is related to the large amplitude ofthe L = 2, AA states in this model. Model E, with intermediate A A components, has the maximum of tzo between that of models C' and Dl. The B(q2) for models C' and D' is similar to that of [12] with minima a t slightly smaller q. However the minimum of B(q2) for model E is at much too small a value (q2 = 1.3 (GeV/c)'). 4. C O N C L U S I O N S
The R-matrix boundary condition model E, with all the relevant isobar channels, reproduces the np(3Sl -3 Dl) scattering phases well up to T L 5 ~0.8~GeV. It also reproduces very well the static properties of the deuteron and A(q2) for q2 _< 6 ( G ~ V / C ) ~It. does not fit t20(q2)as well as the simpler model C' (although it is better than model D'), and ) too small a value. To correct the full model (E) has the first minimum of ~ ( q at~ much for B(q2),the ratio of the AA(7D1) to A A ( ~ D couplings ~) to the NN sector needs to be varied. That, and perhaps a further substitution of N*(1440) coupling for A A coupling to the NN sector may also correct the fit to the maximum of t20(q2). REFERENCES 1. L.C. Alexa et al., Phys. Rev. Lett. 82 (1999) 1374, and references therein. 2. R.G. Arnold et al., Phys. Rev. Lett. 58 (1987) 1723, and references therein. 3. D. Abbot et al., arXive:nucl-ex/0001006, and references therein. 4. P. LaFrance and E.L. Lomon, Phys. Rev. D34 (1986) 1341. 5. P. Gonzdez, P. LaFrance and E.L. Lomon, Phys. Rev. D35 (1987) 2142. 6. W.P. Sitarski, P.G. Blunden and E.L. Lomon, Phys. Rev. C36 (1987) 2479. 7. M. Creutz, Phys. Rev. Lett. 45 (1980) 313. 8. E.P. Wigner and L. Eisenbud, Phys. Rev. 72 (1947) 29. 9. P. LaFrance, E.L. Lomon and M. Aw, nucl-th/9306026. 10. J. Ball et al., Phys. Lett. 320 (1994) 206. 11. H. Feshbach and E.L. Lomon, Ann. Phys. (NY) 29 (1964) 19. 12. P.G. Blunden, W.R. Greenberg and E.L. Lomon, Phys. Rev. C40 (1989) 1541. 13. E.L. Lomon and H. Feshbach, Ann. Phys. (NY) 48 (1968) 94. 14. G. Hohler, E. Piertarinen and I. Sabba-Stefanescu, Nucl. Phys. B144 (1976) 505. 15. M. Gari and W. Kriimpelmann, Phys. Lett. B173 (1986) 10. 16. P.G. Blunden, private communication. The formulas of Blunden and Ian Towner are used here. 17. H. Arenhovel, F. Ritz and T . Wilbois, to be published Phys. Rev. C. This work includes boost corrections not included in the work of P.G. Blunden and I. Towner.
VI. NUCLEON STRUCTURE STUDIED BY HIGH-ENERGY ELECTRONS
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Hadron and Nuclear Physics with Electromagnetic Probes K. Maruyama and H. Okuno (Editors) 2000 Elsevier Science B.V. All rights reserved.
"
The proton and the photon, who is probing whom in electroproduction? Aharon Levy* School of Physics and Astronomy Raymond and Beverly Sackler Faculty of Exact Sciences Tel Aviv University, Tel Aviv, Israel The latest results on the structure of the proton and the photon as seen at HERA are reviewed while discussing the question posed in the title of the talk. 1. INTRODUCTION
The HERA collider, where 27.5 GeV electrons collide with 920 GeV protons, is considered a natural extension of Rutherford's experiment and the process of deep inelastic ep scattering (DIS) is interpreted as a reaction in which a virtual photon, radiated by the incoming electron, probes the structure of the proton. In this talk I would like to discuss this interpretation and ask the question of who is probing whom [I]. The structure of the talk will be the following: it will start with posing the problem, after which our knowledge about the structure of the proton as seen at HERA [2] will be presented followed by a description of our present understanding of the structure of the photon as seen at HERA and at LEP [3,4]. Next, an answer to the question posed in the title will be suggested and the talk will be concluded by some remarks about the nature of the interaction between the virtual photon and the proton [5]. 2. THE QUESTION
- WHO IS PROBING WHOM?
2.1. The process of DIS The process of DIS is usually represented by the diagram shown in figure 1. If the lepton does not change its identity during the scattering process, the reaction is labeled neutral current (NC), as either a virtual photon or a Z0 boson can be exchanged. When the identity of the lepton changes in the process, the reaction is called charged current (CC) and a charged Wi boson is exchanged. During this talk we will discuss only NC processes. Using the four vectors as indicated in the figure, one can define the usual DIS variables: Q2 = -q2, the 'virtuality' of the exchanged boson, x = Q 2 / ( 2 P . q), the fraction of the proton momentum carried by the interacting parton, y = (P . q ) / ( P 9k), the inelasticity, and W2 = (q + P)2,the boson-proton center of mass energy squared. *This work was partially supported by the German-Israel Foundation(GIF), by the U.S.-Israel Binational Foundation (BSF) and by the Israel Science Foundation (ISF). The financial support of my visit to Japan by JSPS is highly appreciated.
proton remnant
Figure 1. A diagram describing the process of deep inelastic scattering (DIS). The four vector of the incoming and outgoing leptons are k and k', that of the exchanged boson is q, and that of the incoming proton is P. The four momentum of the struck quark is xP.
The interpretation of the diagram describing a NC event is the following. The electron beam is a source of photons with virtuality Q2. These virtual photons 'look' at the proton. Any 'observed' structure belongs to the proton. How can we be sure that we are indeed measuring the structure of the proton? Virtual photons have no structure. Is that always true? We know that real photons have structure; we even measure the photon structure function Fz [4]. Let us discuss this point further in the next subsections. 2.2. The fluctuating photon How is it possible that the photon, which is the gauge particle mediating the electromagnetic interactions. has a hadronic structure? Ioffe's argument [6]: the photon can fluctuate into qq pairs just like it fluctuates into e+e- pairs (see figure 2). If the fluctuation time. defined in the proton rest frame as tf Y (2E,)/miq, is much larger than the interaction time, ttnt E r,, the photon builds up structure in the interaction. Here, E, is the energy of the fluctuating photon, m,,- is the mass into which it fluctuates, and r, is the radius of the proton. The hadronic structure of the photon, built during the
remnant photon SYrrent jet
e
Figure 2. Fluctuation of a photon into (a) an e+e- pair, (b) a qq pair.
Figure 3. A diagram describing a DIS process on a quasi-real photon using the reaction e+e- -+ e+ePX.
interaction, can be studied by measuring the photon structure function F; in a DIS type of experiment where a quasi-real photon is probed by a virtual photon, both of which are emitted in e+e- collisions, as described in figure 3. This diagram is very similar to that in DIS on a proton target (figure 1).
2.3. Structure of virtual photons? Does a virtual photon also fluctuate and acquire a hadronic structure? The fluctuation time of a photon with virtuality Q2 is given by tf E (2E,)/(rniq + Q2),and thus at very high Q2 one does not expect the condition tf >> ttnt to hold. However at very large photon energies. or at very low x, the fluctuation time is independent of Q2: tf N 1/(2rnpx).where m, is the proton mass, and thus even highly virtual photons can acquire structure. For instance, at HERA presently W -- 200 - 300 GeV. and since x = Q2/(Q2 W2), x can be as low as 0.01 even for Q2 = 1000 GeV2. In this case, the fluctuation time will be very large compared to the interaction time and the highly virtual photon will acquire a hadronic structure. How do we interprate the DIS diagram of figure 1 in this case? Whose structure do we measure? Do we measure the structure of the proton, from the viewpoint of the proton infinite momentum frame, or do we measure the structure of the virtual photon, from the proton rest frame view? UTho is probing whom? When asked this question. Bjorken answered [7] that physics can not be frame dependent and therefore it doesn't matter: we can say that we measure the structure of the proton or we can say that we study the structure of the virtual photon. I will try to convince you at the end of my talk that this answer makes sense.
+
3. THE STRUCTURE OF THE PROTON
In this section we will refrain from discussing the question posed above and will accept the interpretation of measuring the structure of the proton via the DIS diagram in figure 1. We present below information about the structure of the proton as seen from the DIS studies at HERA. 3.1. HERA With the advent of the HERA ep collider the kinematic plane of x-Q2 has been extended by 2 orders of magnitude in both variables from the existing fixed target DIS experiments, as depicted in figure 4. The DIS cross section for ep --, can be written (for Q2 ZICN
E
Figure 20. Distributions of for different thresholds on the highest transverse energy jet. The open histogram is the prediction of the MC, and the shaded part is the direct photon component of the MC.
Figure 21. Comparison of the photon gluon density, determined from di-jet photoproduction events taken in 1996, with earlier measurements of HI. The curves are the expectations of different parameterizations.
fixed values of the hard scale. which is taken as the highest transverse energy jet [23]. One can go one step further by assuming leading order QCD and Monte Carlo (MC) models to extract the effective parton densities in the photon. An example of the extracted gluon density in the photon [24] is shown in figure 21. The gluon density increases with decreasing x, a similar behaviour to that of the gluon density in the proton. The data have
the potential of differentiating between different parameterization of the parton densities in the photon, as can be seen in the same figure. 4.3. Virtual photons at HERA
One can study the structure of virtual photons in a similar way as described above. In this case, the Q2 of the virtual photon has to be much smaller than the transverse energy squared of the jet, EZ, which provides the hard scale of the probe. Such a study [25] is presented in figure 22, where the dijet cross section is plotted as function of x, for different regions in Q2 and EZ. One sees a clear excess over the expectation of direct photon reactions, indicating that virtual photons also have a resolved part. This fact can
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a*4.5
Figure 23. The ratio of the resolved to direct photon component, as function of the virtuality Q2 (in GeV2) of the photon.
also been seen in figure 23 where the ratio of resolved to direct photon interactions is plotted as function of the virtuality Q2 of the probed photon [26]. One sees that although the ratio decreases with Q2, it remains non-zero even at relatively high Q2 values. 4.4. Virtual photons at LEP The study of the structure of virtual photons in e+e- reactions was dormant for more than 15 years following the measurement done by the PLUTO collaboration [27]. Recently, however. the L3 collaboration at LEP [28]measured the structure function of photons with a virtuality of 3.7 GeV2, using as probes photons with a virtuality of 120 GeV2. In the same experiment. the structure function of real photons was also measured. Both results
a) < Q ~ >= 120 G E V ~ , =o
2-
DATA QPM ...... GRV
-
t)
....
2 X w
2
1-
~3
AGF LRSN
-/&+=??+ +---.+ ........
...
.......
'.'3
,_.-.
,,rr
0
I
0
0.2
0.4
I
I
0.6
0.8
O 1
1
PLUTO
X
b) = 120 G E V ~ , = 3.7 G E V ~
2-
t)
-
2
QPM
21
-.-- GRS
X w
....
F ;
b,
GRS F2
DATA QPM
,
ILa' ..........
0
0
I
I
I
0.2
0.4
0.6
018
X
Figure 24. The effective photon structure function from L3 for (a) a quasi-rea1 photon target and (b) a 3.7 GeV2 photon target. Both photons were probed at a scale of 120 GeV2.
Figure 25. The dependence of the effective photon structure function on the mass P2 of the probed photon,
can be seen in figure 24 and within errors the structure function of the virtual photons is of the same order of magnitude as that of the real ones. The effective structure function is also presented as function of the virtuality of the probed photon P2 in figure 25 and show very little dependence on PZ up to values of 6 GeV2 [27,28].
-
4.5. What have we learned about the structure of the photon? Let us summarize what we have learned so far about the structure of the photon.
At HERA one can see clear signals of the 2-component structure of quasi-real photons, a direct and a resolved part. Virtual photons can also have a resolved part at low x and fluctuate into qq pairs. a
Structure of virtual photons has been seen also at LEP.
5. THE ANSWER
Following the two sections on the structure of the proton and the photon, let us remind ourselves again what our original question was. At low x we have seen that a y* can have structure. Does it still probe the proton in an ep DIS experiment or does one of the partons of the proton probe the structure of the y*?
The answer is just as Bjorken said: at low x it does not matter. Both interpretation are correct. The emphasis is however 'at low x'. At low x the structure functions of the proton and of the photon can be related through Gribov factorization [29]. By measuring one, the other can be obtained from it through a simple relation. This can be seen as follows. Gribov showed [29] that the yy,yp and pp total cross sections can be related by Regge factorization as follows:
This relation can be extended [I] to the case where one photon is real and the other is virtual,
or to the case where both photons are virtual,
Since at low x one has 0 ;;. 9 p 2 ,one gets the following relations between the proton structure function F:, the structure function of a real photon, F2, and that of a virtual photon, F;" :
and
The relation given in equation (10) has been used 1301 to 'produce' F; 'data' from well measured F; data in the region of x 4 GeV2, it is difficult to test the relation. However both data sets have been used for a global QCD leading order and higher order fits [31] to obtain parton distributions in the photon. Clearly there is a , data for such a study. need of more precise direct FY In any case, our answer to the question would be that at low x the virtual photon and the proton probe the structure of each other. In fact, what one probes is the structure of the interaction. At high x, the virtual photon can be assumed to be structureless and it studies the structure of the proton.
I
"
'
!
"
I
l
'
"
1
Figure 26. The photon structure function, as function of x, for fixed Q2 values as indicated in the figure. The full points are direct measurements, and the open triangles are those obtained from F; through the Gribov factorization relation. The full line is the result of a higher order fit and the dashed line is that of a leading order parameterization.
6. DISCUSSION
- T H E STRUCTURE
OF T H E INTERACTION
We concluded in the last section that at low x one studies the structure of the interaction. Let us discuss this point more clearly. We saw that in case of the proton at low x, the density of the partons increases with decreasing x. Where are the partons located? In the proton rest frame, Bjorken x is directly related to the space coordinate of the parton. The distance 1 in the direction of the exchanged photon is given by [6],
Therefore partons with x >0.1 are in the interior of the proton, while all partons with x
p-remnant
47
current jet
Figure 28. A DIS diagram, as seen from the point of view of a photon fluctuating into an asymmetric pair, in which the fast quark becomes the current jet.
is also still contributing and thus both soft and hard components are present. Another way to see this interplay is by looking a t the diagram in figure 28. In a simple QPM picture of DIS, the fast quark from the asymmetric configuration becomes the current jet while the slow quark interacts with the proton in a soft process. Thus the DIS process looks in the y*p frame just like the Q2=0 case. This brings the interplay of soft and hard processes.
7. CONCLUSION In DIS experiments at low x one studies the 'structure' of the y'p interaction. In order to study the interior structure of the proton, one needs to measure the high x high Q2 region. This will be done at HERA after the high luminosity upgrade.
I would like to thank Professors K. Maruyama and H. Okuno for organizing a pleasant and lively Symposium. Special thanks are due to Professor K. Tokushuku and his group for being wonderful hosts during my visit in Japan. Finally I would like to thank Professor H. Abramowicz for helpful discussions.
REFERENCES 1. A. Levy, Phys. Lett. B404 (1997) 369. 2. For a recent review see H. Abramowicz and A. Caldwell, Rev. Mod. Phys. 71 (1999) 1275. 3. J. Butterworth, Structure of the photon to appear in the proceedings of Lepton-Photon 99; [hep-ex/9912030]. 4. R. Nisius and references therein, The photon structure from deep inelastic electronphoton scattering, [hep-ex/9912049].
5. A. Levy, The structure of the Troika: proton, photon and Pomeron, as seen at HERA, Proceedings of the XXIth Workshop on the Fundamental Problems of High Energy Physics and Field Theory, (Eds. V. Petrov and I. Filimonova), p. 7, Protvino 1998, [hep-ph/9811462]. 6. B.L. Ioffe, Phys. Lett. B30 (1969) 123; B.L.Ioffe, V.A. Khoze and L.N. Lipatov, Hard Processes, (Korth-Holland, 1984). 7. J.D. Bjorken, in response to the question asked at DIS94, Eilat, February 1994. 8. See e.g. E. Leader and E. Predazzi, An Introduction to Gauge Theories and the New Physics, (Cambridge University Press, 1982). 9. C.G. Callan and D.J. Gross, Phys. Rev. Lett. 22 (1969) 156. 10. A.D. Martin, R.G. Roberts, W.J. Stirling and R.S. Thorne, Europ. Phys. J. C4 (1998) 463. 11. CTEQ Collab., H.L. Lai et al., Phys. Rev. D55 (1997) 1280. 12. M. Klein, Structure Functions in Deep Inelastic Lepton-Nucleon Scattering, to appear in the proceedings of Lepton-Photon 99, [hep-ex/0001059]. 13. H. Abramowicz and A. Levy, The ALLM parameterization of atot(y*p): an update, DESY 97-251, [hep-ph/9712415]. 14. A. Donnachie and P.V. Landshoff, Phys. Lett. B296 (1992) 227. 15. A. Donnachie and P.V. Landshoff, Zeit. Phys. C61 (1994) 139. 16. H. Abramowicz, E. Levin, A. Levy and U. Maor, Phys. Lett. B269 (1991) 465. 17. H. Abramowicz, L. Frankfurt and M. Strikman, Surveys in High Energy Phys. 11 (1997) 51. 18. C. Amelung for the ZEUS Collab., proceedings of DIS99, Nucl. Phys. B(Proc.Suppl.)79 (1999) 176. 19. A. Donnachie and P.V. Landshoff, Phys. Lett. B437 (1998) 408. 20. ZEUS Collab., J. Breitweg et al., Europ. Phys. J. C7 (1999) 609. 21. J. Zacek for the H1 Collab., proceeding of DIS99, Nucl. Phys. B(Proc.Suppl.)79 (1999) 86. 22. ZEUS Collab., M. Derrick et al., Europ. Phys. J. C1 (1998) 109. 23. ZEUS Collab., paper 540 submitted to HEP99, Tampere, July 1999. 24. J. Cvach for the H1 Collab., proceeding of DIS99, Nucl. Phys. B(Proc.Suppl.)79 (1999) 501. 25. H1 Collab., C. Adloff et al., DESY 98-205, to be published in Europ. Phys. J. C, [hep-ex/9812024]. 26. D. Kcira for the ZEUS Collab., proceedings of Photon99, Freiburg, May 1999. 27. PLUTO Collab., Ch. Berger et al., Phys. Lett. B142 (1984) 119. 28. F.C Erne for the L3 Collab., proceedings of Photon99, Freiburg, May 1999. 29. V.N. Gribov, L.Ya. Pomeranchuk, Phys. Rev. Lett. 8 (1962) 343. 30. H. Abramowicz, E. Gurvich and A. Levy, Phys. Lett. B420 (1998) 104. 31. H. Abramowicz, E. Gurvich and A. Levy, Next to leading order parton distributions in the photon from y*y and y*p scattering, proceedings of ICHEP98, p.885, Vancouver, July 1998. 32. J.D. Bjorken, Final state hadrons in deep inelastic processes and colliding beams, proceedings of the International Symposium on Electron and Photon Interactions at High Energies, p. 281, Cornell, 1971.
Hadron and Nuclear Physics with Electromagnetic Probes K. Maruyama and H. Okuno (Editors) 2000 Elsevier Science B.V. All rights reserved.
'
High-Q2 neutral- and charged-current reactions at HERA Kunihiro Nagano
a
*
aDESY, Notkestrasse 85, 22603 Hamburg, Germany. b~EK-Tanashi,Midori-cho 3-2-1, Tanashi, 188-0002 Tokyo, Japan. The H1 and ZEUS experiments at HERA measured e+p deep inelastic scattering cross sections at high-Q2 both for neutral-current and charged-current interactions using data taken during the 1994- 1997 running periods. Preliminary results of e-p deep inelastic scattering cross sections from the data taken during the recent 1998- 1999 running period are also presented. 1. DEEP INELASTIC SCATTERING AT HERA
Deep inelastic scattering (DIS) of leptons on nuclei has been the key for our understanding of the structure of the nucleon. HERA at DESY is a unique facility of colliding electrons (or positrons) with protons. The large center-of-mass energy, s, of about 300 GeV allows an extension of the explorable kinematic phase space by two orders of magnitude in Q2, the negative four-momentum transfer squared, compared with that covered by the previous fixed-target experiments. The maximum Q2 reaches almost lo5 GeV2, which corresponds to a spatial resolution of about 10-l6 cm. During the running in 1994-1997, HERA collided an e+-beam of 27.5 GeV and a p-beam of 820 GeV. The two collider experiments, H1 and ZEUS, collected 35.6 and 47.7 pb-' of luminosity during this period, respectively. Both the neutral-current (NC) DIS interaction, e+p -+ e+X, and the charged-current (CC) DIS interaction, e'p -+F X , have been studied using these data at high-Q2, typically Q2 > 200 GeV2 [I-31. From 1998 to April 1999, HERA provided an e--beam of 27.5 GeV and a p-beam of 920 GeV. Both H1 and ZEUS collected about 16 pb-l of luminosity during this period. Preliminary e-p -, e-X (NC) and e-p -+ vX (CC) cross sections [4-71 are also presented in this report. The NC DIS cross sections can be written as
where a is the electromagnetic fine structure constant, x is the Bjorken scaling variable, y is the inelasticity parameter and Y+= 1 1(1- y ) 2 . The kinematic variables are related Q 2 = xys. The helicity dependence of the electroweak interactions is mostly contained *The author is financially supported by t h e Japan Society for the Promotion of Science (JSPS).
in the kinematic factors Y*.The dependences on the quark structure of the proton and on the quark coupling to the Z0 boson as well as that on the mass of the Z0 boson are absorbed in the structure functions, F2NC, F t C and F,NC. The structure function F t C is dominated by a parity-violating term and contributes oppositely in e-p and e+p collisions. The evolution of the structure functions are evaluated using the next-to-leading order (NLO) DGLAP evolution equation of QCD. The longitudinal structure function FFC is not zero at NLO QCD and is only relevant at high y, where it contributes to the cross section by up to about 10%. The CC DIS cross sections can be written as
where GF is the Fermi constant, and Mw is the mass of the W* boson. At leading-order QCD, the structure function terms 4ZC can be written as
where d(x, Q2) is, for example, the parton density function (PDF) for the d-quark. The flavor-selecting nature of CC interaction is clearly seen; only down-type quarks (antiquarks) and up-type antiquarks (quarks) participate at leading order in e f p (e-p) CC DIS. The kinematic suppression factor (1 - y ) 2 is multiplied to quarks (antiquarks) in e+p (e-p) collisions. 2. NEUTRAL-CURRENT DIS CROSS SECTION
NC DIS events were required to have an isolated scattered electron with an energy greater than 11 (10) GeV in H1 (ZEUS) analysis. The selection efficiency is on average 80% for ZEUS. The main source of background is due to photoproduction, i.e. yp interaction with photon virtuality nearly equal to zero. The background contamination was estimated to be less than 1% in overall kinematic region, and was statistically subtracted. The resolutions of the kinematic variables are typically 5% for Q2 and 10% for x for ZEUS. The systematic error for the double differential cross section measurement by H1 is about 8%. The luminosity error is 1.5% (1.6%) for H I (ZEUS) and is not included in the systematic errors. 2.1. Single differential cross section da/dQ2 Figure 1 shows the e+p NC cross section da/dQ2 measured by ZEUS together with the preliminary e-p NC cross section, compared to the Standard Model (SM) predictions evaluated with the CTEQ 4D [8] PDF. The efp cross section was measured up to values of Q2=43000 GeV2. A good agreement was observed between data and SM predictions over six orders of magnitude as Q2 varies by two orders of magnitude, although a small excess at the highest Q2 was observed in the e+p cross section. H1 also observed a similar excess at the highest Q2 15000 GeV2 in efp cross section. The difference between e-p
ZEUS NC N -
10
%
Pn
a
1
CTEQ4D NLO e-p E,=820 GeV
N
Q
$ 10
.1
b P
10
.2
1994-97 e'p DATA CTEQ4D NLO e'p E,=820 GeV
I
Figure 1. The NC DIS cross section da/dQ2 measured by ZEUS both for e+p [I] and e-p [4] compared to the SM predictions evaluated with the CTEQ 4D PDF.
and e+p cross sections is clearly visible at large Q2 3000 GeV2. The figure also presents the expected SM e-p cross section with a proton beam energy of 820 GeV, resulting in a small change. The measurement demonstrates that the Z0 interference affects the cross section oppositely in e-p and e+p. 2.2. Double differential cross section d2a/dxdQ2 Figure 2 shows the reduced double differential cross section, CNC, which is defined as
measured by H1 both for e+p and e-p. The cross sections are displayed as functions of Q2 for fixed values of x ,compared to the SM prediction evaluated with the H1 NLO QCD fit [3]. A good agreement was observed with an exception at x = 0.40 in e+p data, where the measured cross section exceeds the prediction at higher Q2 10000 GeV2. This was already observed in the 1994- 1996 data [9]. The SM prediction overshoots the measured cross section at x = 0.65, where only BCDMS [lo] provides low Q2 data used in the NLO fit.
i,
1
2
' ' "
I
l o 4F
I
"
x=0,08 (x9000) 10 3Fx=0. 13 (~3000)
' ' " ' "
' ' '
BCDMS (x 0.97) o NMC --- Hl e'p QCD Fit
--" -
A
' '
'
HI e-p preliminary H1 etp
"
-
-
I
7 $
.-!-5
-.~,#hif-rry
3
Figure 2. The reduced NC DIS cross section, eNc,measured by H1 both for e+p and e-p compared to the SM predictions (solid lines for e+p and dashed lines for e-p) evaluated with the HI NLO QCD fit [ 6 ] .
3. CHARGED-CURRENT DIS CROSS SECTION
The principal signature of CC DIS events is a large missing transverse momentum, to the undetected final-state neutrino. The gT measured in the calorimeter was required to exceed 12 GeV. The overall selection efficiency is 80% and exceeds 90% in the high-Q2 region as estimated by ZEUS. The main source of background is due t o photoproduction with high transverse energy flow. The background contamination was estimated to be less than 2.5% in general, but is as large as about 10% in the lower-Q2 region of Q2 < 400 GeV2 in the ZEUS analysis. The final sample consists of about 1100 events for ZEUS. The resolutions of the reconstructed kinematic variables are typically 20% for Q2 and 8-15% for x achieved by ZEUS. The dominant systematic error is the energy-scale uncertainty of the calorimeter, resulting in a uncertainty of the measured
f i ; due
-1
Charged Current
.
H 1 e-p ds=3zo G prelimnary
~ V
- Standard Model (HI e'p QCD Fit)
Figure 3. The CC DIS cross section du/dQ2 measured by H1 both for e+p and e-p compared to the SM predictions evaluated with the H1 NLO QCD fit [6].
cross sections of typically 10%) but as much as 25% at the highest Q2 and 20% at the highest x. The total systematic error for the double differential cross section measurement of H1 is about 15%. 3.1. Single differential cross section da/dQ2 Figure 3 shows the e+p CC DIS cross section da/dQ2 measured by H1 together with the preliminary e-p NC cross section, compared to the SM predictions evaluated with the H1 NLO QCD fit. The e+p cross section was measured up to Q2 15000 GeV2. The measured cross sections are consistent with the SM predictions. ZEUS also observed a good agreement between data and SM in da/dQ2 both for e f p and e-p. Figure 4(a) shows the ZEUS measurement of the e+p CC cross section daldx for Q2 > 200 GeV2 compared to the SM prediction with the CTEQ 4D PDF. The plot (b) shows the cross section as a ratio divided by the SM prediction with the CTEQ 4D PDF. Additionally, the SM prediction evaluated with the ZEUS NLO QCD fit [ll]is also presented in the figure. The associated hatched error band represents the uncertainty of the SM prediction arising from PDF uncertainty as estimated from the ZEUS NLO QCD fit. This fit was performed to lower-Q2 data both from fixed-target and HERA experiments, not including the high-Q2 data presented in this report. At x 0.3, the data lie above the SM prediction with the CTEQ 4D PDF. The e+p CC cross section is dominated in the high-x region by the contribution from the d-quark, whose density function is poorly constrained from existing experimental data. This is reflected in Figure 4 in the large error band at high x. A possibility of a larger d / u ratio than currently assumed has been discussed in recent years. A modification of the CTEQ 4D PDF according to the prescription [12]: d/u -+ d/u O.lx(x 1) yields doldx, shown in the figure with the
+
+
ZEUS CC 1994-97 ZEUS 94-97 etp CC SM with CTEQ4D
a
ZEUS Preliminary 1998-99
' "
staq
Figure 4. The e+p CC DIS cross section daldx (defined for Q2 > 200 GeV2) measured by ZEUS [2] compared to the SM predictions evaluated with the CTEQ 4D PDF,
I}
h t . syst
Figure 5. The e-p CC DIS cross section daldx (defined for Q2 > 1000 GeV2) measured by ZEUS [5] compared to the SM predictions evaluated with the CTEQ 4D PDF.
dashed line, close to the ZEUS NLO QCD fit. For comparison, the prediction with the MRST [13] P D F is also shown in the figure. Figure 5 shows the preliminary e-p CC cross section daldx defined for Q 2 > 1000 GeV2 measured by ZEUS. A good agreement was observed between data and the SM prediction. It is worth while to note that the e-p CC cross section is dominated by the contribution from the u-quark at high x.
Figure 6. The reduced double differential CC cross section Zrcc measured by H1 as a function of x for fixed values of Q2, compared to the SM predictions with the H1 NLO QCD fit [6].
which is defined as
measured by H I both for e+p and e-p. The cross sections are displayed as functions of x for fixed values of Q2, compared to the SM prediction evaluated with the H I NLO QCD fit. The reduced cross section depends directly on the quark density distributions at leading-order QCD (see Eqs. (5) and (6)). A good agreement between data and the SM prediction was observed both in e+p and e-p; the measurement clearly demonstrates the flavor-selecting nature of the charged-current interaction. 4. HELICITY DEPENDENCE OF NC AND CC INTERACTIONS
In the approximate Bjorken scaling region of x x 0.1, the helicity dependence of structure functions can be separated from the dependence on parton densities. Figure 7 presents the H1 measurement of the structure function terms, q5NC and $CC, as functions of (1 - Y)' at fixed x = 0.13 both for e+p and e-p. The kinematic factor (1 - Y ) ~
x=O. 13
clcctrou
positron
H1 NC e-p preliminary H1 CC e'p preliminary
- H1 e'p
QCD Fit
.---y exchange only
Figure 7. The structure function terms for NC and CC, HI [7].
$NC
and $",
measured by
is related to the lepton scattering angle in the lepton-quark center-of-mass frame, 8*, as: cos4$ = (1 - y)2. In the upper plots, are presented together with SM predictions evaluated with the H1 NLO QCD fit. Also, SM predictions calculated only from y exchange terms are shown in the plots for illustration; the difference between and '$! is due to Z0 exchange. The lower plots show $2'. The difference in the intercepts (at (1 - y)' = 0) can be understood at leading order QCD stemming from the difference between densities of up-type quarks and of up-type antiquarks. And the difference in the slopes can also be understood at leading order QCD as due to the difference between densities of down-type quarks and down-type antiquarks (see Eqs. (5) and (6)).
$zC
$yC
5.
Mw determination
Two electroweak parameters, Mw and GF, enter the CC cross section; GF determines the absolute magnitude of the cross section, and Mw determines the shape of the cross
ZEUS 1994-97
Figure 8. The ZEUS
Mw
and GF fit to the e+p CC DIS cross section da/dQ2 [2].
section. A chi-squared fit to the ZEUS measured e+p CC da/dQ2, treating G F and Mw as free parameters, yielded +0.026 ( s y ~ t+0.016 G F = 1.171f0.034 tat.)-^,^^^ . ) - ~ ,(PDF) ~,, x G~v-~, (9)
Mw
=
80.8+::! (stat.)::::
(syst.)+i:' (PDF) GeV,
(10)
respectively. The central values are obtained with the CTEQ 4D PDF, and the P D F errors quoted are evaluated from the ZEUS NLO QCD fit. The obtained value of G F is in agreement with the value obtained from the muon-decay experiment, implying the universality of the CC interaction over a wide range of Q2. The obtained value of Mw is in agreement with the value obtained from direct measurements at LEP and Tevatron in the time-like region, demonstrating a consistency of the SM assumptions in complementary measurements. Figure 8 displays the fit result as the triangle with the 70% confidence level contour, which is determined only from statistical errors.
Two more fits were performed with more restrictive theoretical assumptions. First, a "propagator-mass" of the exchanged W + boson was extracted by fixing GF as that obtained from the muon-decay experiment [14]. The fit yielded Mw = 8l.4?$:~(stat.)k2.0(syst.)~~:~(~~F) GeV, which is also shown in Figure 8 as the solid point along the GF = 1.16639 x GeV-2 line. H1 performed a similar fit to their CC double differential cross section and obtained Mw = 80.9zk3.3(stat.)i1.7(syst.) GeVf 3.7(theo.) GeV. The SM uncertainty (quoted as "(theo.)" above) was evaluated by varying the assumptions for the H1 NLO QCD fit. Finally, a "Standard Model fit" was performed by ZEUS using the SM relation between GF and Mw, which includes the radiative corrections, namely contribution from t-quark, Z0 boson and Higgs boson. This constraint is also shown in the figure as the heavy solid line. The fit yielded Mw = 80.50?~:~$(stat.)~::$(syst.)f O,~~(PDF)+:::;(AM~, AMH,AMz) GeV. The result is indicated in the figure as the small star, while the large star along with the SM constraint line shows the position of the minimum x2. A great sensitivity to Mw was obtained within the SM framework. REFERENCES 1. ZEUS Collab., J. Breitweg et al. , DESY-99-056, accepted by Eur. Phys. J. 2. ZEUS Collab., J. Breitweg et al. , DESY-99-059, accepted by Eur. Phys. J. 3. H1 Collab., C. Adloff et al. , DESY-99-107, submitted to Eur. Phys. J. 4. ZEUS Collab., contributed paper #549 to EPS'99 conference, Tampere, Finland. 5. ZEUS Collab., contributed paper #558 to EPS'99 conference, Tampere, Finland. 6. HI Collab., contributed paper #157b to EPS'99 conference, Tampere, Finland. 7. M. Erdmann, a talk given at DESY seminar '99. 8. H.L. Lai et al. , Phys. Rev. D 55(1997) 1280. 9. HI Collab., C. Adloff et al. , 2.Phys. C 74(1997) 191. 10. BCDMS Collab., A.C. Benvenuti et al. , Phys. Lett. B 364(1995) 107. 11. M. Botje, DESY 99-038, NIKHEF-99-011. 12. U.K. Yang and A. Bodek, Phys. Rev. Lett. 82(1999) 2467. 13. A.D. Martin et al. , Eur. Phys. J. C4(1998) 463. 14. Particle Data Group, C. Caso et al. , Eur. Phys. J. C3(1998) 1.
Hadron and Nuclear Physics with ElectromagneticProbes K. Maruyama and H. Okuno (Editors) 2000 Elsevier Science B.V. All rights reserved.
Spin structure of the nucleon studied by
HERMES
Yasuhiro Sakemia* on behalf of the HERMES collaboration "Department of Physics, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8551, Japan Spin asymmetries of semi-inclusive cross sections for the production of positively and negatively charged hadrons have been measured in deep-inelastic scattering of polarized positrons on polarized hydrogen and 3He targets in the kinematic range 0.023 < x < 0.6 and 1 GeV2 < Q2 < 10 GeV2 . Polarized quark distributions are extracted as a function of x for up and down quarks. The up quark polarization is positive and the down quark polarization is negative in the measured range. The polarization of the sea is compatible with zero. A measurement of the spin asymmetry in the photoproduction of high-pT hadron pairs is also presented. For h+h- pairs with pkl > 1.5 GeV/c and pk2 > 1.0 GeV/c the measured asymmetry is negative, in contrast to the positive asymmetries typically measured in deep inelastic scattering from protons, and can be interpreted to arise from a positive gluon polarization.
.
1. INTRODUCTION
The HERMES experiment studies the spin structure of the nucleon using semi-inclusive polarized deep inelastic scattering (DIS). In the Quark-Parton Model (QPM), the spin of the nucleon may be decomposed as:
where AX, AG, L,4, and LZGare the contributions from quark and gluon spin and quark and gluon orbital angular momentum. AC can be further divided among the quark flavors, and between valence and sea contributions:
Previous inclusive experiments precisely measured the polarized structure function gl (x), which can be interpreted in the QPM as the sum over all the flavors of polarized parton distribution functions (PDFs) Aq E (qf (x) - qP(x)):
*E-mail address:
[email protected] The function, qf(-), represents the distribution of quarks of flavor q with spin parallel (anti-parallel) to the nucleon spin, and e, is the quark charge in units of the elementary charge. Here, x is the Bjorken scaling variable x = Q2/2Mv which is determined by the scattered lepton's kinematics, where Q2 and v are the negative squared four-momentum and energy of the exchanged virtual photon, and M is the nucleon mass. By measuring gl(x) from various targets and integrating over x, it has been found that AC is only about a third of the nucleon spin, and that the strange sea seems negatively polarized. In the semi-inclusive measurements, hadrons are measured in coincidence with the scattered lepton. Hadron productions in DIS are described with the absorption of a virtual photon by a point-like quark and the subsequent fragmentation into a hadronic final state. Measuring asymmetries in semi-inclusive reactions yields further insights into the individual contributions of the quark flavors to AX. In these reactions, one also detects the properties of the fragments of the nucleon breakup after the initial scattering, which are correlated to the flavor of the struck quark in the reaction. One way to measure AG(x) is by isolation of the leading order QCD photon gluon fusion diagram. In deep inelastic scattering, one can do this via charm production or dijet production. Jet separation is not possible at most fixed target experiments due to the center of mass energy not being high enough. However it is possible to use highp~ hadrons instead of jets. A few phenomenological studies of the sensitivity of spindependent high-pT hadron photoproduction and low-Q2 electroproduction to the polarized gluon distribution AG have been conducted [1][2][3]. This paper reports on the extraction of polarized quark distribution from data taken by the HERMES experiment using the 27.5 GeV beam of longitudinally polarized positrons in the HERA storage ring at DESY, and on the polarized gluon distribution extracted from the spin asymmetry of high-pT hadrons. 2. THE HERMES EXPERIMENT
The HERMES experiment was designed to optimize semi-inclusive DIS measurements [4]. The experiment is located at the DESY laboratory in Hamburg, Germany, where a 27.5 GeV positron (or electron) beam is circulated in the HERA ring. The positron beam is naturally polarized transverse to the beam direction due to an asymmetry in the emission of synchrotron radiation, and longitudinal polarization at the HERMES interaction point is provided by a pair of matched spin rotators. The averaged beam polarization during the run is 55 % with a relative systematic uncertainty in the measurement of 4.0 % (3.4 %) for the 3He(H) data. The HERA beam passes through the center of a 40 cm long cylindrical storage cell containing polarized 3He or 'H.In 1995, the 3He target atoms became polarized through metastability-exchange optical pumping [5]. Polarization is 46 % with a fractional uncertainty of 5 % . In 1996 and 1997, an Atomic Beam Source (ABS) used permanent sextupole magnets and radio-frequency units to select desired hyperfine states of 'H.A Breit-Rabi polarimeter (BRP) monitors the proton polarization, which is 86 % during the run. The HERMES detector is an open-geometry forward spectrometer. A series of tracking chambers placed before and after a dipole magnet determine momenta and directions
of those particles. A threshold ~erenkovdetector, a Transition Radiation Detector, a Preshower Counter, and an Electromagnetic Calorimeter allow separation of electrons from hadrons. The ~ e r e n k o vfurther provides identification of pions above particle momenta of 5.6 and 3.8 GeV/c in 1995 and 1996-7. The data from the polarized 3He and polarized 'H targets is used to extract quark polarizations, and the polarized 'H target data is used to study the gluon polarization, respectively. 3. FLAVOR DECOMPOSITION OF THE POLARIZED QUARK DISTRI-
BUTIONS 3.1. Spin asymmetries of semi-inclusive cross sections In polarized deep-inelastic scattering, one can construct a cross-section asymmetry All of parallel and anti-parallel orientations of the nucleon spin to the lepton beam spin :
Under the assumption that g2(x) is zero, the measured double-spin asymmetry Ail is directly related to the asymmetry of the virtual photoabsorption in nucleon, Al(x), and to the structure function ratio, gl(x)/Fl(x):
where D. y.and q are kinematic factors. For this discussion, the index h of the asymmetry and structure function includes both inclusive positron data and coincident positive and negative hadrons and identified pions (h+, h-, T + , and T-). HERMES has measured asymmetries for inclusive and semi-inclusive h+ and h- reactions from 1995, 1996, and 1997 running. These asymmetries are shown in Figure 1. Inclusive DIS data are selected from events with cuts on Q2 > 1 GeV2, y = v / E < 0.85, and the final hadronic state mass, W2 > 4 GeV2 . The sizes of the inclusive event samples satisfying these requirements were 2.2 x lo6 and 2.3 x lo6 for 3He and 'H respectively. The analyzed semi-inclusive events form a subsample having a detected charged hadron and W2 > 10 GeV2. A cut on the hadron energy fraction, z > 0.2, and the Feynman variable. xf > 0.1, also selects nucleon fragments from the current region. The uncertainties in beam and target polarization measurement uncertainties, uncertainties in R(x, Q2), experimental yield fluctuation in 1995 are the dominant contributions to the systematic uncertainty of the measured asymmetries. 3.2. Interpretation of t h e asymmetries In the QPM and under the assumption of factorization, one sees that the photoabsorption asymmetry probes the helicity difference of the parton distributions Aq(x):
The asymmetries are assumed to be Q2 independent as the experimental values show no significant Q2 dependence. The fragmentation functions DPh(z)are spin independent and represent the probability that a struck quark q fragments into a hadron type h.
Figure 1. Results for the virtual photon asymmetries from HERMES 1995,1996, and 1997 data. The top row shows the asymmetries on the 'H target; the bottom row shows the 3He target. The columns are the inclusive, semi-inclusive h f , and semi-inclusive hasymmetries from left to right. The inclusive asymmetries are compared with the SLAC El43 [6] and El54 [7] measurements. For the semi-inclusive asymmetries with 'H target, a comparison with the SMC [8] results are shown.
This expression can be rearranged by defining a quark purity denoted by Pqh(x,z)for each hadron type h as
P," (x, 2) =
s;;
eiq(x) dzD,h(z) e$ql(x) dzD?/ (2) '
c~~ s;:
Pqhrepresents the probability that a detected hadron h is originated from a struck quark of flavor q in the nucleon. Using these purities, the asymmetries in equation 6 may be rewritten as
The asymmetries, purities, and quark polarizations may be grouped into matrices, and these equations can be solved using matrix algebra,
The quark polarizations are obtained by least square minimization techniques. The quark purities relevant for the HERMES experiment have been estimated with a Monte Carlo simulation of unpolarized DIS. The simulation model is based on LUND string fragmentation [9], CTEQ low Q2 unpolarized parton distributions [lo]. After nuclear corrections 3He asymmetries are related to quark distributions in the proton. In solving equation 9, one would like to separate the spin contributions of six quark flavors: u, dl s , fi, l a n d 3. In fact, statistics and limited sensitivity to the sea quark flavors require a selection of the model on average sea polarization. A simple assumption of flavor independent sea polarization is used here: Aqs - Ads 4s ds
Au,
Ad
Afi
As - As -
s
S
3.3. Extracted polarized quark distributions Using the six experimental asymmetries Ape,Aph+,Aph-, AsHee,AsHeh+,AsHeh- shown in Figure 1, three quark polarizations have been extracted: (Au Aii)/(u ii)(x) , the total up flavor polarization; (Ad + Ad)/(d + d)(x) , the total down flavor polarization; and (Aqs/qs)(x), the average polarization of the sea quarks. After multiplying by x and the unpolarized PDF parameterization, Figure 2 shows the results from this experiment in nine X B ~bins. The extraction has been repeated with GRV parton distributions [ll],with the independent fragmentation model, and with different parameterization for the fit of the LUND string model to HERMES multiplicities; a systematic uncertainty is assigned from the variation of the resulting quark polarizations. For x > 0.3, it is assumed that sea polarization does not contribute significantly to the measured asymmetries; the sea polarization, %(x), is set to zero with a small systematic uncertainty. In Figure 2, the parameterizations of De Florian et al. (0.1 < AG < 0.8, LO) [12] Gehrmann and Stirling (Gluon A, LO) [14], and Gliick et al. (Standard, LO) [13] are provided for comparison. A correction of (1+R) to the De Florian and Gliick parameterization~is necessary for consistency. These parameterizations agree with the HERMES results. Figure 3 compares the extracted valence quark polarization with the similar
+
+
extraction by the SMC experiment. Note that the HERMES definition of the flavor independence of sea polarization differs slightly from the model chosen by SMC; it has been verified that the results are insensitive to the slight differences between the models. Within statistical and systematic uncertainties, the results of both experiments agree, though high precision HERMES proton data is more sensitive to Au, and Afi [15]. Moments of the quark distributions in the measured x range are determined from the area under the measured points. For comparison to previous measurements, the distributions are extrapolated to low x by constant fits to the data; the low x extrapolation is quoted but no value is quoted for the uncertainty due to theoretical ambiguities. The resulting integrals in the measured region are listed in Table 1. The integrals are compared to predictions from Reference 16, which are corrected to fourth order in QCD; these predictions have been extracted from inclusive data assuming SU(3) flavor symmetry. The HERMES values are slightly smaller in magnitude than these predictions.
+
Au Aii Ad+Ad As+As
measured region predictions low-x total integral 0.51 f 0.02 f 0.03 0.04 0.57 f 0.02 f 0.03 0.66 f 0.03 -0.22% 0 . 0 6 f 0.05 -0.03 -0.25% 0 . 0 6 f 0.05 -0.35% 0.03 - 0 . O l f 0 . 0 3 f 0.04 0.00 - 0 . 0 1 f 0.03% 0.04 - 0 . 0 8 f 0.02
Table.1: The first moments of the extracted polarized parton distributions. 4. SPIN ASYMMETRY IN THE PHOTOPRODUCTION OF HADRON PAIRS 4.1. Analysis The cross section asymmetry A l l is given by
where N+ (N-) is the number of hadron pairs observed when the target spin is parallel (anti-~arallel)to the beam spin direction. The luminosities for each target spin state are L' and L,*, the latter being weighted by the product of the beam and target polarization values for each spin state. To identify the hadron pairs of interest, it was required that at least one positive hadron h' and at least one negative hadron h- be observed in the spectrometer. Each of these two hadrons were required to have a momentum greater than 4.5 GeV/c and transverse momentum p~ greater than 0.5 GeV/c. Here, p~ is defined with respect to the positron beam axis. It was required that the invariant mass of the two hadron system (assuming the hadrons to be pions) be greater than 1.0 GeV/c2. This effectively reduced the background arising from diffractive processes, specifically the decays of vector mesons. In order to include statistics from leptoproduction at Q2 x 0, it was not required that the scattered positron necessarily be observed in the spectrometer. The measured All is shown in Figure.4 . A negative asymmetry is observed at high transverse momenta of the negative hadrons. It should be noted that small and positive
Figure 2. Polarized parton distributions for up and down quarks, x(Au AG) and x(Ad + Ad), compared to the parameterizations of world data of g7'P(x). The data points have been evolved to Q2 = 2.5 Gev2 for the comparison.
+
Figure 3. The valence separation of parton distributions at Q2 = 2.5 GeV2 for up x(Auv), down x(Adv) and sea quarks x(Aq,) compared to results from the SMC experiment. The SMC data points have been extrapolated to Q2 = 2.5 GeV2 for the comparison.
asymmetries are expected for DIS from polarized proton target and that diffractive processes are expected to give zero asymmetry. A negative asymmetry is thus an indication for a different contribution to the asymmetry. 4.2. Interpretation The result for the asymmetry is interpreted in a model including contributions from vector meson dominance processes (VMD) and the direct photon processes. The direct photon processes are modelled using the two LO QCD processes: the QCD Compton effect (QCDC) and photon gluon fusion (PGF). The contribution from deep-inelastic scattering has been evaluated to be small. The asymmetries of these processes are related to the measured asymmetry in the following way:
where D is the virtual photon depolarization factor and Ai and fi are, respectively, the asymmetry and fraction of events arising from process i . Under the assumption of a zero spin asymmetry for VMD processes, the first term vanishes. For the small region of the phase selected by the present analysis, the asymmetries for QCDC and PGF are given by:
~ -1 The subprocess asymmetries 6's are directly calculable in LO QCD. They are G p G = (exactly, for real photons and massless participants), and GQcoc x 0.5 (for the average kinematics of this analysis). The polarized quark distributions Aq(x) are relatively well known from inclusive and semi-inclusive polarized DIS measurements. The contributions of the various processes were determined using the PYTHIA Monte Carlo generator [17]. The PYTHIA parameters were chosen according to Ref. [la] while the JETSET fragmentation parameters were tuned to semi-inclusive deep-inelastic scattering data from HERMES. In the region of interest ( pkl > 1.5 GeV/c and pk2 > 0.8 GeV/c ), the model was found to describe the shape of various distributions relatively well, though the normalization of the cross section was found to be underestimated. In this model, the cross section at high p~ is dominated by the photon gluon fusion, so this data was used to extract AG/G. The Monte Carlo showed no leading particle effects (as the background from QCDC was small), despite the fact that the data show a tendency to more negative All for pk- > 1.5 GeV/c , as opposed to large &+. The measured asymmetry (now averaged over the two plots from Figure.4) is compared with predictions from the Monte Carlo with different assumptions for AG/G in Figure.5. The best fit value for AG/G in this model is AG/G = 0.41 f O.lG(stat.) f 0.04(syst.) at an average XG of 0.17 [19].
5. CONCLUSIONS AND OUTLOOK
The first three years of HERMES data taking have yielded high statistics polarized deep-inelastic scattering data on 3He and 'H targets. The polarized measurements yield inclusive and semi-inclusive charged hadron asymmetries, which have been presented as a function of x. In the framework of the Quark-Parton Model these asymmetries are simultaneously fitted to obtain helicity distributions for up, down, and sea quarks. A measurement of the spin asymmetry All in quasi-real photoproduction of high p~ hadron pairs is also presented. When interpreted in a LO QCD model implemented in the PYTHIA Monte Carlo generator, a value of the gluon polarization in the nucleon AG/G is extracted. Further sensitivity to sea polarizations is gained by identification of the different coincident hadron measurements. Identification of pions from hadrons by the HERMES threshold ~ e r e n k o vcounter, may provide better sensitivity to As in the quark polarization extraction [20]. At present HERMES is taking a new data set with high statistics on the deuterium target, which will significantly improve the precision on the d-quark helicity distribution. In 1998, the Threshold ~ e r e n k o vcounter was replaced with a Ring
,
b
o.2
2 1 3
pr>1.5GeV/c
0 ...*......... .+................................................... -0.2 [ -0.4 -0.6 F -0.8 0 . 5
0.75
1
1.25
1.5
1.75
0.6
,
P y > 1 : 5 5
AG/G=-14 I
$ 0.4
-
0.2 : 2
-. -.-._._._._._._._._.-.-.-.-.-.-.-~-.-.j
-t-
4
O
-.:.
.tr
AG/G=O
I
-. -.-.-.-.-.,. . . . . .;i -----a
-0.4
-
-0.6 -0.4 -0.6 -0.8 1 0.5
-
0.75
1
1.25
1.5
1.75 2 ph;*(Ge V/c)
Figure 4. The measured asymmetry: the top plot is A l las a function of pTh- for pk+ ;t 1.5 GeV/c and the bottom plot is Ail as a function of p ~ for~ pk-+ > 1.5 GeV/c. The rightmost point in each plot is identical. Only the statistical error is shown.
- - - GSA ( 0.4)
. . . GSB ( - 0.3) 1 -.-.-. GSC (o tlie isovector giant dipole resonance (I\'GDR): ancl the 2+ phonon to the isovector giant' quatlrul>ole resonance ( I i 7 Q G R ) .An isoscaler mode is not excited since t h e MEC acts as a T-T+ (or T + T - ) operator. Thus. one of the two phonons of this I V G D R z I V G Q R 1state is t h e proton-particle and neutron-hole type while the other neutron-particle and roton on-hole tj.pe. The 1- tu-o-phonon G R escitecl by photo-absorption can be regarded with the C;R of the nest nucleus. as the pliono11 escitation cou~~lecl 1.Suzulii[5] predicted the photo absorption cross sect,ion to escite t h e t>wo-phononGR to be as large as 2.5% of that of the single GDR escitation in "Ca. One may expect to oljspr~.ethe t,\vo-phonon G R in the 'OCa(? , n ) reaction by measuring neutrons ~vhich escape from t h e two-phonon state to one-phonon stat,e.
3. EXPERIMENTS WITH 1.2 GEV CIRCULATING BEAM For esperinle~ltswith GeV electrons, one of the main interests is t'he behavior of t h e nucleon escitation in nuclei. As presented by Yamazaki in this symposium[6], we have studied S l l resonance in nuclear medium at the ES facility of t h e former IXS. \Ve are ) on various lluclear to estend the measurements of ( - , . I / ) ancl ( ~ , p 7 /reactions targets wit11 nlucli Ijetter statistics, in order to see whether t h e resonance property does cliange in nuclear nletliuill or not.
TAGX
SCISSORS
Figure .3. 1.2-Gel7 tagged photon beam course and target station.
In Fig. 3, we show the experimental area for 1.2-GeV tagged photon experiments. The photon tagging systelll is now being in~t~alled.The system consists of a radiator placetl at t h e entrance of the bending magnet (BM4) of the S T B ring and 100 plastic scintillat,ors placed in a snlall room in the bellcling maganet. The radiator made of a thin carbon foil can be moved ancl set at the beam position very quickly: t h e rac1iat)or is removed from the beam position when t,he electron beam is injected and accelerated. The electrons emitting photons are detected with one of t,he plastic scintillators (5 x 5 x 20 m m ) . Each scirltillator is connected with a special light guide which couples t o 3-m long light f i b ~ r . Then, phot,omult,ipliers can b e placed a,t a distance from t h e bending magnet. T h e target position of the ta,gged photon is &out 7.5 m from the radiator. There. two large detector syst,ems will be placed: one is the TAGX sytem, which has been working very well in the ES facility at INS for these 15 years. T h e other is called SCISSORS (Sendai CsI Scintillation Systenl On Radiat,ion Search): a large scintillation detector system: corlsisting of 200 pure CsI crystals. The former system will be used maily for charged part,icle detection, while the latter for y-ray detection. As sllown in Fig. 3, they can be placed at the target position of the tagged photon beam. The 1.2-GeV tagged phot,on system is now being examined together with the improveinent of t,he quality of the 1.2-GeV electron beam circulating the STB ring. The tagged photo11 esperirnent is expected to start in llay, 2000, and, then, LNS will serve not only for lo~r-energynuclear physics but also for GeV energy nuclear physics.
REFERENCES I. ' I Terasalva, . private coininunicat~ion. 2. S . Mordechai et al., Phys. Rev. Lett. 60 (1988) 408. 3 . J . Ritnlan et al.. Phys. Rev. Lett. 70 (1993) 533; R. schmit et al., Phys. Rev. Lett. 70 (1993) 1767. 4. K. Frascaria, Nucl. Phys. A569 (1994) l l l c . .5. T . Suzulci, private communication. 6 H. Yamazaki, paper in this proceedings; T . I'orita et al., Phys. Lett. B in press.
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Hadron and Nuclear Physics with Electromagnetic Probes K. Maruyama and H. Okuno (Editors) 2000 Elsevier Science B.V. All rights reserved.
'
Laser electron photon facility at Spring-8 T . Hottaa* , J.K. Ahna, H. Akimuneb, Y. Asanoc, W.C. Changd, S. DatBe, M. Fujiwaraalf, K. Hicksg, K. lmaii, T . Iwatah, T . lshikawai, H. Kawai", Z.Y. KimJ, T . Kishimotom, N. Kumagaie, S. MakinoP, T. Matsumuraklc, N. Matsuokaa, T . Mibea, M. Miyabei, Y. Miyachih, T . Nakanoa, M. Nomachia, Y. Ohashie, T. Oobao, '~~ Sakaguchim, , T. Sasaki', D. Sekii, H. Ookumae, M. Ooshimaf, C. R a n g a ~ h a r ~ u l uA. H. Shimizua, Y. Sugayaf, M. S ~ m i h a m a ~T). ~~ ,o o ~ a mH.a ~Toyokawae, , A. Wakai", C.W. Wangd, S.C. wangd K. Yoneharab, T . Yoritaa, and M. Yosoia "Research Center for Nuclear Physics, Osaka University,lO-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan bDepartment of Physics, Konan University, Kobe, Hyogo 658-8501, Japan 'Japan Atomic Energy Research Institute, Mikazuki, Hyogo 679-5143, Japan dInstitute of Physics, Academia Sinica, Taipei 11529, Taiwan "Japan Synchrotron Radiation Research Institute, Mikazuki, Hyogo 679-5143, Japan fAdvanced Science Research Center, Japan Atomic Energy Research Institute, Tokai, Ibaraki 319-1195, Japan gDepartment of Physics, Ohio University, Athens, OH 45701, USA hDepartment of Physics, Nagoya University, Nagoya 464-8602, Japan 'Department of Physics, Kyoto University, Kyoto 606-8502, Japan JDepartment of Physics, Seoul National University, Seoul 151-742, Korea kDepartment of Physics, Yamagata University, Yamagata, Yamagata 990-8560, Japan 'Department of Physics, University of Saskatchewan, Saskatoon, S7N 5E2, Canada "Department of Physics, Osaka University, Toyonaka, Osaka 560-0043, Japan "Center for Integrated Research in Science and Engineering, Nagoya University, Nagoya 464-8603, Japan "Department of Physics, Chiba University, Inage, Chiba 263-8522, Japan PWakayama Medical College, Wakayama, Wakayama 641-0012, Japan At Spring-8, we built a 2.4 GeV photon beamline with 350 nm laser photon backscattering off the circulating 8 GeV electron beam. A detector system, optimized to carry out the photoproduction of meson near threshold, is nearly completed.
+
1. LASER ELECTRON PHOTON FACILITY AT SPRING-8
High energy photon beams produced by laser-induced backward Compton scattering off the circulating electrons (laser electron photon) is utilized for nuclear physics studies at various synchrotron radiation facilities in the world [1,2]. Spring-8, the world's highest energy third-generation synchrotron radiation facility, enables us to produce the highest *Corresponding author. E-mail:
[email protected] Experimental Hutch
0
10
20rn
Figure 1. The Laser Electron Photon facility at Spring-8 (LEPS)
energy laser electron photon beam. We have constructed the laser electron photon (LEP) facility a t BL33LEP, one of the 61 beamlines (Fig. 1). At Spring-8, 8 GeV electron beam is circulating with I,, = 100 mA. The laser beam is injected from the Laser Hutch to the straight section. The Compton scattering of the laser photon with an 8 GeV electron produces GeV photons which can be used for the experiments of quark nuclear physics. The maximum energy of the LEP is determined by the electron energy and the laser wavelength. The LEPS facility produces the maximum photon energy of 2.4 GeV with 350 nm Ar laser. At present, Spring-8 is the only facility providing LEP beams above yp -+ q5p threshold. When shorter wavelength (200 nm) laser is used in near future, the maximum photon energy becomes higher than 3 GeV. One of the advantages of LEP beam is its flat intensity distribution for the Compton photons. Thus, experiments do not suffer from high intensity low energy background, a common problem with Bremsstrahlung beams. Another feature is high polarization of photons. If laser photons are 100 % polarized, the LEP is also polarized at the maximum energy. The polarization of the photon decreases a t lower photon energy. However, by changing the laser wavelength, highly polarized photons in wide energy range can be obtained. The position and polarization of the laser can be monitored on both sides of the interaction region; at the Laser Hutch and the end of the beamline. When the electron energy is high, LEPs are emitted in a narrow cone in the electron beam direction. At the LEPS facility, photons in the energy region of interest ( E , 2 1.5 GeV) are within the scattered angle less than 0.15 mrad, which result in beam size of about 1 cm at the target point in the Experimental Hutch. The energy of the LEP is determined by measuring recoil electron with a tagging system. The bending magnet of the storage ring is used for analyzing the momentum of recoil electron. The tagging system is a position detector located inside the ring at the downstream end (along the electron beam) of the bending magnet. It consists of two layers of 500 pm thick silicon strip detectors with 100 pm pitch and two layers of plastic scintillator array. Photons of 1.5 to 3.5 GeV can be tagged by measuring recoil electron
with the momentum range between 6.5 and 4.5 GeV. Simulation results show that the photon energy resolution is expected to be around 15 MeV, and it is mainly determined by energy and angular spread of incident electrons. 2. BEAM COMMISSIONING
The detailed design of the beamline started in 1997. In 1998, some accelerator components were modified to inject a laser beam against the electron, to extract a LEP, and to utilize the tagging system. The laser injection system was completed by the first quarter of 1999. The first LEP beam at Spring-8 was produced on July lst, 1999. A PWO calorimeter was used for the measurement of the beam energy. Fig. 2 shows the measured energy spectrum of the beam. The tagging system was also tested. Fig. 3 shows
0
0.5
1
1.5
2
2.5
3
3.5
4
E, (GeV)
Figure 2. The energy spectrum of the LEP measured by PWO calorimeter.
0
5
lo
15 20 25 position (rnrn)
30
35
Figure 3. The energy of the LEP measured by PWO versus the hit position of recoil electron at the tagging counter.
the correspondence between the energy measured by the PWO calorimeter and the hit position of recoil electron at the tagging counter. While this measurement shows that there is one to one correspondence between the hit position and the photon energy, the resolution of PWO is not good enough to estimate the tagger energy resolution. We plan to measure the e+e- pair conversion of the LEPS beam with the magnetic spectrometer and the detector system to get an estimate of the tagger resolution. As the first stage of the laser system, The UV multi-line photon of Ar laser (-350 nm) was used to produce the LEPs. The integrated intensity at that time was about 2 x lo6 photons/sec with 5 W laser power and 100 mA electron beam current, which is about 5 times lower than we expected. We have found some problems on the reflectivity of the mirror and we expect the intensity reaches 1 x lo7 photons/sec after the beam commissioning.
3. PHYSICS
At the LEPS, many experiments have been proposed and discussed. One of the experiment to be done at the first stage is 4 photoproduction. The photoproduction of vector mesons (VMs) are described by the process that y fluctuates into VM which is well known as the vector meson dominance. Then the VM is scattered diffractively by Pomeron exchange [3]. In the framework of Regge theory, Pomeron was introduced to describe the universal rise of hadronic cross section at high energies. Nowadays Pomeron exchange can be understood as multi-gluon exchange process [4]. In the 4 photoproduction, the meson exchange process is strongly suppressed by OZI rule and the cross section from threshold to HERA energy region is mainly due to Pomeron exchange [ 5 ] . It means that the gluon exchange process at low energy can be studied only by 4 photoproduction. As an example, it is suggested that precise measurement of unpolarized cross section will clarify O+ glueball contribution [6].By using polarized photon and/or polarized target, we can study the contributions of different processes, including sS knockout from a nucleon, in terms of helicity amplitudes [7]. 4. DETECTORS
Figure 4. The LEPS detector.
The LEPS detector setup is shown in Fig. 4. The detector is designed for the measurement of the q5 photoproduction in the forward direction. It consists of silicon strip vertex detectors (SVTX), multi-wire drift chambers (MWDC), a dipole magnet, and a time-of-flight (TOF) wall.
The dipole magnet has 135 cm wide and 55 cm high opening and the length of the pole along the beam is 60 cm. The field strength is 1 T at the center. The SVTX consists of 2 planes (x and y) of single-sided silicon strip detectors (SSDs). The thickness of each SSD is 300 pm and the strip pitch is 120 pm. With the SVTX we measure the positions and energy loss for particle identification. A MWDC (DC1) with 5 planes (x, x', y, y', u) is located upstream and a pair of MWDC (DC2 and DC3) are downstream of the magnet. The sensitive area is 80 cm wide x 30 cm high for the DC1 and 200 cm wide x 80 cm for the DC2 and DC3. Each of the DC2 and DC3 has 5 planes; x, x', y, y' and u (for the DC2) or v (for the DC3). The TOF wall is used to measure times of flight of particles from the target. The TOF wall consists of 40 plastic scintillator bars of dimensions: 4 cm thick x 12 cm wide x 2 m long. The timing resolution of better than a = 100 psec has been achieved. The TOF start signal with a = 12 psec is made from an RF signal of 8 GeV storage ring. As a standard setup, the TOF wall is located at 3 m from the dipole magnet and a flight length of a charged particle is about 4 m. In this case, K/n separation of up to 2 GeVlc momentum is expected at better than 4a level. Each detector components has been tested separately by using the LEP beam and Bremsstrahlung y from the ring. An integrated test of the detector has been just started. For other experiments, a photon calorimeter, made up of 252 lead scintillating fiber detectors of 14 radiation lengths, covers from 30 - 100" in laboratory system. Also, a superconducting solenoid magnet is being built for the future development of polarized target. 5. S U M M A R Y The laser electron photon beam with the maximum energy of 2.4 GeV has been successfully produced at a newly developed beamline, BL33LEP at Spring-8. The beam intensity will be increased up to 1 x l o 7 /sec after the beam commissioning. The detector system has been constructed and the tests are underway. The first test experiment of $ photoproduction is planned for early 2000. REFERENCES 1. A. M. Sandorfi, J. LeVine, C. E. Thorn, G. Giordano and G. Matone, IEEE Trans. Nucl. Sci. 30, 3083 (1983) 2. C. Schaerf, Nucl. Phys. News 2 (1992) No. 1 7-8 3. T . H. Bauer, R. D. Spital, D. R. Yennie and F. M.Pipkin, Rev. Mod. Phys. 50, 261 (1978) 4. A. Donnachie and P. V. Landshoff, Phys. Lett. B296, 227 (1992) 5. M. Derrick et al. [ZEUS Collaboration], Phys. Lett. B377, 259 (1996) 6. T. Nakano and H. Toki, Exciting Physics with New Accelerator Facilities, World Scientific, 1997 7. A. I. Titov, Y. Oh and S. N. Yang, Phys. Rev. Lett. 79, 1634 (1997)
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Hadron and Nuclear Physics with Electromagnetic Probes K. Maruyama and H. Okuno (Editors) 2000 Elsevier Science B.V. All rights reserved.
'
MUSES project at RIKEN RI beam factory T. Katayama
a)b,
K. Maruyama
a
and M. Wakasugi
"Center for Nuclear Study, Graduate School of Science, University of Tokyo 2-1, Hirosawa, Wako-shi, Saitama 351-01, Japan bRIKEN (The Institute of Physical and Chemical Research) 2-1, Hirosawa, Wako-shi, Saitama 351-01, Japan We are proposing to construct an accelerator system of storage ring and collider at RIKEN Radio Isotope Beam Factory (RIBF). This accelerator complex is named as MUSES(Mu1ti Use Experimental Storage rings). MUSES consists of several rings. One is an Accumulator Cooler Ring (ACR) where an electron cooling device and a stochastic cooling device will be installed to cool down the RI beams. The accumulated and cooled RI beams will be transported to the Double Storage Ring (DSR). The DSR is a new type of collider where the collision of RI beams with electron beams is planned to study the electromagnetic structure of unstable nuclei. In the present paper, the outline of the MUSES project will be described as well as plans of two typical experiments. 1. OUTLINE OF MUSES PROJECT
The Radioactive Isotope Beam Factory (RIBF) is an extension of present heavy ion accelerator facility at RIKEN [I]. The construction of the RIBF is separated into two phases. The first phase is scheduled from 1997 to 2002. In this construction phase, an intermediate ring cyclotron (IRC, Knumber=950), a super-conducting ring cyclotron PLAN VlEVl
OF RI-BEAM FACTORY
Figure 1. Plan view of RIBF and MUSES
(SRC, K=2500), RI beam separators (Big RIPS) and an experimental hall will be completed. The second phase is scheduled from 2001 to 2008, when the MUSES (Multi-Use Experimental Storage rings) project will be completed. The MUSES is an accelerators complex consisting of RI beam separator (RIPS-M), an accumulator cooler ring (ACR), a booster synchrotron ring (BSR), a 300-MeV electron linac (e-linac) and double storage rings (DSR)[2]. Figure 1 shows a plan view of the RIBF. Heavy ion beams from the RRC (Riken Ring Cyclotron K=540) are boosted up to 400A MeV for light ions and lOOA MeV for heavy ions by IRC and SRC. With this heavy ion beam, we can produce RI beams for all elements using projectile-fragmentation process. Details of the SRC and the IRC are described elsewhere [3, 41. 1.1. RI beam separator At the downstream of the SRC, we will construct two RI beam separators. The separator (Big RIPS) provides RI beams for the experimental halls, and another (RIPS-M) for the MUSES system. The primary beam is supplied for three separators with time sharing technique (see Fig. 3). A pulsed beam with the peak beam intensity of 100 (the particles pA (ppA) is supplied for RIPS-M, and the maximum duty factor is beam duration of 30 psec and the interval of 30 msec). DC beams with the intensity of 1 ppA are supplied for the Big RIPS. About 3000 radioactive isotopes including about 1000 new isotopes can be used for experiments with those separators. The RIPS-M used for the MUSES has a momentum acceptance of f 2.5 % and an angular acceptance of f 10 mrad. The momentum spread of the RI beams from the RIPS-M is expected to be f 0.5 %. Since this value is too large from the point of view of cooling time in the ACR, we will place debuncher at 80-m downstream of the RIPS-M. The momentum spread is reduced to 0.15 % with the debuncher system. The maximum RF voltage for the debuncher is required to be 4.23 MV.
+
1.2. Accumulator cooler ring (ACR) Figure 2 shows schematic view of the MUSES system. The RI beams are injected into the ACR by means of a multi-turn injection method (about 30 turns per one injection).
Figure 2. Schematic view of MUSES accelerator system
la1 Primary Beam from SRC
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Figure 3. Time chart of
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The transverse horizontal acceptance of the ACR is 125 nmm.mrad and the momentum acceptance is k 2 %. Injected RI beam is stacked by controlling the RF voltage and the frequency in the ACR. After RF stacking process, the beam is cooled down in both transverse and longitudinal directions by combination of a stochastic cooling and an electron cooling methods. A cycle of the injection, i.e. the multi-turn injection, the RF stacking and the cooling, is repeated until the number of stored particles reaches to the equilibrium number which depends on the intrinsic lifetime of the RI and the space charge limit. This cycle is shown in Fig. 3(b). If we do not need the cooling, only the stacking process takes about 30 msec. This is why the maximum duty factor of primary beam for the RIPS-M is lop3. Figure 4 shows typical results of analytical calculation of the stochastic cooling time vs. number of stored particle, assuming the pickup impedance of 100 R, the temperature of the pre-amplifier of 20 Kelvin, the system band width of 2 GHz, and the output power
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of main amplifier of 10 kW. Since the number of radioactive isotopes for one injection cycle is expected to be less than l o 7 particles, we can find that the cooling times in both directions are less than 100 msec. On the other hand, typical results of the electron cooling simulations shown in Fig. 5 tell us that the cooling time is roughly less than 1 sec. In this case, the electron beam temperature is 20 meV and 0.05 meV in transverse and longitudinal directions, respectively, and the electron beam current is 4 A. The stochastic cooling method is suitable to be used as pre-cooling for injected hot beams, which has large momentum spread and large emittance, because this is, in principle, the feedback system. The electron cooling method is effective for the pre-cooled beams. The combination of the stochastic cooling and the electron cooling makes the cooling time shorter than that for the case of only the electron cooling. The total cooling time is expected to be, roughly speaking, less than 1 sec for all RI beam.
Figure 5. Simulation results of electron cooling in ACR
The ACR is not only an accumulation/cooling ring but also an experimental ring. We provide an electron cooler, Schottky devices, four dispersive positions in arc section (the maximum dispersion of 4.52 m), two achromatic straight sections where the internal targets can be placed, so that the ACR can be used for various experiments. 1.3. Booster synchrotron ring (BSR) The accumulated/cooled RI beam is extracted from the ACR and injected into the BSR to boost up to the required energy, and the beam is immediately transported to the DSR as shown in the time chart of Fig. 3(c). The BSR has a circumference of 179.7 m, the maximum magnetic rigidity of 14.6 Tm, a repetition rate of 1 Hz and the acceleration time of 0.3 sec. The ion beams can be accelerated up to 1.4 GeV for proton and 0.8A GeV for uranium at maximum. Relatively wide range of RF frequency of 25-53 MHz is required to boost up to the maximum energy. Two kinds of extraction methods are provided, which are a fast (one turn) extraction and a slow extraction using 113 resonance technique. The fast extraction is for transporting the beams to the DSR, and the slow extraction is used for experiments at the experimental halls. The BSR can accept not
only ion beams coming from the ACR but also an electron beam. The electron beam can be accelerated from 300 MeV up to the required energy. 1.4. 300-MeV electron linac Depending on the way of use of the electron beam at the DSR, either the single bunch or the full bunch operation mode is chosen in the BSR. Corresponding to that, the operation mode of the e-linac is also changed to the short-pulse mode (1-nsec pulse length, 1Ampere peak current) or the long-pulse mode (5-pet pulse length, 100-mA peak current). The e-linac is driven by the RF frequency of 2856 MHz and the length is about 30 m including a SW type of pre-buncher, a T W type of buncher, and 5 constant gradient type of acceleration tube.
1.5. Double storage ring (DSR) As shown in Fig. 2, the DSR is a new type of experimental storage ring that consists of vertically stacked two rings, which are called e-ring and Ion-ring, respectively. It has a circumference of 269.5 m and two colliding points in long straight sections, which are called the colliding section and the merging section, respectively. The colliding section is prepared for the collision experiments with the crossing angle of 20 mrad. The RIelectron collision experiment, which is described later, is planned at this colliding section. The betatron function of the RI beams and the electron beam at the colliding point are designed to be 10 cm and 2 cm, respectively. On the other hand, the merging section with the crossing angle of 175 mrad is for the ion-ion merging experiments. In this section, we can make low energy collision experiments such as a fusion reaction. In this straight section, the RI-X-ray colliding section is also provided. An undulator is installed as a source of high brilliant X ray in this section as describing later. The DSR has different operation modes corresponding to different types of collision experiments. They are the colliding mode and the merging mode for the RI (ion) beams. For the electron beam, we will have the small emittance operation mode required to produce high brilliant X ray, and the large emittance operation mode required to get larger luminosity for the RIelectron collision experiment. According to requirements for the small emittance mode, a double bend achromatic (DBA) lattice is adopted in the arc section and the emittance of order of lo-' mrad is presently designed. On the other hand, the emittance for the mrad. Details of design of the DSR is large emittance mode is designed to be about described in Refs. [5, 61. 2. EXPERIMENS AT DSR
2.1. RI-electron collision experiment One of unique experiments at the DSR is the RI-electron collision [7].We are interested in the elastic scattering (e,el)from which we can determine the nuclear charge distribution of RI's. Physical interests are the proton skin structure, the neutron skinlhalo structure, difference in collective structure between protons and neutrons etc. in RI's which have unbalanced numbers of protons and neutrons. This experiment allows us to make systematic study on these problems. This kind of study, which has been performed only for light elements, can be extended to heavier elements. In our estimation, required range of momentum transfer for electron scattering is q=0.5
Figure 6. Expected yield of the electron scattering experiments
- 2 fm-I
to determine the charge distribution around the nuclear surface. This corresponds to the scattering angle from 10 deg. to 60 deg. in the laboratory frame for the case of electron beam energy of less than 1 GeV and the RI beam energy of less than 1A GeV. The essential point of this experiment is that how large luminosity can we have at the DSR. According to our calculation [a], we can precisely determine the nuclear charge distributions for RI's for the case of the luminosity of more than ~ m - ~ s - lthat , corresponds to isotopes having the life time of more than 1 min. Figure 6 shows examples of the expected yield of the electron scattering for one-week beam time. Here we assume the electron beam current in the DSR is 500 mA. 2.2. RI-X-ray collision experiment The purpose of this experiment is to determine the mean square nuclear charge radii and the electromagnetic moment by means of isotope shift measurements in the 2s2P (so called D l transition) atomic transition of the Li-like RI ions [7, 9, 101. We provide an undulator and an X-ray spectrometer as a monochromatic X-ray source placed near merging section in the e-ring as shown in Fig. 7. Output X-ray from the spectrometer is injected again into the Ion-ring and collides with the circulating Li-like RI ions at the detector position. Requirements on the X ray for this experiment are as follows. The X-ray energy is 30 - 800 eV to excite the D l transition of Z>40 elements, and the
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energy resolution is about AEx/Ex=10-4. The X ray intensity should be at least 1012 photons/sec/O.Ol % b.w. at the RI-X-ray colliding section. If we satisfy these conditions, we can measure the isotope shift for quite small number of RI beam stored in the DSR, even if only one ion is in the DSR. To get the required specifications of the X ray, we have to store 500-mA electron beam in the small emittance operation mode of the DSR. The specifications for the small emittance mode is the same like the third-generation synchrotron-light source. In such machine, instability of the electron beam is always big problem especially at lower energy. The instability is caused by the ring-broadband impedance and the narrow-band impedance at the high-Q cavities. We are now investigating the instability and designing the vacuum tube and cavities of the DSR.
3. CONCLUDING REMARKS In this paper, the outline of the MUSES accelerator system and typical experiments proposed at the DSR are described. The key issue of the e-RI collision experiment is the available luminosity. It is found that the luminosity can be reached up to ~m-~sec-l for the nuclei of which the lifetime is 1 min. To get such high luminosity, the important factor of the accelerator aspect is the cooling-stacking of the RI beam in the ACR, optimized lattice structure of the DSR, synchronous collision at the colliding point, and the beam-beam effects. Optimization on these problems are now in progress. For another experiment, RI-X-ray collision, the most important thing is how to get stable and largecurrent of the electron beam under the low-emittance operation mode in the DSR. The special design is needed for the components of the vacuum tube and cavities, and effective feedback system has to be installed in the DSR. The phase 1 of the RIBF project is under the construction and the phase 2, MUSES project, is expected to start the construction from 2001.
REFERENCES 1. Y. Yano et al., Proc. of PAC97 (1998) 930. 2. T. Katayama et al, Nucl. Phys. A626 (1997) 545c. 3. A. Goto et al., Proc. of 16th Cyclotron and Their Applications, to be published. 4. T. Kawaguchi et al., Proc. of PAC97 (1998) 3419. 5. N. Inabe et al., Proc. of PAC97 (1998) 1400. 6. N. Inabe et al., Proc. of EPAC98 (1998) 897. 7. I. Tanihata, Nucl. Phys. A588 (1995) 253c. 8. Y. Batygin et al., RIKEN-AF-AC-10 (1998). 9. M. Wakasugi et al., Proc. of EPAC96 (1997) 611. 10. M. Wakasugi et al., Proc. of EPAC98 (1998) 1017.
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Hadron and Nuclear Physics with Electromagnetic Probes K. Maruyama and H. Okuno (Editors) 2000 Elsevier Science B.V.All rights reserved.
'
Summary of the symposium "KEK-Tanashi, Tanashi-shi, Tokyo 188: Japan First of all, on behalf of the organizing committee of the symposium, I would like to express our sincere thanks to all the speakers and participants for their stimulating talks and lively discussions, which made this meeting so fruitful. 1. AIM OF THE SYMPOSIUM
The aim of this symposium, which we have intended to achieve, is to discuss recent experimental and theoretical developments of hadron and nuclear physics and future directions of these fields. Especially, emphases are placed on the hadron and nucleus studies with electron and photon beams. The reason why we have chosen these topics is closely related to the history of our 1.3GeV electron synchrotron at Tanashi. The operation of the synchrotron was terminated in June this year after 37-years service for particle and nuclear physics. The main research areas in recent days at the electron synchrotron were studies of photonuclear reactions by using a high-duty tagged-photon beam and a large-acceptance magnetic spectrometer TAGX. Physics topics were the study of meson and baryon properties in nuclear medium and nucleon-nucleon correlations in nucleus. In this symposium, we intended to summarize these experimental results from our electron synchrotron together with recent experimental and theoretical developments of hadron and nuclear physics. Also we wanted to look forward the future of these fields which will be opened by new electron accelerator facilities around the world. 2. HISTORY OF THE KEK-TANASHI ELECTRON SYNCHROTRON
First, I want to introduce our electron synchrotron briefly. Figure 1 shows a picture of our electron synchrotron at early 60's. The diameter of the ring is about 10 m and the ring is composed of eight magnets. This synchrotron is a combined-function-type strong-focussing machine, which is very compact and easy to operate. The Tanashi electron synchrotron was constructed in 1961 as a first high- energy accelerator in our country. The experiment started in 1966 at the maximum energy of 1.3 GeV. In those days. a quark model was proposed by Gell-Mann and Zweig in order to explain the mass spectra of baryon and meson resonances. Therefore, the main experimental topics were the study of electromagnetic structure of nucleon resonances through meson photoproductions. Especially, in 1970's and 1980's, the polarization measurements such as the polarized-beam asymmetry, the polarized target asymmetry and the recoil-proton
Figure 1. KEK-Tanashi 1.3-GeV electron synchrotron.
polarization were extensively measured for each single-pion production process. Through the partial wave analysis, photo-coupling parameters of nucleon resonances were analyzed, which contributed to establish the quark structure of nucleon resonances. In addition, it should be mentioned that this electron synchrotron played a critically important role in the pioneering work of synchrotron radiation physics in our country in 1960-1970. Studies of elementary processes of meson photoproduction on nucleon ended after the main stream of Japanese high-energy community moved to the experiments at the KEK 12-GeV proton synchrotron at Tsukuba. Since then physics interests were shifted to the study of nucleus. The experiments of inelastic scattering of electrons, pion production on nucleus and photodisintegration of nucleus were made sporadically in 1970's and 1980's. Some interesting results were obtained as for the shell structure of nuclei, nucleon-nucleon correlation inside nucleus: the meson exchange current, etc.. However, accuracies of the experimental results were limited due to the small duty factor of the accelerator. Also experiments on these processes were limited to the inclusive measurements, where only a single particle in the final state was detected.
3. HIGH-DUTY TAGGED PHOTON BEAM AND TAGX SPECTROMETER In order to overcome these difficulties, we have developed a high-duty tagged-photon beam and a large-acceptance magnetic spectrometer called TAGX. These fascilities were intended to be used primarily for the detailed study of photonuclear reactions in the
Bremsstrahlung Beam
Tagglng System
y 3 Area TAG, Spectrometer
y 2 Area
Figure 2. Layout of the accelerator and experimental apparatus.
1-GeV region. The floor plan of the synchrotron 1990's is shown in Fig.2. Since the main magnets of our electron synchrotron was excited with a sinusoidal wave form at a repetition rate of 21 Hz, the beam spill was limited to be about 1 msec which corresponded to the duty factor of 2%. This was a standard for this type of the synchrotron. So, at various laboratories, experimentalists wished to improve the duty factor by introducing a beam stretcher ring, a microtron and a superconducting linear accelerator. In our case, we applied a very simple method to excite the all beam transport magnets to follow the wave form of the synchrotron magnet. As a results, we could obtain the duty factor of 20% . This is quite optimum from the viewpoint of the data aquisition rate when we use a large-acceptance spectrometer like TAGX, where a large number of detector components are used. I do not go into details of TAGX because speakers on the
TAGX experiments already presented the detailed description of the TAGX spectrometer. The TAGX experiments concentrated on the study of light nuclei starting from deuterium, 3He and 4He, to 12C. The physics topics are categolized as 1. Photodisintegration of light nuclei, 2. Pion production from nuclei, 3. K+ photoproduction, 4. p0 mass modification in nuclear medium, where the results of these experiments were presented a t this symposium. These experiments were the first pioneering work which used a large-acceptance spectrometer for the photon beam. 4. TOPICS AT THE SYMPOSIUM
These TAGX experiments in mind, we organized the symposium topics as follows; 1. Mesons in Nuclear Medium, 2. Nucleon Resonances in Nuclei, 3. Strangeness Physics, 4. NN Correlations and Few-body Physics, 5. Nucleon Structure Studied by High-energy Electrons, 6. New Facilities. As for the topics 1-4, the TAGX collaboration presented their results. We wanted to discuss these results together with recent experimental and theoretical developments. The topics 5 is a study of nucleon structure studied by high-energy electrons, in which our KEK-Tanashi group led by S. Yamada is working at HERA. A deep understanding of the proton structure including a spin structure is a key issue in QCD. Therefore it was very interesting to hear a beautiful review talk by A. Levy on the recent understanding of proton structure learned from high-energy collider experiments. Since the energy range covered by this symposium is very wide, from MeV t o 100 GeV, it is beyond my ability to summarize all these physics topics. So I just want to say a few words on the limited items, Mesons in Kuclear Medium. At the opening session of this symposium, T . Hatsuda made a nice introductory talk on hadron and nuclear physics from a QCD point of view. In his summary, he claimed that QCD is a theory of everything in strong interactions, from soft, no-perturbative QCD to hard, perterbative QCD. He pointed out that three physics issues which should be investigated are; 1. QCD at extreme conditions (high temperature and high density), 2. Interplay between hard and soft QCD, 3. In-medium hadrons. In order to study hadrons in nuclear matter, the spectral function is a direct link between the theory and experiments. The particles to be looked at are vector mesons p, w and 4, and a scaler meson a. After Hatsuda's talk, R. Rapp described vector mesons in medium and dileptons in heavy-ion collisions from the theorist point. One example of calculation shows the expected spectral function for p, w and $. Then the question is how t o measure these
spectral functions in real experimental conditions. At this symposium, the status of three experiments are presented; 1. KEK-Tanashi ES: yA+pO+A* (p0-+7r+n-), 2. KEK-PS: P A + ~ A *, 3. GSI-HADES: e+e- pair spectroscopy, w in nuclei. These experiments are still in a preliminary stage or in progress. Although there are some hints on mass modification, more detailed experimental study is necessary for detailed comparison with QCD theories. 5. NEW FACILITIES AND FUTURE DIRECTIONS
Although our electron synchrotron has left from the hadron and nuclear physics arena, various new electron facilities came in with a unique beam characteristics. Today, W. Hersman told us the recent activities at Jefferson Lab. Yesterday, Mainz activities and Bonn ELSA activities were introduced by P. Grabmyer and E. Paul. These facilities are characterized by the continuous electron beam of 100% duty factor. The physics objectives which our electron synchrotron has pursued will be investigated more deeply and more extensively at these facilities. In our country, also two new electron accelerator facilities came into operation this year as learned from this afternoon talk by J. Kasagi and T. Hotta. These are the 1.2-GeV stretcher/booster ring at Tohoku and Laser-backscattered photon beam facility at Spring8. We are very glad to hear that the experiments started just at the time of our electron synchrotron's retirement. In addition to these, the symposium attendee heard with a great interest from T . Katayama (M.Wakasugi) about the RIKEN project of the unstable nuclear beam facility. This may open new frontiers on the study of unstable nuclei with electromagnetic probes. At KEK-Tsukuba, the hadron and nuclear physics experiments have long been investigated at the 12-GeV proton synchrotron. Results are presented by H. En'yo for 4 production, J. Chiba for A resonance in nuclei and 0. Hashimoto for the spectroscopy of hypernuclei. These studies by use of the hadron beam are complementary to the studies with the electromagnetic probes. A proposed JHF, Japan Hadron Facility, which is presented by our IPNS Director S. Yamada, is expected to deliver a good quality of pion, kaon and antiproton beams in future. I believe that these new facilities will play a key role in understanding the true nature of hadron and nucleus in the next century. At the end of the symposium, I would like to thank all the speakers and participants, especially participants from abroad, for their enthusiastic contribution to the success of the symposium. I hope everybody have spent a fruitful three days at Tanashi surrounded by a Japanese cultural environment. Please bring back nice memories on Tanashi symposium to your home. Also I would like to thank the advisory committee and organizing committee members, and symposium secretaries for their help in organizing this symposium. Without their sincere help, this symposium could not be realized.
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Symposium Program Oct. 25 (Mon.) Registration (8:30 - 9:20) Opening Session (9:20 - 1050) Chairperson; A. Masaike (Fukui IT), Scientific secretary; K. Oyama (Tokyo) S. Sugimoto (KEK-Tanashi) Opening address S. Yamada (KEK) Future projects of KEK T. Hatsuda (Kyoto) An introductory talk to the symposium
- Coffee Break (1050 - 11:10) Session I (Mesons in Nuclear Medium; 11:lO - 12:30, 14:OO - 15:20) Chairperson; K. Yazaki (Tokyo Woman's Christian), Scientific secretary; K. Oyama (Tokyo) (30+10) R. Rapp (SUNY) Vector mesons in nuclear medium K. Maruyama (CNS, Tokyo) p0 meson in the nucleus (30+10) - Lunch (12:30 - 14:OO) -
H. En'yo (Kyoto) J. Friese (TU Munchen)
$ production at KEK HADES at GSI
Session I1 (Nucleon Resonances in Nuclei and Related Topics; 15:20 - 16:10, 16:40 - 18:20) Chairperson; K. Nakai (Science U Tokyo), Scientific secretary; Y. Umemoto (Nara Women's) H. Yamazaki (Tohoku) S, ,(1535) resonance in nuclei (20+5) J. Chiba (KEK) Delta in nuclei excited by hadronic processes (20+5) - Coffee Break (16:lO - 16:40) -
G. Huber (Regina)
I.T. Cheon (Yonsei) H. Utsunomiya (Konan) I.A. Pshenichnov (INR)
Physics of double Delta excitation in the deuteron and 3 ~ e (20+5) Vector meson contribution to pion photoproduction on the nucleon (20+5) Photoneutron cross section measurement on ' ~ by e inverse Compton scattering of laser photons
(20+5)
Nuclear disintegration induced by virtual photons at heavy-ion colliders
(20+5)
- Reception (18:30 - 20:OO) -
Oct. 26 (Tue.) Session I11 (Strangeness Physics; 9:00 - 10:20, 10:40 - 12:20 ) Chairperson; T. Motoba (Osaka E-C), Scientific secretary; M. Yamaguchi (Ehime) K. Maeda (Tohoku) K' photoproduction on nuclei E.Ya. Paryev (INR) Comment T. Mart (Indonesia) Theoretical aspects of strangeness electroproduction H. Yamamura (Okayama US)
Hyperon polarization in kaon photoproduction from the deuteron
(20+5) (10) (20+5) (15+5)
- Coffee Break (10:20 - 10:40) E. Paul (Bonn) 0 . Hashimoto (Tohoku) H. Kohri (Osaka)
Physics of associated strangeness production experiments (30+ 10) at ELSA Retrospect and prospect of hypernuclear physics (30+10) Spin-orbit splitting of ' ',c (15+5)
Session I V (N-N Correlations and Few-body Physics; 1350 - 15:55, 16:25 - 17:30) Chairperson; T. Suzuki (Fukui), Scientific secretary; N. Naka (Ehime) T. Suda (RIKEN) e 4 ~at eTAGX Photodisintegration reactions of ' ~ and (SO+ 10) G. Orlandini (Trento)
S. Hirenzaki (Nara Women's)
Photonuclear cross sections of three-nucleon systems and the role of three-nucleon forces (30+10) e Quasi-deuteron picture for %Ie and 4 ~Photodisintegration (20+5)
- Coffee Break (15:55 - 16:25) P. Grabmayr (Tiibingen)
Two-nucleon emission experiments at Mainz searching (30+10) for NN correlations
E.L. Lomon (MIT)
Quark substructure and isobar effects on deuteron form factors (20+5) - Banquet (19:OO - 21:OO) -
Oct. 27 (Wed.) Session V (Nucleon Structure Studied by High-Energy Electrons; 9:00 - 10:40, 11:OO - 12:05) Chairperson; H. Abramowicz (Tel Aviv), Scientific secretary; M. Chiba (Tohoku) A. Levy (Tel Aviv) The proton and the photon: who is probing whom in (50+10) electroproduction ? K. Nagano (KEWDESY) ~ i ~neutralh - and ~ charged-current ~ reactions at HERA (30+10)
- Coffee Break (10:40 - 11:OO) Y. Sakemi (Tokyo IT) W. Meyer (Bochum)
Spin structure of the nucleon studied by HERMES A first measurement of the GDH-sum rule at MAMI
(30+10) (20+5)
- Lunch (12:05 - 13:35) Session VI (New Facilities; 13:35 - 15:55) Chairperson; J.C. Kim (Seoul National), Scientific secretary; Y. Miyake (Osaka) W. Hersman (New Hampshire) Topics of few-body physics at J-Lab Y. Yamaguchi (Tokyo) Use of the polarized photon beams at high energies J. Kasagi (Tohoku) Nuclear physics experiments with 1.2-GeV STB ring at LNS, Tohoku T. Hotta (RCNP, Osaka) LEPS at Spring-8 T. Katayama (TokyoIRIKEN) MUSES Project
(15+5) (15+5) (15+5)
Closing Session (1555 - 16:15) Chairperson; Y. Sumi (Hiroshima Int.), Scientific secretary; Y. Miyake (Osaka) H. Okuno (KEK-Tanashi) Summary talk
(20)
Oct. 28 (Thu.) Tour of KEK-facilities on request
(50+10) (15+5)
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List o f Partichants Abramowicz, Halina
Tel Aviv University
[email protected] Akaishi, Yoshinori
KEK
[email protected] Asakawa, Masayuki
Nagoya University
[email protected] Cheon, 11-Tong
Yonsei University
itcheon @phya.yonsei.ac.kr
Chiba, Junsei
KEK
[email protected] Chiba, Masami
Tohoku University
[email protected] Emura, Tsuneo
Tokyo Univ. of Agriculture and Technology
[email protected] En'yo, Hideto
Kyoto University
[email protected] Endo, Ichita
Hiroshima University
[email protected] Endo, Satoru
Hiroshima University
[email protected] Friese, Juergen
Technische Universitaet Muenchen
[email protected] Grabmayr, Peter
University of Tuebingen
[email protected] Hashimoto, Osamu
Tohoku University
[email protected] Hatsuda, Tetsuo
Kyoto University
hatsuda@ruby,scphys.kyoto-u.acjp
Hersman, Bill
University of New Hampshire
[email protected] Hirenzaki, Satoru
Nara Women's University
[email protected] Hotta, Tomoaki
Osaka University
[email protected] Huber, Garth
University of Regina
[email protected] Inuzuka, Masahide
Tokyo Metropolitan University
[email protected] Jeong, Moon Taeg
Dongshin University
[email protected] Kasagi, Jirohta
Tohoku University
[email protected] Katayama, Takeshi
University of Tokyo
[email protected] Kim, Jong Chan
Seoul National University
jckim@phya,snu.ac.kr
Kishimoto, Tadafurni
Osaka University
[email protected],ac.jp
Kohri, Hideki
Osaka University
[email protected],osaka-u.ac.jp
Komatsubara, Takeshi
KEK
[email protected] Lee, Su Houng
Yonsei University
[email protected] Levy, Aharon
Tel Aviv University
levy @alzt.tau.ac.il
Lomon, Earle
Massachusetts Institute of Technology
[email protected] Maeda, Kazushige
Tohoku University
[email protected] Mart, Terry
University of Indonesia
Maruyarna, Koichi
University of Tokyo
Masaike, Akira
Fukui University of Technology
Meyer, Werner
Ruhr Uni. Bochum
Miyake, Yoichiro
Osaka University
Monmatsu, Osarnu
KEK
Motoba, Toshio
Osaka Electro-Communication University
Murata, Yojiro
Musashino Women University
Muto, Masayuki
KEK
Nagano, Kunihiro
KEWDESY
Naka, Norio
Ehime University
Nakai, Kozi
Science University of Tokyo
Nakayoshi, Kazuo
KEK
Niki, Kazuaki
KEK
Nomura, Toru
KEK
Oka, Makoto
Tokyo Institute of Technology
Okuno, Hideki
KEK
Orlandini, Giuseppina
University of Trento
Oyama, Ken
University of Tokyo
Paryev, Eduard
INR, Russian Academy of Sciences
Paul, Ewald
University of Bonn
Pshenichnov, Igor
INR, Russian Academy of Sciences
Rangacharyulu, Chary
University of Saskatchewan
Rapp, Ralf
SUNY
Sakaguchi, Atsushi
Osaka University
Sakamoto, Koh
Kanazawa University
Sakerni, Yasuhiro
Tokyo Institute of Technology
Sasaki, Atsushi
Akita University
Shibata, Seiichi
Kyoto University
Shibata, Toshiaki
Tokyo Institute of Technology
Suda, Toshimi
Institute of Physical and Chemical Research
[email protected] Sugimoto, Shojiro
KEK
[email protected] Sumi, Yoshio
Hiroshima International University
[email protected] Suzuki, Toshio
Fukui University
[email protected] Tamae, Tadaaki
Tohoku University
[email protected] Tezuka, Hirokazu
Toyo University
[email protected] Tokushuku, Katsuo
KEK
[email protected] Ueda, Tamotsu
Ehime University
[email protected] Ukai, Kumataro
KEK
ukai@tanashi,kek.jp
Umemoto, Yukiko
Nara Women's University
[email protected] Utsunomiya, Hiroaki
Konan University
[email protected] Wada, Yoshichika
Meiji Pharmaceutical University
[email protected] Yamada, Sakue
KEK
[email protected] Yamaguchi, Masahiro
Ehime University
[email protected] Yamaguchi, Yoshio
University of Tokyo
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[email protected] Yamamura, Hisahiko
Okayama University of Science
[email protected] Yamashita, Haruo
Shizuoka Cancer Center
[email protected] Yamazaki, Hirohito
Tohoku University
[email protected] Yamazaki, Yuji
KEK
[email protected] Yazaki, Koichi
Tokyo Woman's Christian University
[email protected] Yoshida, Katsuhide
Hiroshima University
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