Nucleus-Nucleus Collisions: Proceedings of the Conference Bologna 2000

THE SCIENCE AND CULTURE SERIES — ADVANCED SCIENTIFIC CULTURE Series Editor: A. Zichichi

NUCLEUS-NUCLEUS COLLISIONS PROCEEDINGS OF THE CONFERENCE: BOLOGNA 2000 STRUCTURE OF^HE NUCLEUS AT THE DAWN OF THE^NTURY

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I0RI, M. BRUNO, A. VENTURA, D. VRETENAR World Scientific

NUCLEUS-NUCLEUS COLLISIONS PROCEEDINGS OF THE CONFERENCE: BOLOGNA 2000 STRUCTURE OF THE NUCLEUS AT THE DAWN OF THE CENTURY

THE SCIENCE AND CULTURE SERIES — ADVANCED SCIENTIFIC CULTURE Series Editor: A. Zichichi, European Physical Society, Geneva, Switzerland Series Editorial Board: P. G. Bergmann, J. Collinge, V. Hughes, N. Kurti, T. D. Lee, K. M. B. Siegbahn, G. 't Hooft, P. Toubert, E. Velikhov, G. Veneziano, G. Zhou

1.

Nucleus-Nucleus Collisions Bologna 2000. Structure of the Nucleus at the Dawn of the Century

2.

Nuclear Structure Bologna 2000. Structure of the Nucleus at the Dawn of the Century

3.

Hadrons, Nuclei and Applications Bologna 2000. Structure of the Nucleus at the Dawn of the Century

NUCLEUS-NUCLEUS COLLISIONS PROCEEDINGS OF THE CONFERENCE: BOLOGNA 2000 STRUCTURE OF THE NUCLEUS AT THE DAWN OF THE CENTURY

Bologna, Italy

29 May - 3 June 2000

Editors

Giovanni C. Bonsignori Mauro Bruno Dipartimento di Fisica dell'Universita di Bologna and INFN-Sezione di Bologna, Italy

Alberto Ventura Ente Nuove Tecnologie, Energia e Ambiente and INFN Bologna, Italy

Dario Vretenar Physics Department, University of Zagreb, Croatia

Series Editor A. Zichichi

V f e World Scientific M

New Jersey • London • Singapore • Hong Kong

Published by World Scientific Publishing Co. Pre. Ltd. P O Box 128, Farrer Road, Singapore 912805 USA office: Suite IB, 1060 Main Street, River Edge, NJ 07661 UK office: 57 Shelton Street, Covent Garden, London WC2H 9HE

British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library.

NUCLEUS-NUCLEUS COLLISIONS Proceedings of the Conference: Bologna 2000 Structure of the Nucleus at the Dawn of the Century Copyright © 2001 by World Scientific Publishing Co. Pte. Ltd. All rights reserved. This book, or parts thereof, may not be reproduced in any form or by any means, electronic or mechanical, including photocopying, recording or any information storage and retrieval system now known or to be invented, without written permission from the Publisher.

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ISBN 981-02-4731-1

Printed in Singapore.

CONTENTS

Foreword The Editors

xv

Welcome Address

xix

F. Rover si-Monaco Committees and Sponsors Photographs

xxi xxiii

Opening Lecture Antimatter — Past, Present and Future A. Zichichi

3

Introductory Talk What Can Nuclear Collisions Teach Us about the Boiling of Water or the Formation of Multi-Star Systems? D. H. E. Gross Section I.

53

Quark and Gluon—Plasma Phase Transition and Relativistic Heavy-Ion Reactions

Results from Pb-Pb Collisions at CERN SPS L. Riccati and E. Scomparin

63

The Search for the QGP: A Critical Appraisal H. Satz

71

Strange Baryon Signals of a New State of Matter in Lead-Lead Collisions at the CERN SPS F. Antinori for the WA97 Collaboration

73

vi

Cooper-Mesons in the Color-Flavor-Locked Superconducting Phase of Dense QCD A. Wirzba

83

Charmonium Suppression in P b - P b Collisions and Quark-Gluon Deconfinement A. B. Kurepin for the NA50 Collaboration

91

Particle Production in P b + P b Collisions at 158 GeV/Nucleon in the NA49 Detector H. Strobele for the NA4-9 Collaboration

99

Exploring the Chiral Phase Transition in High-Energy Collisions J. Randrup

105

Nuclear Collective Flow in Heavy Ion Collisions at SIS Energies N. Bastid for the FOPI Collaboration

111

Hadron Observables from Hadronic Transport Model with Jet Production at RHIC Y. Nara

115

Simultaneous Heavy Ion Dissociation at Ultrarelativistic Energies /. A. Pshenichnov, J. P. Bondorf, S. Masetti, I. N. Mishustin and A. Ventura

119

Direct Photon Production in 158A GeV 2 0 8 P b + 2 0 8 P b Collisions T. Peitzmann for the WA98 Collaboration

123

Deformation and Orientation Effects in Uranium-on-Uranium Collisions at Relativistic Energies Bao-An Li

129

Subthreshold Heavy-Meson and Antiproton Production in the Nucleus-Nucleus Collisions A. T. D'Yachenko

133

Pion Imaging at the AGS S. Y. Panitkin for the E895 Collaboration

137

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Directed Flow in 4.2A GeV/C C+C and C+Ta Collisions J. Milosevic and L. J. Simic

145

Fragmentation of Very High Energy Heavy Ions M. Giorgini and S. Manzoor

149

Section II.

Liquid—Gas Phase Transitions in Nuclear Matter

Phase Transition in Finite Systems Ph. Chomaz, V. Duflot and F. Gulminelli

155

Present Status and Future Prospects of Investigations of the Liquid-Gas Phase Transition C.-K. Gelbke

167

Critical Phenomena in Finite Systems A. Bonasera, T. Maruyama and S. Chiba

179

Experimental Signals of the First Phase Transition of Nuclear Matter B. Borderie

187

Nuclear Fragmentation, Phase Transitions and their Characterization in Finite Systems of Interacting Particles J. M. Carmona, N. Michel, J. Richert and P. Wagner

197

Topology and Phase Transitions: Towards a Proper Mathematical Definition of Finite N Transitions M. Pettini, R. Franzosi and L. Spinelli

203

Introduction to the Phase Transition Discussion Ph. Chomaz

209

Finite Nuclear Fragmenting Systems: An Experimental Evidence of a First Order Liquid-Gas Phase Transition M. D'Agostino, F. Gulminelli, Ph. Chomaz, M. Bruno, F. Cannata, N. Le Neindre, R. Bougault, M. L. Fiandri, E. Fuschini, F. Gramegna, I. Iori, G. V. Margagliotti, A. Moroni, G. Vannini and E. Verondini

215

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Phase Transition in XE+SN Central Events between 32 and 50A MeV N. Le Neindre, R. Bougault, Ph. Chomaz, F. Gulminelli and the INDRA Collaboration

221

Nuclear Caloric Curve: Influence of the Secondary Decays on the Isotopic Thermometers A. H. Raduta and A. R. Raduta

227

Phase Transition Signals in Thermally Excited Nuclei V. E. Viola for the E900 Collaboration

231

Hydrogen Cluster Multifragmentation and Percolation Models F. Gobet, B. Farizon, M. Farizon, M. J. Gaillard, J. P. Bucket, M. Carre, P. Scheier and T. D. Mark

237

Single Quasiparticle Entropy in Excited Nuclei with T < 1 MeV M. Guttormsen, M. Hjorth-Jensen, E. Melby, J. Rekstad, A. Schiller and S. Siem

243

Section III.

Nuclear Caloric Curve and Thermodynamics of Heavy Ion Collisions

Surveying Temperature and Density Measurements in Nuclear Calorimetry G. Raciti for the ALADIN Collaboration

249

Caloric Curve of Fragmenting Systems C O. Dorso and A. Chernomoretz

257

Thermodynamics of Explosions G. Neergaard, J. P. Bondorf and I. N. Mishustin

263

Microcanonical Investigation of the Recent Nuclear Caloric Curve Experimental Evaluations A. H. Raduta and A. R. Raduta

269

Can We Determine the Nuclear Equation of State from Heavy Ion Collisions? T. Gaitanos, H. H. Wolter, C. Fuchs and A. Faessler

273

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Thermodynamical Description of Heavy Ion Collisions T. Gaitanos, H. H. Wolter and C. Fuchs Section IV.

279

Statistical and Dynamical Aspects of Fragmentation

Exact Solution of the Statistical Multifragmentation Model and the Liquid-Gas Mixed Phase K. A. Bugaev, M. I Gorenstein, I. N. Mishustin and W. Greiner

285

What Can We Learn from Nuclear Matter Instabilities? V. Baran, M. Colonna, M. Di Toro, M. Zielinska-Pfabe and H. H. Wolter

293

Energetic Proton Emission and Reaction Dynamics in Heavy Ion Reactions Close to the Fermi Energy R. Coniglione, P. Sapienza, E. Migneco, C. Agodi, R. Alba, G. Bellia, M. Colonna, A. Del Zoppo, P. Finocchiaro, V. Greco, K. Loukachine, C. Maiolino, P. Piattelli, D. Santonocito, P. G. Ventura, N. Colonna, M. Bruno, M. D'Agostino, M. L. Fiandri, G. Vannini, P. F. Mastinu, F. Gramegna, I. Iori, L. Fabbietti, A. Moroni, G. V. Margagliotti, P. M. Milazzo, R. Rui, F. Tonetto, Y. Blumenfeld and J. A. Scarpaci

299

New Results on Preequilibrium 7-Ray Emission and GDR Saturation on Reactions at 25A MeV G. Cardella, F. Amorini, A. Di Pietro, P. Figuera, G. Lanzalone, J. Lu, A. Musumarra, M. Papa, G. Pappalardo, S. Pirrone, F. Rizzo and S. Tudisco

305

The Onset of Mid-Velocity Emissions in Symmetric Heavy Ion Reactions E. Plagnol, J. Lukasik and the INDRA Collaboration

309

Contribution of Prompt Emissions to the Production of Intermediate Velocity Light Particles in the 3 6 Ar+ 5 8 Ni Reaction at 95 MeV/Nucleon P. Pawlowski, B. Borderie and the INDRA Collaboration

313

Proton Emission Times in Spectator Fragmentation C. Schwarz for the ALADIN Collaboration

317

Experimental Evidence for Spinodal Decomposition in Multifragmentation of Heavy Systems M. F. Rivet for the INDRA Collaboration

321

Non-Equilibrium Effects on a Second-Order Phase Transition V. Latora and A. Rapisarda

327

Multifragmentation of Expanding Nuclear Matter S. Chikazumi, T. Maruyama, K. Niita, A. Iwamoto and S. Chiba

331

Contemporary Presence of Dynamical and Statistical Intermediate Mass Fragment Production Mechanisms in Midperipheral Ni+Ni Collisions at 30 MeV/Nucleon P. M. Milazzo, M. Sisto, G. V. Margagliotti, R. Rui, G. Vannini, M. Bruno, M. D'Agostino, N. Colonna, C. Agodi, R. Alba, G. Bellia, M. Colonna, R. Coniglione, A. Del Zoppo, P. Finocchiaro, C. Maiolino, E. Migneco, P. Piattelli, D. Santonocito, P. Sapienza, F. Gramegna, P. F. Mastinu, L. Fabbietti, I. Iori, A. Moroni and M. Belkacem

335

External Coulomb and Angular Momentum Influence on Isotope Composition of Nuclear Fragments A. S. Botvina

341

Section V.

Intermediate Energy Heavy-Ion Reactions

Isospin Fractionation in Excited Nuclear Systems S. J. Yennello, M. Veslesky, E. Martin, R. Laforest, D. Rowland, E. Ramakrishnan, A. Ruangma and E. M. Winchester

347

The Disappearance of Flow and the Nuclear Equation of State G. D. Westfall

353

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The Backtracing Procedure in Nuclear Physics P. Desesquelles

359

Ni+Ni Collisions at 32 MeV/U: Experimental Insight with the INDRA Multidetector A. M. Maskay, P. Lautesse, P. Desesquelles, E. Gerlic, J. L. Laville and the INDRA Collaboration

363

Two-Fragment Correlation Function for Quasi-Projectile and Mid-Rapidity Emission Z. He, R. Roy, L. Gingras, Y. Larochelle, D. Ouerdane, L. Beaulieu, P. Gagne, X. Qian, C. St-Pierre, G. C. Ball and D. Horn

367

Fermion Interferometry in Ni-Induced Reactions at E/A=45 MeV R. Ghetti

371

Investigation of an Angular Distribution of in Peripheral and Central Nucleus-Nucleus at the Momentum of 4.2A GeV/c M. K. Suleymanov, O. B. Abdinov, Z. N. S. Angelov, A. C. Vodopyanov and

375

Protons Collisions Y. Sadigov, A. A. Kuznetsov

REVERSE Experiment at Laboratori Nazionali del Sud E. Geraci for the REVERSE Collaboration

379

REVERSE: The First Experiment with the Chimera Detector G. Politi for the REVERSE Collaboration

383

Non Equilibrated IMF Emission in Heavy Ion Collisions around the Fermi Energy S. Piantelli, L. Bidini, M. Bini, G. Casini, P. R. Maurenzig, A. Olmi, G. Pasquali, G. Poggi, S. Poggi, A. A. Stefanini and N. Taccetti

387

The Complete Fusion and the Competitive Processes in the 3 2 S+ 1 2 C Reaction at E( 3 2 S)=20 MeV/A S. Pirrgjne, G. Politi, G. Lanzalone, S. Aiello, N. Arena, Seb. Cavallaro, E. Geraci, F. Porto and S. Sambataro

391

A Simple Pulse Shape Discrimination Method Applied to Silicon Strip Detector J. Lu, F. Amorini, G. Cordelia, A. Di Pietro, P. Figuera, A. Musumarra, M. Papa, G. Pappalardo, F. Rizzo and S. Tudisco Section VI.

395

Reaction Mechanisms around the Barrier. Fusion and Fission in Heavy-Ion Reactions

Cross-Sections for Coulomb and Nuclear Breakup of Three-Body Halo Nuclei E. Garrido, D. V. Fedorov and A. S. Jensen

401

Multinucleon Transfer Reactions Studied with the PISOLO Spectrometer L. Corradi, A. M. Stefanini, A. M. Vinodkumar, S. Beghini, G. Montagnoli, F. Scarlassara and G. Pollarolo

405

Study of the Cluster Emission Barrier in 1 2 C+ 2 0 8 Pb Elastic Scattering and Possible Observation of Quasimolecular Configuration A. A. Ogloblin, K. P. Artemov, Yu. A. Glukhov, A. S. Dem'yanova, V. V. Paramonov, M. V.Rozhkov, V. P. Rudakov and S. A. Goncharov

409

Very Strong Reaction Channels at Barrier Energies in the System 9Be+ 209 Bi C. Signorini, A. Andrighetto, J. Y. Guo, L. Stroe, A. Vitturi, M. Ruan, M. Trotta, F. Soramel, K. E. G. Lobner, K. Rudolph, I. Thompson, D. Pierroutsakou and M. Romoli

413

Nuclear Rainbows, Nuclear Matter and the 1 6 0 + 1 6 0 System W. von Oertzen, A. Blazevic, H. G. Bohlen, D. T. Khoa, F. Nouffer, P. Roussel-Chomaz, W. Mittig and J. M. Cassandjian

419

Bremsstrahlung by Nonrelativistic Particles in Matter A. V. Koshelkin

427

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Pronounced Airy Structure in Elastic 1 6 0 + 1 2 C Scattering at Eiab=200 MeV Y. A. Gloukhov, A. S. Dem'yanova, A. A. Ogloblin, M. V. Rozhkov, S. A. Goncharov, R. Julin and W. Trzaska

431

Fusion Energy Thresholds Predicted with an Adiabatic Nucleus-Nucleus Potential J. Wilczynski and K. Siwek- Wilczynska

435

Near-Barrier Fusion of 36 S+ 90 - 96 Zr: What is the Effect of the Strong Octupole Vibration of 96 Zr? L. Corradi, A. M. Stefanini, A. M. Vinodkumar, S. Beghini, G. Montagnoli, F. Scarlassara and M. Bisogno

441

Dynamical Model of Fission Fragment Angular Distribution V. A. Drozdov, D. 0. Eremenko, 0. V. Fotina, S. Yu. Platonov and O. A. Yuminov

445

Fusion-Fission Reaction 1 2 C, 1 6 O+ 2 0 8 Pb at Subbarrier Energies S. P. Tretyakova, M. G. Itkis, E. M. Kozulin, N. A. Kondratiev, I. V. Pokrovskii, E. V. Prokhorova, L. Calabretta, C. Maiolini, A. Ya. Rusanov and T. Yu. Tretyakova

449

Sub-Barrier Fusion with a Halo Nucleus: The 6 He Case M. Trotta, J. I. Sida, N. Alamanos, F. Auger, A. Drouart, D. J. C. Durand, A. Gillibert, C. Jouanne, V. Lapoux, A. Lumbroso, F. Marie, S. Ottini, C. Volant, A. Andreyev, D. L. Balabanski, N. Coulier, G. Georgiev, M. Huyse, G. Neyens, R. Raabe, S. Ternier, P. van Duppen, K. Vivey, C. Borcea, J. D. Hinnefeld, A. Musumarra, A. Lepine and R. Wolski

453

Concluding Remarks Impact of Nuclear Science on Modern Society R. A. Ricci

459

Author Index

465

FOREWORD

The International Nuclear Physics Conference " Bologna 2000. Structure of the Nucleus at the Dawn of the Century", held from May 29 to June 3, 2000 in Bologna, Italy, in the Aula Magna of the University and in the medieval Palazzo dei Notai, was attended by more than 340 scientists from 43 countries and five continents. It was the first time that an International Nuclear Physics Conference of this size had taken place in Bologna. The Conference was organized by the Physics Department of the University and by the Bologna Section of the Italian National Institute for Nuclear Physics (INFN). It was sponsored, among others, by the Research Directorate General of the European Commission, the University of Bologna, the Italian Physical Society (SIF), the INFN National Laboratories in Legnaro (LNL) and Catania (LNS), the European Centre for Theoretical Studies in Nuclear Physics and Related Areas (ECT*) in Trento, the Italian National Agency for New Technologies, Energy and the Environment (ENEA). The idea for this Conference emerged from the desire to discuss perspectives in fundamental Nuclear Physics. Some of the most important European research facilities are located in Italy, a country with a long and rich tradition in this field. The Conference took place at a particularly symbolic moment when, celebrating the 900"* anniversary of its famous University, Bologna was designated as European City of Culture for the Year 2000. The aim of the Conference was also a cultural contribution to these celebrations and to the commemoration of the 200"1 anniversary of the death of Luigi Galvani, one of Bologna's most renowned scholars. In 1997, when Thessaloniki was European City of Culture, the Aristotle University organized a very successful Conference: "Advances in Nuclear Physics and Related Areas". The Bologna event was organized as the second in a hopefully long series of Nuclear Physics Meetings in the European Cities of Culture. This International Conference Bologna 2000 was devoted to a discipline which has seen a strong revival of research activities in the last decade. New experimental results and theoretical developments in the field of Nuclear Physics will certainly produce important contributions to our knowledge and understanding of Nature's fundamental building blocks. The interest aroused by the Conference in the scientific community is clearly reflected in the large number of participants. They represented the most important nuclear physics laboratories in the world. The generous grants from our sponsors allowed the financial support for a considerable number of

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participants who, otherwise, would not have been able to attend. The Conference covered five major topics of modern nuclear physics: nuclear structure, nucleus-nucleus collisions, hadron dynamics, nuclear astrophysics and trans-disciplinary and peaceful applications of nuclear science. The scope of the Conference was to present a review of recent progress in the field and to provide a forum for the discussion of current and future research projects. The Conference consisted of plenary and parallel sessions. The plenary sessions focused on comprehensive reviews, concepts and perspectives. Specialized talks on recent developments were presented in the parallel and poster sessions. More than 200 participants, among them many young researchers, had the opportunity to present their work in oral contributions. Two guided tours helped participants and accompanying persons to discover Bologna and the historical roots of its culture and traditions. On the last day most participants joined the excursion to Ravenna and its magnificent Byzantine monuments. We sincerely hope that all the participants enjoyed the Conference and their stay in Bologna. Acknowledgements We are very grateful to the University of Bologna and to its Rector Fabio Roversi Monaco for the generous financial support and the hospitality extended to the Conference in the beautiful Aula Magna of Santa Lucia. We would like to thank the Rector for his warm welcome address. The Pro-rector of the University Ettore Verondini gave an important and encouraging contribution to the organisation of the Conference. We thank professor Enzo Iarocci, President of the National Institute for Nuclear Physics, for his welcome address. Our special thanks go to Professor Antonino Zichichi, distinguished senior member of the Department of Physics of the University of Bologna. His promotional activities, his help in the publishing of the Proceedings, and finally his extremely interesting opening lecture, made an essential and animating contribution to the success of the Conference. The Conference was closed with the most interesting review of the past and the future of Nuclear physics by Professor Renato Angelo Ricci, former President of the Italian Physical Society. We acknowledge the help of the Department of Physics of the University. We are very grateful to its Director, Professor Attilio Forino, for his numerous suggestions, encouragement and also generous financial help which was essential in the initial stages of the organisation of the Conference. Our spe-

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cial thanks go to the Bologna section of the INFN and its Director Professor Paolo Giusti for the financial and logistic support. The Research Directorate General of the European Commission, the framework of Human Potential Programme, High-Level Scientific Conferences (under contract no. HPCF-CT-1999-00131), generously supported the participation of young researchers from the European Community and Associate States. It is a pleasure to thank the President of ENEA, Nobelist Carlo Rubbia, and the head of the ENEA Accelerator-Driven-System Project, Giuseppe Gherardi, for the financial and scientific support offered by ENEA to the Conference. Essential for the success of the Conference was the dedicated work of the Conference Secretariat: Barbara Simoni and Luisa De Angelis, the Conference Agency: Angela Rizzi, and the kind assistance of Erika Giorgianni. Special thanks go to Angela Belluzzi from the Office of the Rector of the University for her help in organising the Conference venue. The commitment of ECT* Trento and its Scientific Secretary Renzo Leonardi was invaluable in the organisation and during the Conference. The very efficient work of Ines Campo and Rachel Weatherhead during the five days of the Conference, and the kind assistance of Mauro Meneghini should be particularly mentioned. We express our warmest thanks to the Directress of the Centro Documentazione delle Donne, Mrs Grazia Negrini, for the kind hospitality extended to the Conference in the magnificent Palazzo dei Notai. We are very grateful to the municipality of Bologna for hosting the welcome party in the splendid Sala d' Ercole. The visit to the Museo Morandi and to the Town Hall art collections was particularly appreciated. The financial contribution of the "Fondazione Carisbo" is gratefully acknowledged. Further financial support was provided by the listed in the following pages. Informative brochures were kindly provided by the University of Bologna, by Professor Giorgio Giacomelli, the LNS and LNL and the " Assessorato alia Cultura" of the Bologna Province. The plenary sessions were recorded and broadcasted in real time via Internet by the INFN informatics and multimedia! groups. We express our warmest thanks to the Director Paolo Mazzanti and to Gianfranco Artusi, Daniela Bortolotti, Roberto Giacomelli, Alessandro Italiano, Franco Martelli, Gianluca Peco, Ombretta Pinazza, Giuliano Scandellari, Stefano Zani. The facilities and services provided by the Physics Department are gratefully acknowledged. We thank Attilio Capponi, Nino Muzzupappa, Romano Serra, Simonetta Salsini and Giuseppe Zaccarelli. The work of Paolo Finelli was invaluable in preparing the proceedings of

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the Conference. Our warmest thanks go to the colleagues and friends, members of the Organizing Committee, and to the International Advisory Committee, for the excellent job done in preparing the scientific programme and selecting the speakers. The plenary and parallel sessions were run very smoothly and efficiently by our Session chairpersons. Finally, we would like to thank all the participants whose enthusiasm, interest and scientific ideas represent the main contribution to the success of the Conference. They presented an exciting variety of new results, demonstrating the vitality of Nuclear Physics and shedding light on future perspectives at the turn of the century.

The Editors

WELCOME ADDRESS FABIO ROVERSI MONACO Rector of the University of Bologna Ladies and Gentlemen, distinguished Colleagues, It is my pleasure to warmly welcome all the participants in this Conference. The Bologna studium, as you know, is the oldest of the western world: here, the very concept of University as an institution based on independent scientific research and teaching first came into being. In 1988 the University of Bologna celebrated its nine-hundreth anniversary and, on that occasion, the Magna Charta Universitatum, i.e. the Universal Declaration of Academic rights, was signed by more than 400 Rectors of Universities all over the world. A few months ago, following the same inspiration, an Observatory on Academic Freedom was founded, with the purpose of controlling the respect of the fundamental principles on which scientific research and teaching are based. This Conference opens at the turn of a millennium in a particularly symbolic moment: the year 2000, when Bologna takes its turn as Cultural Capital of Europe. A century is passing that saw a tremendous progress in understanding the ultimate structure of matter: technological and social consequences have deeply affected the whole mankind in good and evil. Only one hundred years ago Henry Becquerel and Marie Curie discovered radioactivity. Some ten years later, Ernest Rutherford proved the existence, at the core of the atom, of a very small nucleus, where the same kind of energy is stored as the one that feeds our life from the sun. Even the atoms that make up our bodies were synthesized by nuclear reactions in the core of stars and supernova explosions: there is thus an impressive link between macro- and microcosm. Nevertheless, we assist today to a growing disaffection for science, mainly due to nuclear weapons and improper use of nuclear energy. Let me remind you that the term nuclear was recently dropped from such medical applications as Nuclear Magnetic Resonance. Does this mean that we must give up studying Nature? The Magna Charta states that the future of mankind depends to a large extent on cultural, scientific and technical development. This means that we must go on investigating the deepest problems Nature faces us with, but, at the same time, we must recover the sense of Nature as a common mother. What we need is a new sense of ethic with a direct commitment of scientists for peace, justice, development and mutual understanding.

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It is the first time a Nuclear Physics Conference of large size takes place in Bologna. In 1977, when Thessaloniki was the Cultural Capital, the Aristotle University organized a very successful Conference " Advances in Nuclear Physics and Related Areas". I hope we are participating in the beginning of a long series of Nuclear Physics meetings to be held in the Cultural Capitals of Europe. Starting from Enrico Fermi, Italy has a long and rich tradition in this field, and the contribution of Bologna to the advances of nuclear physics covers the main trends in experimental and theoretical fundamental research, carried on mainly by University and INFN, in collaboration with major institutions and laboratories all over the world. It ranges from light nuclei to heavy-ion physics, from nuclear models to hadron structure with a prominent emphasis on high-energy physics and fundamental interactions. Let me mention now a few words about some troubles that nuclear physics is actually suffering in Bologna. While experimental nuclear physics is represented by many active groups, theoretical nuclear physics runs the risk of disappearing, since young people are moving towards other fields of research where more positions are available. This is a pity in many respects: firstly, a long tradition is disappearing and the richness of knowledge and experience in this field will be definitely lost in a short time; secondly, even if they are considered less fundamental than subnuclear physics, nuclear problems show many interesting and puzzling facets, which may stimulate imagination and development of new models and techniques. For these reasons, I think that the Physics Department, the Faculty of Sciences and INFN should make a serious effort in term of positions - professors and researchers - to avoid the loss of such a relevant cultural legacy. The aim of this Conference is to promote the exchange of ideas and future collaboration among nuclear physicists coming from developed countries and developing regions. The high quality of the programme, with special emphasis on new directions and opportunities, the quality of contributions and the level of speakers, who are among the most prominent scientists in this field, the large number of participants, illustrate the vitality of nuclear science and are favourable auspices for the success of this International Conference. Let me conclude by warmly thanking Giovanni Carlo Bonsignori, Mauro Bruno and the whole organizing committee for their engagement in arranging this meeting. Finally, I wish the Bologna-2000 International Conference on Structure of the Nucleus at the Dawn of the Century the best success, and I hope you will enjoy not only the conference, but also your stay in Bologna and its University.

Conference Organization CONFERENCE CHAIRMEN:

G. Bonsignori (Bologna), M. Bruno (Bologna)

SCIENTIFIC SECRETARIES:

A. Ventura (ENEA/Bologna), D. Vretenar (Zagreb)

INTERNATIONAL ADVISORY COMMITTEE M. Arnould (Bruxelles) J. Aysto (Jyvaskyla) C. Baktash (Oak Ridge) D. Bazzacco (Padova) J.-F. Berger (CEA/Bruyeres-le-Chatel) J.-P. Blaizot (CEA/Saclay) S. Born (Pavia) J. Bondorf (Copenhagen) R. Bougault (Caen) T. Bressani (Torino) R. Broglia (Milano) P. Chomaz (Caen) C. Ciofi degli Atti (Perugia) M. Di Toro (Catania) C. Fahlander (Lund) G. Fiorentini (Ferrara) B. Frois (CEN Saclay) K. Gelbke (NSCL/East Lansing) W. Gelletly (Surrey) G. Gherardi (ENEA/Bologna)

D.H.E. Gross (Berlin) M. Grypeos (Thessaloniki) W. Haxton (Seattle) F. Iachello (New Haven) A.V. Ignatyuk(Obninsk) M. Ishihara (Tokyo) F. Kaeppeler (Karlsruhe) P. Kienle (Munchen) R. Malfliet (Trento) I. Massa (Bologna) V.Metag (Giessen) E. Migneco (Catania) Y.Ts. Oganessian (Dubna) T. Otsuka (Tokyo) R.A. Ricci (Padova) P. Ring (Munchen) F.-K. Thielemann (Basel) A.W. Thomas (Adelaide) A. Zichichi (Bologna)

LOCAL ORGANIZING COMMITTEE S. Lunar di (Padova) A. Mengoni (ENEA/Bologna) A. Pagano (Catania) M. Savoia (Bologna) A. Vitturi (Padova)

F. Cannata (Bologna) A. Covello (Napoli) G. De Angelis (Legnaro) P. Finocchiaro (Catania) R. Leonardi (Trento) G. Lo Bianco (Camerino)

XXI

XX ii

SPONSORS • Dipartimento di Fisica dell'Universita' di Bologna • Istituto Nazionale di Fisica Nucleare - Sezione di Bologna • Universita' degli Studi di Bologna • Provincia di Bologna • INFN - Laboratori Nazionali del Sud - Catania • INFN - Laboratori Nazionali di Legnaro - Legnaro PD • ENEA • Research Directorate General of the European Commission (Under contract no. HPCF-CT-1999-00131) • ECT* - European Centre for Theoretical Studies in Nuclear Physics and Related Areas • Societa' Italiana di Fisica • Fondazione Carisbo, Bologna • Banca di Roma, Roma • Bologna 2000 Committee • EG&G ORTEC Italia • CAEN - Nuclear Physics - Viareggio LU • 3M Italia • Consorzio Marchio Storico dei Lambruschi Modenesi • Consorzio tutela Parmigiano Reggiano • Consorzio produttori di aceto balsamico tradizionale di Modena • Alcisa Bologna • Consorzio tutela aceto balsamico • Monari e Federzoni - Bomporto (Modena)

#'

11 of the positive and of the negative muons was also considered a further proof for the validity

9 of C invariance. This belief went on with the discoveries of the antiproton (p) by Segre et al. [13], of the antineutron (n) by Piccioni et al. [14] and even with the discovery (see later) of the second neutral "strange" meson (called at the time 9° ) by Lederman et al. [15]. While the validity of C invariance appeared to be experimentally established on firm grounds, and the CPT theorem was discovered, a "new era" started. Let us recall what the "old era" was. The edifice of antimatter was built on the validity of C invariance. In fact, as mentioned above, if C invariance holds in all physics processes, then the existence of the electron is sufficient to guarantee the existence of the antielectron, and the existence of a particle that of its antiparticle. Dirac in his Nobel Lecture [16] proposed, in addition to the existence of all antiparticles, that of antimatter, antistars and antigalaxies. The "new era" had an important (later proved to be false) component in the domain of the crisis for all known RQFTs. Before the "new era" started, there was a period of great enthusiasm and success in the description of: i) electromagnetic interactions (QED), thanks to the discovery of the renormalization; ii) weak and iii) nuclear interactions, respectively, thanks to the Fermi theory of the weak processes and to the Yukawa prediction of the ii-meson. After these original successes, serious difficulties came in. In QED with the Landau poles, in weak interaction with the unitarity at 300 GeV being violated, and in the field of strong interactions with the proliferation of mesons and baryons. This proliferation was totally out of any theoretical understanding in terms of a RQFT. Moreover a new mathematical formalism came into the limelight: the Scattering matrix. The S-matrix was the negation of the "field" concept and needed only analyticity, unitarity and crossing. The "new era" had its real fundamental contribution in the sector of the breakdown of all Symmetry Operators. In 1953 the ( 9 - x ) puzzle found by R.H. Dalitz [17] prompted T.D. Lee and C.N. Yang [18] to discover that there were no experimental checks for the validity of the Symmetry Operators C and P in the field of weak interactions. Within a year, C.S. Wu et al. discovered [19] that weak interactions do not obey C and P invariance, but CP seemed to be conserved [20]. By the end of 1957 the world appeared to have, as counterpart, the antiworld made like our own world with all charges and parity reversed. So, antimatter, antistars and antigalaxies could still exist with just one little additional detail: all intrinsic charges had to be reversed, together with the parity of each state. But in 1964, J. Christenson, J.W. Cronin, V.L. Fitch and R. Turlay [21] discovered that the combined Symmetry Operator CP was not valid in the decay of the neutral K-meson. T.D. Lee, R. Oehme and C.N. Yang (LOY) [22], before the experimental discovery by

10 C.S. Wu et al. [19] that, in weak interactions, the two Symmetry laws C and P were fully violated, pointed out that the existence of two "strange" mesons, 9j with decay mode 2 it and lifetime x =. 10

sec, 0 2 with decay mode 3 jt and much longer lifetime, could not be

considered a proof for the validity of either C invariance or P invariance or CP invariance. The (1957) LOY paper [22] anticipated the 1964 discovery [21], i.e. that the CP Symmetry is violated. In fact the LOY paper established, on firm grounds, the clear distinction between meson-antimeson mixing and CP invariance [see Appendix 3]. The breakdown of CP implied that also T had to be broken if CPT had to remain a fundamental invariance. For some founding fathers of our physics, the time reversal invariance was sacred. Thus CPT had to be broken. After all, why should we believe in CPT if RQFTs were encountering big difficulties? The "new era" had no need for the validity of the Symmetry Operators nor, apparently, for the description of the fundamental forces in terms of a RQFT. What about matter-antimatter symmetry? The discovery of CP violation was announced at the first High Energy Conference, held in the USSR, where I was reporting on the results obtained at CERN with the most intense beam of negative particles implemented in order to study the time-like structure of the proton [23, 24], to search for a new Heavy Lepton [25] in addition to the muon, and to study the existence of nuclear antimatter at a level ten times better than previous searches. The purpose of this negative beam was to compete with the physics of "Bubble-chamber" which dominated nearly all the physics research activities of the time. The breakdown of the Symmetry Operators (C, P, CP) and the apparent triumph of the S-matrix were coupled with the lack of evidence for the first example of nuclear antimatter (the antideuteron) found not to be there at the 10

level: i.e. no antideuteron

observed despite the 10 pions, produced in the same interactions. Question. If C invariance is broken, what is expected to happen to the existence of antimatter? The answer — as said before — is not that antimatter should not be there, but that its existence is granted by the validity of CPT. And what about the well established equality between x^+ and x^- ? The answer is the same. The equality X^i+

— T^-

is granted by the validity of CPT invariance. Another question. Since CP invariance does not hold in K-physics, what happens to matter-antimatter symmetry? Is it broken? The answer is no, since — as mentioned earlier — matter-antimatter symmetry, despite the breaking of CP invariance, is granted by the validity of CPT. In other words it does not matter if the Symmetry Operators, C, P, and CP

11

are broken. The existence of antimatter depends on the validity of CPT. But the validity of CPT was (and is) deeply rooted in RQFT. Here comes the key point: the theoretical description of the strong interactions. No one knew how to describe the nuclear forces in terms of a mathematical formalism of the RQFT type. We have said that those were years of triumph for the negation of RQFT: i.e. the S-matrix theory. Since this was the status of the strong interactions, the existence of the antideuteron could not be "theoretically" predicted. In fact, the strong interactions seemed to be dominated by the S-matrix theory and the Symmetry Operators C, P and CP were all experimentally found to be broken with the T Symmetry Operator imagined to have a privileged role. All this conspired towards a breakdown of CPT invariance. Table 1 illustrates the basic steps, during seven decades, from the antielectron and the properties of an antiparticle to the properties of antimatter and the unification of all gauge forces (QED, QFD, QCD). It is the unification of all forces at E G U = 10 16 GeV, and eventually at the Planck scale, which creates problems to the validity of CPT [11]. Let me elaborate a bit on these developments, starting with the discovery of the first example of nuclear antimatter (or, better, "antinuclear matter": see later). For this to happen, two basic technical developments were needed. One was the most intensive beam of negative particles we had built at CERN. The other was the most precise Time-Of-Flight (TOF) device we had implemented [26] to reach a time accuracy of ± 100 psec (psec s 10" sec). These technical developments allowed us to improve, by an order of magnitude, the search for nuclear antimatter. And this is how — contrary to the general theoretical trend of the time and to the experimental failure to observe the first example of nuclear antimatter — we were able to prove that the antideuteron was produced at the level of 10" , thus providing a firm basis for the validity of the CPT symmetry in the theory of the nuclear forces, despite the fact that no one knew how to theoretically describe these forces. This was in 1965. The following years were even more dramatic for the validity of a mathematical description of the strong interactions in terms of a RQFT. Not only the Smatrix theory was dominating but an experimental discovery at SLAC came in. It was called "scaling" [27]: its physical implication was (and is) that the proton had to have elementary constituents (called partons: quarks and gluons) and that these, at high energy, had to be free inside a proton. On the other hand, it was experimentally found at CERN [28] that, in violent (pp) collisions, there were no free quarks produced. The general trend in the theoretical interpretation of these experimental results was that RQFT was not able to describe the physics of the strong interactions. The proof that the first example of nuclear antimatter exists was — during those years — the only source for confidence in a RQFT

12 description of the strong interactions. There were exceptions in the theoretical front. In the volume "The Discovery of Nuclear Antimatter" [26] Luciano Maiani says: «The discovery of the antideuteron gave more confidence to the search of a field theoretical basis for the strong interactions, that today seems so obvious to us». The most interesting of these searches was the one by G. 't Hooft who discovered in 1971 that the (3-function has negative sign for nonAbelian RQFT. By now — after many decades — our understanding of the nuclear forces is not in terms of a fundamental RQFT but in terms of an "effective" theory which should be derived from the fundamental one - QCD: i.e. the non-Abelian force acting between quarks and gluons. This is a RQFT; however, the discovery of the renormalization group equations (RGEs) generates the running with energy of the three couplings (ctj a 2 a 3) of the gauge forces (QCD, QFD, QED) towards a unique point at E G U = 1016 GeV where a

GU ~ 74 [10]

anl

^ eventually at the Planck scale. Here the CPT theorem loses its

foundation [11], therefore all gauge forces, despite being examples of Relativistic Quantum Field Theories (RQFTs), have no fundamental reasons to obey CPT invariance, if their common origin is at the Planck scale. How the loss of such a theoretical fundamental reason affects the low energy domain, where the colour-neutral-QCD-binding nuclear forces act, is not known. Only high precision experiments on the mass difference Am, between nuclei and antinuclei, as for example, deuteron-antideuteron, Am - , can determine these consequences. At present all we know is very simple: antinuclear matter exists despite that the theoretical foundations whose existence it was based upon have vanished.

13

SEVEN

DECADES FROM THE ANTIELECTRON TO ANTIMATTER AND THE UNIFICATION OF ALL GAUGE FORCES

• The validity of C invariance from 1927 to 1957 After the discovery by Thomson in 1897 of the first example of an elementary particle, the Electron, it took the genius of Dirac to theoretically discover the Antielectron thirty years after Thomson. 1927 -> Dirac equation [1]; the existence of the antielectron is, soon after, theoretically predicted. Only a few years were needed, after Dirac's theoretical discovery, to experimentally confirm (Anderson, Blackett and Occhialini [12]) the existence of the Dirac antielectron. 1930-1957 -* Discovery of the C operator (charge conjugation) [H. Weyl and P.A.M. Dirac [2]]; discovery of the P Symmetry Operator (Parity) [E.P. Wigner, G.C. Wick and A.S. Wightman [3, 4]]; discovery of the T operator (time reversal) [E.P. Wigner, J. Schwinger and J.S. Bell [5, 6, 7, 8]]; discovery of the CPT Symmetry Operator from RQFT (1955-57) [9]. 1927-1957 -» Validity of C invariance: e+ [12]; p [13]; n [14]; K° — 3JI [15] but see LOY [22]. • The new era starts: C * ; P * ; CP * ( , ) 1956 -» Lee & Yang P * ; C * [18]. 1957 -* Before the experimental discovery of P * & C * , Lee, Oehme, Yang (LOY) [22] point out that the existence of the second neutral K-meson, K 2 -» 3JI , is proof neither of C invariance nor of CP invariance. Flavour antiflavour mixing does not imply CP invariance. 1957 -» C . S . W u e t a l . P x ; C * [19]; CP ok [20]. 1964 — K° - . 2it M K L : CP * [21]. 1947-1967 -» QED divergences & Landau poles. 1950-1970 -» The crisis of RQFT & the triumph of S-matrix theory (i.e. the negation of RQFT). 1965 -» Nuclear antimatter is (experimentally) discovered [29]. See also [26]. 1968 -» The discovery [27] at SLAC of Scaling (free quarks inside a nucleon at very high q 2 ) but in violent (pp) collisions no free quarks at the ISR are experimentally found [28]. Theorists consider Scaling as being evidence for RQFT not to be able to describe the Physics of Strong Interactions. The only exception is G. 't Hooft who discovers in 1971 that the p-function has negative sign for non-Abelian theories [30]. 1971-1973 -» p = - ; 't Hooft, Gross & Wilczek. The discovery of non-Abelian gauge theories. Asymptotic freedom in the interaction between quarks and gluons [30]. 1974 -» All gauge couplings a, o^ c^ run with q 2 but they do not converge towards a unique point. 1979 -» A.P. & A.Z. point out that the new degree of freedom due to SUSY allows the three couplings a a . a , , to converge towards a unique point [31]. 1980 -» QCD has a "hidden" side: the multitude of final states for each pair of interacting particles: ( e + e _ ; pp ; up; Kp; vp; pp; etc.). The introduction of the Effective Energy allows to discover the Universality properties [32] in the multihadronic final states. 1992 -» All gauge couplings converge towards a unique point at the gauge unification energy: E G U = 10 16 GeV with aGV = ^ [33, 3 4 ] . 1994 —• The Gap [35] between E G U & the String Unification Energy: E s u ~ E P l a n c k . 1995 -» CPT loses its foundations at the Planck scale (T.D. Lee) [11]. 1995-1999 -» No CPT theorem from M-theory (B. Greene) [36]. 1995-2000 -* A.Z. points out the need for new experiments to establish if matter-antimatter symmetry or asymmetry are at work. (*)

The symbol ^ stands for "Symmetry Breakdown". Table 1

14 3—

Definitions. Intrinsic, confinement and binding masses. Matter and antimatter Richard Feynman would have said: «let us first agree on what we are talking about*.

In the field of "matter-antimatter" this is badly needed. We have learned during these seven decades since the discovery of the Dirac equation that matter consists of fundamental fermions (leptons and quarks) which show up as free objects if they are leptons or manifest in a confined state which can be made of a quark antiquark pair (mesons) or a three quark state (baryons). These states are confined by QCD colour forces. During many decades mesons and baryons have been called "elementary particles". We know they are not elementary but composite of quarks and gluons, nevertheless we can go on calling them "particles". Baryons and mesons interact in such a way as to build up the nuclei of the atoms. Matter and antimatter are made of some constituents and their antis, respectively. They are listed in Table 2. If a basic symmetry has to exist between matter and antimatter, from this symmetry necessarily follows the symmetry between the constituents and their antis. The reverse is not true. If a basic symmetry principle exists between the constituents and their antis, this does not imply that the symmetry can be extended to matter and antimatter. In fact, in order to have matter, we need to put together the constituents of which matter consists. In order to have antimatter, we need to put together the anticonstituents of which antimatter consists. The way we put constituents together depends on the interactions which are responsible for their existence and for their properties, including the mutual interactions between them. The same is true for the anticonstituents. charges;

antimatter consists of mass plus anticharges.

Matter consists of mass and The properties of matter and

antimatter include all these fundamental problems. If we apply the CPT operator to matter we will get mass plus anticharges. If the charges are of nuclear origin we will have antinuclear charges with the same mass, if CPT invariance holds. If not, the mass associated with the antinuclear charges will be different from the mass associated with the nuclear charges. We have also the case where the mass is associated with the electric charges.

All atoms are made of electrons glued

electromagnetically to nuclei. The antielectrons will be electromagnetically glued to the antinuclei. There is therefore an electromagnetic binding which ensures the existence of all atoms. The masses produced by the electromagnetic binding are negative, like the nuclear binding. The negative masses of nuclear and electromagnetic origin should be the same for

15

CONSTITUENTS AND ANTICONSTITUENTS FUNDAMENTAL FERMIONS AND ANTIFERMIONS Leptons (£) vP \ Ve

(

V(X

V

)

l

/

Quarks

and

HL

HL

;

c

(q)

/u \

I )

(t

Id ,

,s ,

UJ

t

\

© Antileptons (£)

Antiquarks

and

PARTICLES esons and Baryo ns (q q q)

(qq) and

ANTIPARTICLES Ai itimesons and Antibarycjns (qqq )

( qq)

NUCLEI

AND ANTINUCLEI

=

D

;

(p5)

( ann) =

H

;

(piin)

; P n)

( ppn) a

He

;

(ppn)

^ D 3 B

-

3

H

He

(q)

16 atoms and antiatoms only if CPT invariance holds for the electromagnetic and the nuclear forces. There is a difference between electromagnetic and nuclear forces. The electromagnetic forces are (fundamental forces)'*' described by a Relativistic Quantum Field Theory (RQFT), (QED); the nuclear forces are secondary effects originated by the fundamental colour force QCD acting between quarks and gluons inside the "particles". The nuclear forces mimic the existence of nuclear charges and of a nuclear field whose source was believed to be "elementary", the nucleon. We know that the nucleon is not elementary but composed of a ( q q q ) triplet. The quanta of the nuclear field were also thought to be elementary, the JI -mesons. We now know that not only the Jt-meson, but all mesons, are not elementary entities but composed of a (q q) pair. Nuclei exist if protons and neutrons can be bound together by nuclear forces. Proton and neutrons (nucleons) are QCD colour neutral, nevertheless these residual QCD effects generate attraction between nucleons. These attractive forces — before QCD was discovered — were called "Nuclear Forces" and, as mentioned above, were supposed to be originated by "Nuclear" charges. We know now that nuclear charges do not exist as fundamental charges. Their effects are the residues of QCD, once we go from the QCD colour-full world of quarks and gluons to the QCD colour neutral world of baryons and mesons. Nevertheless if we suppose that the QCD colour neutral residual effects behave as if nuclear charges exist, and apply the C operator to a "nuclear" charge, it reverses the sign of this "charge". If C invariance holds, the mass associated with the opposite nuclear charge remains the same as before.

Thus

an

antinucleus, composed of a number of antinucleons (larger or equal to two), has all nuclear charges reversed. The antinuclear charges are associated with the mass of the antinuclei. Thus the antideuteron is an example of antinuclear matter. When the anticharges are of electromagnetic nature, the appropriate terminology would be: antielectromagnetic matter. Since matter is made of mass plus charges, the terms antinuclear

and

antielectromagnetic specify the reversal of the charges and there is no reason to specify further this reversal with the term "anti" repeated for matter. Antinuclear antimatter is redundant, but to eliminate the redundancy the most appropriate choice is "antinuclear matter".

Certainly not "nuclear antimatter", as it is in the present-day terminology

(see

Appendix 2). The word "matter" should be used to specify the elements of the Mendeleev table. If

(*)

They originate from the fundamental gauge forces SU(2)xU(l) broken at the Fermi scale (see for example [30]).

17

matter was made only of Hydrogen, we would not need the nuclei. Hydrogen is the only element whose nucleus is a single particle, the proton, and the atom is obtained by adding to it a fundamental fermion, the electron. Matter^ consists of nuclei surrounded by electron clouds. In order to have an atom of iron we need the leptonic cloud (made of electrons) and the nucleus (made of baryons glued by mesons). To have anti-iron we need the fundamental antifermions (positrons), the antiparticles (antimesons and antibaryons) and the property that these antiparticles must interact like the particles in order to have the antinucleus of the iron. We cannot define antimatter as the fundamental antifermions and the antiparticles. We need to specify the property which allows all antielements of the Mendeleev table to exist. This property (in addition to the electromagnetic binding) is the nuclear binding which allows antiprotons and antineutrons (antinucleons) to form the antinuclei. Thus it seems appropriate to use the word antimatter when we refer to the specific need to build the antielements of the Mendeleev table. The nucleus of the anti-Hydrogen atom is an antiparticle (the antiproton). The simplest element where an antinucleus is needed is the antideuterium. This is the reason why the antideuteron ( p n ) is referred to as being the first example of "nuclear antimatter"; (as mentioned earlier, "antinuclear matter" would be more appropriate). To be more precise, in order to build matter it is necessary to have quarks and leptons (fundamental fermions), mesons and baryons (particles) and nuclei (protons and neutrons glued together). As shown in Table 2 there are three classes of constituents to which masses are associated: i) the fundamental fermions (quarks and leptons): f; i.e. "intrinsic" masses; ii) the particles (mesons and baryons): p A ; i.e. "intrinsic" plus "confinement" masses; iii) the nuclei (Deuteron, Tritium, Helium etc.): N ; i.e. "intrinsic", "confinement" plus the nuclear "binding" masses. To these correspond three types of anticonstituents. The three types of mass differences Am follow from these three classes and are: i) Amf7 (example: the mass-difference quark-antiquark, Am - ). ii) Am„P P= (example: the mass-difference meson-antimeson, A mK^K; or the >• r A

A

mass-difference proton-antiproton Am _). iii) Am^-: (example: the mass-difference deuteron-antideuteron, AmDg). An antiatom of iron would be made of an antinucleus surrounded by a cloud of (*)

The word "matter" should not be used to indicate "particles" (mesons and baryons) or fundamental fermions: quarks and leptons.

18

antielectrons. To its mass, all types of masses will contribute: "intrinsic", "confinement" and "binding" (nuclear plus electromagnetic). In fact the mass of the antiatom of iron would have: i) a component which is in the antinuclear mass of the anti-iron nucleus; and ii) a component which is in the antielectromagnetic mass of the antielectrons surrounding the antielectric charges of the antiprotons in the nucleus (in addition to other possible effects originating in the antielectromagnetic properties of the antineutrons in the same nucleus). If all forces, including both the residual colour-neutral QCD forces, called nuclear forces, and the QED forces, obey CPT symmetry, the mass of the anti-iron element of the Mendeleev table will be exactly equal to the mass of our iron. Thus matter and antimatter would be identical. But if CPT breaking mechanisms are active in any of the processes needed to build up the antiatom of iron, the matter-antimatter identity will be lost. In Table 3 there is a synthesis of all quantities needed to build Matter and Antimatter. In the column "masses", it is shown where the various masses come in.

19

Table 3: If the Hydrogen atom were the only element in the Mendeleev table, then the only form of antimatter would be the anti-Hydrogen atom. There are, by now, 118 elements in the Mendeleev table and the first element, Hydrogen, is the only example of atom made of two elementary particles: the proton and the electron. In order for (what we call) matter to exist, it is necessary that protons and neutrons can attract each other to form nuclei.

20 4—

What we know about the sources of the three types of masses To sum up, our understanding of the physical quantity called mass can be described in

terms of three sources: i)

the "intrinsic" mass of the fundamental fermions (quarks and leptons); the origin of these "intrinsic" masses is not known but is certainly not due to Quantum ChromoDynamics (QCD).

ii)

the "confinement" masses : m B a g , due to QCD confining colour forces; these effects produce positive masses.

iii) the "binding" masses which can be either of electromagnetic or of nuclear origin. These effects produce negative masses. All these sources of masses have different origins. Two of them, the QCD "confinement" masses and the electromagnetic "binding" masses, are associated with well known forces, QCD and QED. The other two ("intrinsic" and "nuclear binding") have some peculiar features. We briefly review all of them. "Intrinsic mass". It has still not been established how the masses of the fundamental fermions (quarks and leptons) are generated and what is the energy scale where this mechanism becomes operative. It could be the so-called "Higgs" mechanism and it could also be a new force of nature operating at the Planck scale. If it is at the Planck scale, here the CPT theorem loses its foundations [11] and it cannot be excluded that a CPT breaking occurs. Since no one knows either the energy scale where the masses of the fundamental fermions are generated, or if quark and lepton masses have the same origin, it cannot be excluded that at the energy scale where these masses are generated — for example the SUSY breaking scale — the CPT invariance breaks down. It is therefore worthwhile to investigate if the "intrinsic" masses obey CPT invariance. "Confinement mass". We will assume for simplicity that the "confinement" masses respect CPT invariance. Since QCD is a Relativistic-Local-Quantum-Field-Theory (RQFT), it could appear reasonable to expect that QCD respects CPT invariance [37]. But the convergence of the gauge forces at the Planck scale creates problems with the foundations of CPT [11]. On the other hand, the "confinement" mass is a QCD non-perturbative phenomenon and no one knows how to deal with these divergent effects. The "binding" masses have two origins. Electromagnetic "binding" masses. All atoms exist because of the electromagnetic binding effects, which produce negative masses.

Their origin is in Quantum

ElectroDynamics (QED). If QED were a RQFT without the problems created by the

21 convergence of the gauge forces at the Planck scale, here again it would be reasonable to expect no CPT breaking effects. Nuclear "binding" masses. All nuclei exist because of the nuclear "binding" forces. These are residual QCD effects, acting between neutral colour quark-triplets (qqq), the nucleons, and neutral colour quark-antiquark pairs (q q), the mesons. Nuclear physics is based on the interaction between nucleons mediated by mesons. Before the discovery of QCD [32], nuclear physics was thought to be an example of a RQFT; by now we know that the origin of nuclear forces is the non-Abelian theory describing the interaction between quarks and gluons, Quantum Chromodynamics, QCD; nevertheless no one has been able so far to derive the nuclear forces from QCD. What is clear is that what we call nuclear processes represent the transition from the QCD world (coloured quark and gluons) to the QCD colour neutral world of baryons and mesons. Since no one is able to describe this transition, it is probably wise to perform some experimental checks on the validity of CPT invariance in the nuclear "binding" masses, which are negative. The conclusion is that, out of the four types of masses, two of them appear to be associated with problems still to be understood. These two are: i) the exact origin of the intrinsic quark masses and ii) the nuclear binding (negative) masses which are associated with the transition from the QCD colour-full world of quarks and gluons to the colour neutral world of mesons and baryons. This is why it is important to check CPT invariance in the field of "intrinsic" and of nuclear "binding" masses. Let us start focusing our attention on the "intrinsic" quark masses and on their CPT invariance. The first point to clarify is the basic difference between a meson and a baryon.

22 5 —

Meson-antimeson and baryon-antibaryon mass differences A meson is made of a quark-antiquark pair, while a baryon consists of a quark triplet.

This difference is particularly relevant when mesons-antimesons and baryons-antibaryons are used to check CPT invariance via the measurement of the masses which make up the mesons, the antimesons, the baryons and the antibaryons. The first input we need is how to construct the mass of a meson and the mass of a baryon. The simplest proposal is to assume that the mass of these bound states is the sum of the masses of the various components, as it is the case for the "binding" masses' '. For the "binding" masses, "electromagnetic" and "nuclear", it is taken for granted that the "binding" masses (always negative) are the result of the sum of all masses of the free particle states minus the mass of the bound state, be it a molecule, an atom or a nucleus. When dealing with molecules and atoms, the intrinsic "leptonic" masses come in. These are the only type of "intrinsic" masses accessible to direct measurements since leptons exist as free states. Quarks exist only as bound states of mesons and baryons and their masses can only be indirectly determined. When dealing with mesons and baryons, life is not easy because we have no way to directly measure the masses of their constituents, quarks and antiquarks, since they do not exist as free states. What about the linear versus the quadratic expressions for the fermionic and bosonic masses?

In our Lagrangians the fermionic masses appear linearly, but the bosonic ones

quadratically: the origin being in their respective equations of motion. Since there is no theory describing CPT breaking effects in the masses of either fermions or bosons, we will consider the linear case for both fermions and bosons and, for bosons, also the quadratic expression. The linear case for bosons will be in § 6. The reason is that we want to have at least a preliminary idea on what could be the consequences of CPT breaking for the masses of mesons and antimesons, baryons and antibaryons. In fact, as pointed out earlier, a meson is a boson and it is composed of a quark and an antiquark, while its antipartner is again an antiquark-quark pair. So in the bosonic state we have the presence of both a quark and its anti. We will see that, if the CPT breaking has some simple property, its effects on the masses of quarks and antiquarks may be washed out for mesonic states and be present in baryonic states, since a baryon has no antiquarks in its

(*)

We are aware of the fact that this is a severe simplification. Masses originate from the Hamiltonian which is an Operator, not a C number.

23 composition. This means that we cannot neglect the basic structure of a meson and of a baryon when searching for CPT breaking effects in their masses. It is generally stated that the CPT invariance in the field of masses has its best check on the very high precision determination of the mass difference between the neutral K-meson, K , and its antipartner, K . There is in fact an upper limit oni this this mass difference | mKo - mR0 |

10 = Am KR * 4 x l(T-10 eV/c 2

(1)

which is derived from the mass difference [38] between the long-lived and the short-lived neutral K-mesons: (m K0 ~ n v )

-

AmKLKs = (3.491 ± 0.009) x 10"6 eV/c2 .

(2)

When the result (1) is compared with the value of the K -meson mass, the result is -

^

*

ID"18 •

(3)

K

If the result (3) were not linked to the mesonic mass, this would allow to conclude that CPT invariance is checked at the level of a few parts in 10 18 parts in the field of massmeasurements. This is not the case; it could very well happen that the result (3) be exactly zero, despite CPT invariance not being valid in the field of "intrinsic" and of nuclear "binding" masses. The reason being that a meson is composed of a quark-antiquark pair, and its antipartner is again an antiquark-quark pair. To avoid this cancellation, the best way to check possible CPT breaking effects is by comparing states composed of quarks, such as a baryon (qqq), with states composed of antiquarks, such as an antibaryon (qqq). If we compare baryon (qqq) and antibaryon (qqq) masses, the possibility of cancellation effects, which are present when comparing a meson (qq) with its antistate (qq), does not exist. According to a QCD model proposed by G. 't Hooft [39], the mass squared of a meson, (qq) pair, could be expressed by the sum of the intrinsic quark masses times a QCD factor, K,?™ n e m e n , coming from QCD confining forces acting on the quarks. For a baryon, quark triplet (qqq), the 't Hooft model [39] would suggest again the sum of the various quark masses plus a mass term F 0 ™ i n e m e n due to the QCD confining effects. It can be argued that mesonic as well as baryonic masses are linked to eigenstates and that the eigenvalue of the given mass-matrix could have strong contributions from offdiagonal elements of the same mass-matrix. On the other hand, it cannot be excluded that CPT breaking effects have consequences that are fundamentally different for a mesonic and a baryonic state. As emphasized above, a meson is made with a quark-antiquark pair, (qq),

24 while a baryon consists of a quark triplet (qqq). If our simple hypothesis is followed by nature, then care is needed when dealing with the experimental results on CPT invariance coming from the mass difference measured by comparing the mass of a meson and its antipartner. Following G. 't Hooft [39], the masses of a meson (qfL) (and of its antistate (c^q,)) are given by c™

\ 2 _ ,

(mq.q.) where m mand

• „

\ ^Confinement

= (m q . + m - )

=

the mass of the quark flavour "i"

=

the mass of the antiquark flavour " j "

KOCD

=

K ^

a factor — with dimension of mass — originated by the QCD

confinement forces needed to keep the (qjCh) pair into the bound state called meson. We assume that K ^ i n e m e n t is CPT invariant^. If this is the case, when we work out the meson-antimeson mass difference we have: \2 m qi

qj)

/„

\2T

r/

\

/

M ^Confinement

" ( m q i q j ) ] = [( m qi " mq;) + ( ^ .

=

" « , . )] K Q C D

[(Amqi-)-(Am-jq.)]^inemeni.

If CPT invariance holds for the "intrinsic" mass differences, we expect Am q = zero and Am q j - = zero . If CPT breaking effects are present in the intrinsic quark masses, we expect Am

qiqi

"

°

and Am q .-. *

0 .

Suppose that CPT breaking lowers the antiquark masses (assuming m

<m-

would

not change the conclusion) m qi

>

m-.

and suppose that the CPT breaking is "flavour" independent. In this case Am

(*)

qiqi

= ^qjqj =

Am

"

We make this assumption in spite of the fact that KJ°" ",emenl does contain higher order contributions from the quark masses. Nevertheless we are assuming for simplicity that these contributions in K are CPT invariant.

25 If this is true, we expect the difference between the mesonic and the antimesonic masses to be given by r

m

2

[ m(q;qj) "

m

2

i

r

.

(q iqj )] = [ ^

A

"

Am

i ^Confinement

1 KQCD

=

ZeTO

'

This cancellation effect does not exist when we compare baryonic and antibaryonic states. In fact, the masses of a baryon and of its antibaryonic partner are expected to be, following 't Hooft [39] : m m

_

/ -,

-.

,

q iqiqj

=

(2mq. + m q .)

,

+

FQCD

qiqiqj

_ =

/~

~ Confinement

where K™ D ' nemen is a mass term factor due to the QCD confinement forces which act on the quark triplet (qqq). As before we assume the same flavour invariant CPT breaking and no CPT breaking for F Confinement The mass difference is given by: (

|K_>-

|K L ) .

The recent discovery that E'/E^O [50] rules out the milliweak proposal [51], i.e. that K L is a mixture of K2 with a per mill fraction of Kj as the sole source of CP violation.

44 References and Notes [1]

P.A.M. Dirac, "The Quantum Theory of the Electron", Proc. Roy. Soc. (London) A117. 610 (1928); "The Quantum Theory of the Electron, Part IP', Proc. Roy. Soc. (London) A118. 351 (1928).

[2]

H. Weyl, " Gruppentheorie und Quantenmechanik", 2nd ed., 234 (1931).

[3]

E.P. Wigner, " Unitary Representations Math., 40, 149(1939).

[4]

G.C. Wick, E.P. Wigner, and A.S. Wightman, "Intrinsic Particles", Phys. Rev. 88,101 (1952).

[5]

E.P. Wigner, "Uber die Operation der Zeitumkehr in der Quanten-mechanik", Gott. Nach. 546-559 (1931). Here for the first time an anti-unitary symmetry appears.

[6]

E.P. Wigner, Ann. Math. 40,149 (1939).

[7]

J. Schwinger, Phys. Rev. 82, 914 (1951).

[8]

J.S. Bell, "Time Reversal in Field Theory", Proc. Roy. Soc. (London) A231, 479-495 (1955).

[9]

To the best of my knowledge, the CPT theorem was first proved by W. Pauli in his article "Exclusion Principle, Lorentz Group and Reflection of Space-Time and Charge", in "Niels Bohr and the Development of Physics" [Pergamon Press, London, page 30 (1955)], which in turn is an extension of the work of J. Schwinger [Phys. Rev. 82, 914 (1951); "The Theory of Quantized Fields. II", Phys. Rev. 9 1 , 713 (1953); "The Theory of Quantized Fields. Ill", Phys. Rev. 9 1 , 728 (1953); "The Theory of Quantized Fields. VI.", Phys. Rev. 94, 1362 (1954)] and G. Luders, "On the Equivalence of Invariance under Time Reversal and under Particle-Antiparticle Conjugation for Relativistic Field Theories" [Dansk. Mat. Fys. Medd. 28, 5(1954)], which referred to an unpublished remark by B. Zumino. The final contribution to the CPT theorem was given by R. Jost, in "Eine Bemerkung zum CPT Theorem" [Helv. Phys. Acta 30, 409 (1957)], who showed that a weaker condition, called "weak local commutativity" was sufficient for the validity of the CPT theorem.

[10]

The Simultaneous Evolution of Masses and Couplings: Consequences on Supersymmetry Spectra and Thresholds F. Anselmo, L. Cifarelli, A. Petermann and A. Zichichi, Nuovo Cimento 105A. 1179 (1992); see also A. Zichichi in "Subnuclear Physics - The first fifty years", O. Barnabei, P. Pupillo and F. Roversi Monaco (eds), a joint publication by University and Academy of Sciences of Bologna, Italy (1998); World Scientific Series in 20th Century Physics, Vol. 24 (2000). See also References [33], [34], [35].

of the Inhomogeneous Lorentz Group", Ann.

Parity

of

Elementary

45 [ll]

Are Matter and Antimatter Symmetric? T.D. Lee, in Proceedings of the "Symposium to celebrate the 30th anniversary of the Discovery of Nuclear Antimatter", L. Maiani and R.A. Ricci (eds), Conference Proceedings 53, page 1, Italian Physical Society, Bologna, Italy (1995).

[12]

The Positive Electron C D . Anderson, Phys. Rev. 43, 491 (1933); Some Photographs of the Tracks of Penetrating Radiation P.M.S. Blackett and G.P.S. Occhialini, Proc. Roy. Soc. A139. 699 (1933).

[13]

Observation of Antiprotons O. Chamberlain, E. Segre, C. Wiegand, and T. Ypsilantis, Physical Review 100. 947 (1955).

[14]

Anti-Neutrons Produced from Anti-Protons in Charge Exchange Collisions B.Cork, G.R. Lambertson, O. Piccioni, W. A. Wenzel, Physical Review 104, 1193 (1957).

[15]

Observation of Long-Lived Neutral V Particles K. Lande, E.T. Booth, J. Impeduglia, L.M. Lederman, and W. Chinowski, Physical Review 103,1901 (1956).

[16]

Theory of Electrons and Positrons P.A.M. Dirac, Nobel Lecture, December 12,1933.

[17]

On the Analysis of r-Meson data and the Nature of the x-Meson R.H. Dalitz, Phil. Mag. 44,1068 (1953); Isotopic Spin Changes in r and 6 Decay R.H. Dalitz, Proceedings of the Physical Society A69, 527 (1956); Present Status ofx Spin-Parity R.H. Dalitz, Proceedings of the "Sixth Annual Rochester Conference on High Energy Nuclear Physics", Interscience Publishers, Inc., New York, 19 (1956); for a detailed record of the events which led to the (6-x) puzzle see R.H. Dalitz "Kaon Decays to Pions: the t-d Problem" in "History of Original Ideas and Basic Discoveries in Particle Physics", H.B. Newman and T. Ypsilantis (eds), Plenum Press, New York and London, 163 (1994).

[18]

Question of Parity Conservation in Weak Interactions T.D. Lee and C.N. Yang, Phys. Rev. 104, 254 (1956).

[19]

Experimental Test of Parity Conservation in Beta Decay C.S. Wu, E. Ambler, R.W. Hayward, D.D. Hoppes, Phys. Rev. 105,1413 (1957); Observation of the Failure of Conservation of Parity and Charge Conjugation Meson Decays: The Magnetic Moment of the Free Muon R. Garwin, L. Lederman, and M. Weinrich, Phys. Rev. 105, 1415 (1957);

in

46 Nuclear Emulsion Evidence for Parity Non-Conservation in the Decay Chain J.J. Friedman and V.L. Telegdi, Phys. Rev. 105, 1681 (1957).

jti~n+e+

[20]

On the Conservation Laws for Weak Interactions L.D. Landau, Zh. Eksp. Teor. Fiz. 32, 405 (1957).

[21]

Evidence for the 2 n Decay of the K°2 Meson J. Christenson, J.W. Cronin, V.L. Fitch, and R. Turlay, Physical Review Letters 113. 138 (1964).

[22]

Remarks on Possible Noninvariance under Time Reversal and Charge Conjugation T.D. Lee, R. Oehme, and C.N. Yang, Physical Review 106, 340 (1957).

[23]

Search for the Time-Like Structure of the Proton M. Conversi, T. Massam, Th. Muller and A. Zichichi, Phys. Lett. 5, 195 (1963).

[24]

The Leptonic Annihilation Modes of the Proton-Antiproton System at 6.8 (GeV/c)2 Timelike Four-Momentum Transfer M. Conversi, T. Massam, Th. Muller and A. Zichichi, Nuovo Cimento 40, 690 (1965).

[25]

C.S. Wu, T.D. Lee, N. Cabibbo, V.F. Weisskopf, S.C.C. Ting, C. Villi, M. Conversi, A. Petermann, B.H. Wiik and G. Wolf The Origin of the Third Family, O. Barnabei, L. Maiani, R.A. Ricci and F. Roversi Monaco (eds), Academy of Sciences, Bologna University, INFN, SIF, Rome (1997); and World Scientific Series in 20th Century Physics, Vol. 20 (1998).

[26]

The Discovery of Nuclear Antimatter L. Maiani and R.A. Ricci (eds), Conference Proceedings 53, Italian Physical Society, Bologna, Italy (1995); see also A. Zichichi in "Subnuclear Physics - The first fifty years", O. Barnabei, P. Pupillo and F. Roversi Monaco (eds), a joint publication by University and Academy of Sciences of Bologna, Italy (1998); World Scientific Series in 20th Century Physics, Vol. 24 (2000).

[27J

The first report on "scaling" was presented by J.I. Friedman at the 14th International Conference on High Energy Physics in Vienna, 28 August-5 September 1968. The report was presented as paper n. 563 but not published in the Conference Proceedings. It was published as a SLAC preprint. The SLAC data on scaling were included in the Panofsky general report to the Conference where he says «... the apparent success of the parametrization of the cross-sections in the variable v / q 2 in addition to the large cross-section itself is at least indicative that point-like interactions are becoming involved». "Low q2 Electrodynamics, Elastic and Inelastic Electron (and Muon) Scattering", W.K.H. Panofsky in Proceedings of 14th International Conference on High Energy Physics in Vienna 1968, J. Prentki and J. Steinberger (eds), page 23, published by CERN (1968). The following physicists participated in the inelastic electron scattering experiments: W.B. Atwood, E. Bloom, A. Bodek, M. Breidenbach, G. Buschhorn, R. Cottrell, D. Coward, H. DeStaebler, R. Ditzler, J. Drees, J. Elias, G. Hartmann, C. Jordan, M. Mestayer, G. Miller, L. Mo, H. Piel, J. Poucher,

47 C. Prescott, M. Riordan, L. Rochester, D. Sherden, M. Sogard, S. Stein, D. Trines, and R. Verdier. For additional acknowledgements see J.I. Friedman, H.W. Kendall and R.E. Taylor, "Deep Inelastic Scattering: Acknowledgements", Les Prix Nobel 1990, (Almqvist and Wiksell, Stockholm/Uppsala 1991), also Rev. Mod. Phys. 63, 629 (1991). For a detailed reconstruction of the events see J.I. Friedman "Deep Inelastic Scattering Evidence for the Reality of Quarks" in "History of Original Ideas and Basic Discoveries in Particle Physics", H.B. Newman and T. Ypsilantis (eds), Plenum Press, New York and London, 725 (1994). [28]

Quark Search at the ISR T. Massam and A. Zichichi, CERNpreprint,

June 1968;

Search for Fractionally Charged Particles Produced in Proton-Proton Collisions at the Highest ISR Energy M. Basile, G. Cara Romeo, L. Cifarelli, P. Giusti, T. Massam, F. Palmonari, G. Valenti and A. Zichichi, Nuovo Cimento 40A, 41 (1977); and A Search for quarks in the CERN SPS Neutrino Beam M. Basile, G. Cara Romeo, L. Cifarelli, A. Contin, G. D'Ali, P. Giusti, T. Massam, F. Palmonari, G. Sartorelli, G. Valenti and A. Zichichi, Nuovo Cimento 45A, 281 (1978). [29]

Experimental Observation of Antideuteron Production T. Massam, Th. Muller, B. Righini, M. Schneegans, and A. Zichichi, Cimento 39,10 (1965).

Nuovo

[30]

A. Zichichi in "Subnuclear Physics - The first fifty years", O. Barnabei, P. Pupillo and F. Roversi Monaco (eds), a joint publication by University and Academy of Sciences of Bologna, Italy (1998); World Scientific Series in 20th Century Physics, Vol. 24 (2000).

[3l]

New Developments in Elementary Particle Physics A. Zichichi, Rivista del Nuovo Cimento 2, n. 14,1 (1979). The statement on page 2 of this paper, ^.Unification of all forces needs first a Supersymmetry. This can be broken later, thus generating the sequence of the various forces of nature as we observe them», was based on a work by A. Petermann and A. Zichichi in which the renormalization group running of the couplings using supersymmetry was studied with the result that the convergence of the three couplings improved. This work was not published, but perhaps known to a few. The statement quoted is the first instance in which it was pointed out that supersymmetry might play an important role in the convergence of the gauge couplings. In fact, the convergence of three straight lines (a~J a~2 a ~ 3 ) w i m a change in slope is guaranteed by the Euclidean geometry, as long as the point where the slope changes is tuned appropriately. What is incorrect about the convergence of the couplings is that, with the initial conditions given by the LEP results, the change in slope needs to be at M S U S Y ~ 1 TeV as claimed by some authors not aware in 1991 of what was known in 1979 to A. Petermann and A. Zichichi.

48 [32]

V.N. Gribov, G. 't Hooft, G. Veneziano and V.F. Weisskopf "The Creation of Quantum ChromoDynamics and the Effective Energy", L.N. Lipatov (ed), a joint publication by the University and the Academy of Sciences of Bologna, Italy (1998); World Scientific Series in 20th Century Physics, Vol. 25 (2000).

[33]

The Effective Experimental Constraints on MsuSY ond MGUT F. Anselmo, L. Cifarelli, A. Petermann and A. Zichichi, Nuovo Cimento 1817(1991).

104A,

[34]

The Simultaneous Evolution of Masses and Couplings: Consequences on Supersymmetry Spectra and Thresholds F. Anselmo, L. Cifarelli, A. Petermann and A. Zichichi, Nuovo Cimento 105A. 1179(1992).

[35 J

A Study of the Various Approaches to MGUT and aour F. Anselmo, L. Cifarelli and A. Zichichi, Nuovo Cimento 105A, 1335 (1992).

[36]

String Theory: the Basic Ideas B. Greene, Erice Lectures - Discussion 1999 in "Basics and Highlights Fundamental Physics", A. Zichichi (ed), World Scientific (to be published).

in

[37]

No matter what the physics processes are, if their mathematical formulation can be expressed in terms of a relativistic, local, quantized, field theory (RQFT), these processes have to obey CPT invariance. Thus to violate this fundamental invariance of nature corresponds to break the basic conceptual structure of a RQFT. It took many decades to discovery CPT invariance, as discussed in Reference 9. And many other decades were needed to discover the convergence of the gauge forces at the Planck scale.

[38]

C. Caso et al., Particle Data Group, Eur. Phys. J. C3,1 (1998).

[39]

G. 't Hooft, The Physics of Instantons in the pseudoscalar and vector meson mixing, Hep-th/9903183, to be published in the volume "Quark flavour mixing in meson physics"; and private communication.

[40]

Precision Mass Spectroscopy of the Antiproton and Proton Using Simultaneously Trapped Particles G. Gabrielse, A. Khabbaz, D.S. Hall, C. Heimann, H. Kalinowsky and W. Jhe, Phys. Rev. Lett. 82, 3198 (1999).

[41]

As discussed in § 3, the nuclear forces mimic the existence of nuclear charges. If despite the non-existence of "nuclear charges", CPT invariance holds, the mass associated with the opposite nuclear charges is the same as the one associated with the original nuclear charges. Thus an antinucleus must have the same mass as the nucleus. And this means that the nuclear "binding" masses obey CPT invariance. The mass of the deuteron has in its value all transition processes needed for the QCD colour-full world of quarks and gluons to become the world made of mesons and

49 baryons. The equality of the "binding" masses in nuclei and antinuclei is the proof that CPT invariance holds in all these processes. [42]

G. 't Hooft, The Creation of Quantum ChromoDynamics, in "The Creation of Quantum ChromoDynamics and the Effective Energy", page 25, V.N. Gribov, G. 't Hooft, G. Veneziano and V.F. Weisskopf; N.L. Lipatov (ed), Academy of Sciences and University of Bologna, INFN, SIF, joint publication (1998); World Scientific Series in 20th Century Physics, Vol. 25 (2000); T.D. Lee, Rev. Mod. Phys., Vol. 47, 267 (1975); T.D. Lee, Nucl. Phvs..A553. 3c (1993); and A. Zichichi, INFN - Report on the ALICE project for LHC to the INFN-Comm. Ill, Rome - Italy, May 1999.

[43]

A. Sakharov, JEPTLett.,5,

24 (1967).

[44]

The Discovery of Nuclear Antimatter and the Origin of the AMS Experiment S.C.C. Ting, in Proceedings of the Symposium to celebrate the 30th anniversary of the Discovery of Nuclear Antimatter, L. Maiani and R.A. Ricci (eds), Conference Proceedings 53, 21, Italian Physical Society, Bologna, Italy (1995).

[45]

Evidence for the Existence of New Unstable Elementary Particles G.D. Rochester and C.C. Butler, Nature 160, 855 (1947).

[46]

Behavior of Neutral Particles under Charge Conjugation M. Gell-Mann and A. Pais, Phys. Rev. 97,1387 (1955).

[47]

Unitary Symmetry and Leptonic Decays N. Cabibbo, Phys. Rev. Lett. 10, 531 (1963).

[48]

Weak Interactions with Lepton-Hadron Symmetry S.L. Glashow, J. Iliopoulos and L. Maiani, Phys. Rev. D2, 1285 (1970).

[49]

CP-Violation in the Renormalizable Theory of Weak Interaction M. Kobayashi and T. Maskawa, Prog. Theor. Phys. 49, 652 (1973).

[50]

Observation of direct CP violation in KL s -* mi decays A. Alavi-Harati et al., Phys. Rev. Lett. 83.', 22 (1999).

[51]

Violation of CP invariance and the possibility of very weak interactions L. Wolfenstein, Phys. Rev. Letters 13, 562 (1964); T.D. Lee and L. Wolfenstein, Phys. Rev. 138, B1490 (1965); see also L. Wolfenstein in "Theory and Phenomenology in Particle Physics", Proceedings of the 1968 Erice School, 219, Academic Press, New York and London (1969), A. Zichichi (ed).

Introductory Talk

W h a t can nuclear collisions teach us about the boiling of water or the formation of multi-star systems D.H.E. Gross Hahn-Meitner-Institut Berlin, Bereich Theoretische Physik,Glienickerstr. 100 14109 Berlin, Germany and Freie Universitat Berlin, Fachbereich Physik Phase transitions in nuclei, small atomic clusters and self-gravitating systems demand the extension of thermo-statistics to "Small" systems. The main obstacle is the thermodynamic limit. It is shown how the original definition of the entropy by Boltzmann as the volume of the energy-manifold of the N-body phase space allows a geometrical definition of the entropy as function of t h e conserved quantities. Without invoking the thermodynamic limit the whole "zoo" of phase transitions and critical points/lines can be unambiguously defined. The relation to the Yang-Lee singularities of the grand-canonical partition sum is pointed out. It is shown that just phase transitions in non-extensive systems give the complete set of characteristic parameters of the transition including the surface tension. Nuclear heavy-ion collisions are an experimental playground to explore this extension of thermo-statistics

Thermodynamicists will certainly answer our question by: "Nothing" and yet: 1

There is a lot t o add t o classical equilibrium statistics from our experience with "Small" systems:

Following Lieb 1 extensivity a and the existence of the thermodynamic limit N —• oc\N/v=const are essential conditions for conventional (canonical) thermodynamics to apply. Certainly, this implies also the homogeneity of the system. Phase transitions are somehow foreign to this: The essence of first order transitions is that the systems become inhomogeneous and split into different phases separated by interfaces. In general are phase transitions consequently represented by singularities in the grand-canonical partition sum (Yang-Lee singularities). In the following we show that the micro-canonical ensemble gives much more detailed insight. There is a whole group of physical many-body systems called "Small" in the following which cannot be addressed by conventional thermo-statistics: • nuclei,

"Dividing extensive systems into larger pieces, the total energy and entropy are equal to the sum of those of the pieces. 53

54

• atomic cluster • polymers • soft matter (biological) systems • astrophysical systems • first order transitions are distinguished from continuous transitions by the appearance of phase-separations and interfaces with surface tension. If the range of the force or the thickness of the surface layers is such that the number of surface particles is not negligible compared to the total number of particles, these systems are non-extensive. It is common to all these examples, that the systems are non-extensive. For such systems the thermodynamic limit does not exist or makes no sense. Either the range of the forces (Coulomb, gravitation) are of the order of the linear dimensions of these systems, and/or they are strongly inhomogeneous e.g. at phase-separation. Before inventing any new type of non-extensive thermodynamics or entropy like the one introduced by Tsallis 2 we should realize that Boltzmann's definition:

1 S - k * InW 1 with

W(E,N,V)=e0tr6(E-HN)

^E-H^ =

(1)

lm06iE'HN)-

(eo a suitable small energy constant) does not invoke the thermodynamic limit, nor additivity, nor extensivity, nor concavity of the entropy S(E,N) (downwards bending). This was largely forgotten since hundred years. We have to go back to pre Gibbsian times. It is a purely geometrical definition of the entropy and applies as well to "Small" systems. Moreover, the entropy S(E, N) as defined above is everywhere single-valued and multiple differentiate. There are no singularities in it. This is the most simple access to equilibrium statistics. We will explore its consequences in this contribution. Moreover, we will see that this way we get simultaneously the complete information about the three crucial parameters characterizing a phase transition of first order: transition temperature Ttr, latent heat per atom qiat and surface tension aSUrf- Boltzmann's famous epitaph above contains everything what can be said about equilibrium thermodynamics in its most condensed form. W is the volume of the sub-manifold at sharp energy in the 6iV-dim. phase space.

55

2

Relation of the topology of S(E, N) to the Yang-Lee singularities

In conventional thermo-statistics phase transitions are indicated by singularities of the grand-canonical partition function Z(T, n,V), V is the volume. See more details in 3>4>5'6 Z(T,(i,V)=

— diV -[E-fiN-TS{E)]/T JJo £ o i7 0 ° d e d n e -V[ e -Mn-T.(«,„)]/T = Y1 eo JJo ^ const.+lin.+quadr.

(2)

in the thermodynamic limit V —> oo\N/v=constThe double Laplace integral (2) can be evaluated asymptotically for large V by expanding the exponent as indicated in the last line to second order in Ae, An around the "stationary point" es,ns where the linear term vanish: 1

as

dE a OS A* _ T dN P dS T ~ dv

(3)

the only term remaining to be integrated is the quadratic one. If the two eigen-curvatures Ai < 0, A2 < 0 this is then a Gaussian integral and yields: T/2 Z{T,H,V)

=

rr°°

}-.e-V[e,-»n,-Ts(e.,n.)}/T

eo Z{T,n,V) F(T,n,V) V

//

^

^

^[X^+X^/2

(5)

e~F^^

=

(4)

JJo

, Tln(Vdet(es,ns)) < \nV es - ims - Tss + *— + o(-^-).

(6)

Here det(e s , ns) is the determinant of the curvatures of s(e, n). det(e,n) =

d2s

d2(

df

dnfie

dedn

dri2

— A1A2,

Ai > A2

(7)

In the cases studied here A2 < 0 but Ai can be positive or negative. If det(e s ,n s ) is positive (Ai < 0) the last two terms in eq.(6) go to 0, and we

56

Figure 1: MMMC simulation of the entropy s(e) (e in eV per atom) of a system of 1000 sodium atoms with realistic interaction at an external pressure of 1 atm. At the energy per atom e\ the system is in the pure liquid phase and at e$ in the pure gas phase, of course with fluctuations. The latent heat per atom is qiat — ez — e,\- Attention: the curve s(e) is artifically sheared by subtracting a linear function 25 + e * 11.5 in order to make the convex intruder visible. s(e) is always a steeply monotonic rising function. We clearly see the global concave (downwards bending) nature of s(e) and its convex intruder. Its depth is the entropy loss due to the additional correlations by the interfaces. From this one can calculate the surface tension aSUTj /Ttr = Assurf * N/Nsurf. The double tangent is the concave hull of s(e). Its derivative gives the Maxwell line in the caloric curve T{e) at Ttr

obtain the familiar result f(T,/j,,V) = es — /m s — Tss. I.e. the curvature Ai of the entropy surface s(e, n, V) decides whether the grand-canonical ensemble agrees with the fundamental micro ensemble in the thermodynamic limit. If this is the case, Z(T, /z, V) is analytical and due to Yang and Lee we have a single, stable phase. Or otherwise, the Yang-Lee singularities reflect anomalous points/regions of \\ > 0 (det(e,n) < 0). This is crucial. As det(e s ,n s ) can be studied for finite or even small systems as well, this is the only proper extension of phase transitions to "Small" systems. 3

The physical origin of the wrong (positive) curvature Ai of s(es, ns)

Before we proceed with the general micro-canonical classification of the various types of phase transitions we will now discuss the physical origin of convex (upwards bending) intruders in the entropy surface. As nuclear matter is not accessible to us we should investigate atomic clusters and compare their thermodynamic behavior with that of the known bulk. In the figure (1) we compare the "liquid-gas" phase transition in sodium clusters of a few hundred atoms with that of the bulk at 1 atm..

57

^0

Ttr [K] qiat [eV] Na

Sboil

&ssurf ly

e.ff V/Ttr

200 940 0.82 10.1 0.55 39.94 2.75

1000 990 0.91 10.7 0.56 98.53 5.68

3000 1095 0.94 9.9 0.45 186.6 7.07

bulk 1156 0.923 9.267 CO

7.41

Table 1: Parameters of the liquid-gas transition of small sodium clusters (MMMCcalculation) in comparison with the bulk.

In the table (1) we show in comparison with the known bulk values the four important parameters, transition temperature Ttr, latent heat per atom qiat, the entropy gain for the evaporation of one atom Sb0u as proposed by the empirical Trouton's rule (~ 10) and A s s u r / , the surface entropy per atom as defined above. Nei, is the average number of surface atoms of the clusters. cr/Ttr is the surface tension over the transition temperature.

4

Phase transitions in the micro-ensemble: The topology of the determinant of curvatures of s ( e , n ) :

Now we can give a systematic and generic classification of phase transitions which applies also to "Small" systems and their relation to the Yang-Lee singularities: • A single stable phase by d(e, n) > 0 (Ai < 0). Here s(e,n) is concave (downwards bended) in both gdirections. Then there is a one to c one mapping of the grand-canonical «-»the micro-ensemble. In the two examples on the right the order parameter, the direction vi of the eigenvector of largest curvature Ai was simply assumed to be the energy.

enerffv

58

• A transition of first order with phase separation and surface tension is indicated by d(e,n) < 0 (Ai > 0). s(e,n) has a convex intruder (upwards bended) in the direction vi of the largest curvature. The whole convex area of {e,n} is mapped into a single point in the canonical ensemble.. I.e. if the curvature of S(E, N) is Xi > 0 both ensembles are not equivalent. • A continuous ("second order") transition with vanishing surface tension, where two neighboring phases become indistinguishable. This is at points where the two stationary points move into one another. I.e. where d(e, n) = 0 and v^=o • Ve! = 0. These are the catastrophes of the Laplace transform E -> T. Here v\=0 is the eigenvector of d belonging to the largest curvature eigenvalue A = 0 (—> order parameter). • Finally, a multi-critical point where more than two phases become indistinguishable is at the branching of several lines in the {e, n}-phasediagram with d = 0, V d = 0. An example showing all these possible types of phase transitions in a small system is discussed in 3 ' 4 , 5 5

On the statistical formation of multi-star systems under rotation

Having discussed the micro-canonical description of phase transitions in "Small" systems we sketch here the relevance of our new formulation of thermo-statistics for astrophysical systems. A finite self-gravitating system is controlled by its accessible phase space and its topology: • Due to the long-range gravity the system is non-extensive. • Because there is no heat-, no angular momentum bath a microcanonical treatment under the observation of the conservation laws is neccessary. Attractive gravity leads to a collaps (phase transition of first order). • Under rotation the collapsed system may multi-fragment and multi-star systems will be formed.

59

• Also the fragmentation of Shoemaker-Levy 9 may be viewed as statistical multifragmentation under the linear stress of Jupiter's gravitation field. With the analogy to the long-range Coulomb force one may study many aspects by the study of collision of heavy nuclei. E.g.: • At higher rotation the system prefers larger moment of inertia. This leads to more symmetric heaviest fragments. For the astro-problem higher angular momentum leads to double or multiple stars instead of a mono star. • For heavy-ion collisions, rotation leads to higher radial energy of fragments, larger variance in the statistical fragmentation which must be distinguished from from radial flow ! 6

Conclusion

Instead of using the boiling of water we used the boiling of sodium to demonstrate the physics of first order phase transitions and especially the surface tension. "Small" systems show clearly how to determine all important characteristica of first order transitions: transition temperature, latent heat, and surface tension. This is possible just because these systems are not extensive and their entropy s(e,n) has convex intruders from which the surface entropy can be determined. In the thermodynamic limit this fact becomes more obscured than illuminated. We also mentioned collapsing of self-gravitating and rotating hydrogen clouds towards multi-star systems. As far as the structure of the phase space is concerned there is a striking analogy to the phase space of fragmenting hot nuclei under rotation. The experimental study of the latter is also interesting in view of this analogy. One of the main differences is of course that the latter are experimentally accessible. Another important question is whether there is a kind of statistical equilibrium established in the astrophysical systems and if what the equilibration mechanism might be. Here our learning process has just started. 1. Elliott H. Lieb and J. Yngvason. The physics and mathematics of the second law of thermodynamics. Physics Report,cond-mat/9708200, 310:196, 1999. 2. C. Tsallis. Possible generalization of Boltzmann-Gibbs statistics. J.Stat.Phys, 52:479, 1988. 3. D.H.E. Gross. Microcanonical thermodynamics: Phase transitions in "Small" systems. To be published in Lecture Notes in Physics. World Scientific, Singapore, 2000.

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4. D.H.E. Gross and E. Votyakov. Phase transitions in "small" systems. Eur.Phys.J.B, 15:115-126, (2000), http://arXiv.org/abs/condmat/?9911257. 5. D.H.E. Gross. Micro-canonical statistical mechanics of some nonextensive systems. Invited talk to the International Workshop on Classical and Quantum Complexity and Nonextensive Thermodynamics (Denton-Texas, April 3-6, 2000) http://arXiv.org/abs/astro-ph/9condmat/0004268. 6. D.H.E. Gross. Phase transitions in "Small" systems - a challenge for thermodynamics. Invited talk to CRIS 2000, 3rd Catania Relativistic Ion Studies, Phase Transitions in Strong Interactions: Status and Perspectives, Acicastello, Italy, May 22-26, 2000 http://arXiv.org/abs/condmat/?0006087.

Section I Quark and Gluon-Plasma Phase Transition and Relativistic Heavy-Ion Reactions

RESULTS FROM P b - P b COLLISIONS AT C E R N SPS

L. R I C C A T I a n d E. S C O M P A R I N INFN

- Sezione

di Torino,

Via P. Giuria 1, 1-10125

Torino

(Italy)

The CERN heavy ion program, started in 1986, has accumulated an impressive amount of experimental results on the study of high energy density nuclear matter. One experiment (NA50) has reported evidence for deconfinement of quarks and gluons from the study of the J/ip suppression pattern measured in Pb-Pb collisions. T h e enhancement of multistrange hyperon production observed by WA97 proves that strangeness is equilibrated on a very short timescale, a behaviour expected in case of transition to a deconfined state. Finally, the study of hadron yields has allowed to establish the velocity of the fireball expansion, and to calculate the freeze-out temperature of the created system. All the experiments agree in concluding that at least in central Pb-Pb interactions a state with many of the characteristics foreseen for a Quark-Gluon Plasma(QGP) has been created.

1

INTRODUCTION

The aim of heavy ion experiments in the CERN SPS energy range (A/S ~ 20 GeV) is the study of the phase transition from ordinary nuclear matter towards a state where quarks and gluons behave as free particles, and possibly reach thermal kinematic distributions (QGP). Lattice QCD calculations estimate the occurrence of this transition at T~170 MeV 1 . As will be discussed later, such a temperature is actually reached in the collisions studied at the SPS. Therefore this is the ideal environment for the study of this transition, which can be experimentally detected looking at specific signatures that at the end of the system evolution still keep memory of the early stage of the reaction. The study of ultrarelativistic ion collisions at CERN started in 1986; at that time 1 6 0 ions were first accelerated to 60 GeV/nucleon, and shortly afterwards 32 S ions at 200 GeV/nucleon became available. With the delivery of Pb ions in 1994 the program entered its crucial phase. In this short review, I will try to focus on the aspects of the experimental results which have proven to be more relevant. In particular, after short considerations on the description of the event geometry, I will discuss the state of our knowledge on the J/ip suppression and on the strangeness enhancement, proposed long time ago as signatures of deconfinement, and on the evolution of the produced hadronic final state. 63

64

2

THE SPS EXPERIMENTAL PROGRAM

At present, still four experiments are taking data at the SPS Pb beam. One of them, NA49, is a large acceptance spectrometer for the study of hadron production, which can access a large variety of signals, from strangeness to HBT interferometry. The other experiments focus on specific signals relevant to the search for QGP: NA50 studies dimuon physics, with emphasis on charmonium suppression, NA57 explores the strange hyperon production, and NA45 (CERES) measures the dielectron yield in the invariant mass region below 1 GeV/c 2 . In this short review many important topics will be unfortunately omitted. 2.1

Event geometry

There is a rather general consensus on the determination of the centrality variables (impact parameter b, number of participant nucleons Npart) as a function of the related measured quantities, namely the charged hadron multiplicity Nch, the transverse energy ET around midrapidity, and the zero degree energy EZDC- Assuming that Nch and ET are directly proportional to Npart or, similarly, that EZDC scales linearly with the number of spectator nucleons (wounded-nucleon model, WNM) it is possible to describe with great accuracy the measured distributions in the frame of the Glauber model of the nucleusnucleus collision. A typical example can be seen in Fig. 1.

55 •a

10

4

-5

10

0

200

400

600

800

1000 1200

Figure 1: Fit of the NA57 charged hadron multiplicity distribution with the wounded-nucleon model.

In this way it is possible to calculate the relationship between the measured quantities and Npart, and therefore to compare the results of various experiments as a function of a common centrality variable. The study of the

65

ET distributions is also important since it allows, in the frame of the Bjorken model, to estimate the energy density e reached in the collision. The estimates give e ~ 3 GeV/fm 3 for central Pb-Pb interactions, a value significantly higher than the ones predicted for the occurrence of the phase transition towards a QGP (of the order of 1 GeV/fm 3 ). 2.2

Charmonium suppression

Up to now, the most evident indication that SPS experiments have reached the threshold energy density needed for deconfinement comes from the measured charmonia suppression pattern. In case of phase transition to QGP, color screening prevents charmonium binding. Furthermore, one expects a hierarchy of suppression, due to the different binding energies of the cc states; the loosely bound ip' and Xc should start to be suppressed at a lower temperature with respect to the strongly bound J/ip state. This signature has been studied in detail for Pb-Pb collisions by the NA50 experiment. Their set-up consists essentially of a dimuon spectrometer complemented by various centrality detectors. The results are summarized in Fig. 2 and 3. The quantity plotted on the vertical axis is the ratio between the measured J/tp and Drell-Yan cross section. Since Drell-Yan is a hard process whose cross section scales with the number of nucleon-nucleon collisions, the ratio (JJ/^/ODY represents the J/ip cross section per nucleon-nucleon collision. Being the ratio of two measured yields, this quantity is free from most systematical errors, such as detector inefficiencies and flux uncertainties. In Fig. 2 (TJ/^/CTDY for p-A, S-U and Pb-Pb collisions is shown as a function of the so-called L variable, the mean length of nuclear matter seen by the cc pair in its way through the colliding nuclei. While p-A, S-U, and peripheral (L 8 fm) Pb-Pb collisions follow an exponential scaling law, which has been shown to be compatible with the nuclear absorption of a pre-resonant cc state, the central Pb-Pb points suddenly deviate from this behaviour, indicating the onset of an anomalous J/ip suppression. In Fig. 3 the Pb points alone are shown as a function of the measured transverse energy ET- Using an upgraded analysis method, which allows to calculate the Drell-Yan ET distribution starting from the huge sample of minimum bias events (the so-called "minimum bias analysis"), it is possible to observe a two-step suppression pattern. In a deconfinement scenario this behaviour can be explained assuming that the Xc state (not directly identified by NA50, which only sees its decay J/ip from the Xc —> J/ip + 7 process) melts for Pb-Pb collisions producing ET >40 GeV and that the J/ip state melts when ET >90 GeV. On the contrary, the conventional hadronic models actually available cannot reproduce this pattern (see 2

66

and references therein). Finally, the study of the V' suppression pattern has revealed that this resonance is already suppressed in peripheral S-U collisions. However it is so loosely bound that it can be easily broken by soft interaction with comoving hadrons and therefore its study is not directly relevant for the investigation of deconfinement. •f 3

O

90 80

•

?™ £!

60

T3

50

f« ca

p(450"GeV/c)-p,d(NA51)

* {-.{SW-'CeVAiS-A',, ^W,U, (NA*«>

•

tt

S{32x200"GeV/c)-U(NA38)

* e inn2(fSxl5UH.VfcM'L- '.NA50I

t

: ^k t

20

0^ = 5.8 ± 0.6 mb

% _

4 ,

2.5

5

7.5

10

Lffm) Figure 2: The ratio J/tp/DY as a function of L, for p-A, S-U and Pb-Pb collisions.

0

20

40

60

80

100

120

140

ET(GeV)

Figure 3: The ratio J/tp/DY as a function of ET, for Pb-Pb collisions. Circles represent the results obtained with measured Drell-Yan events, squares and triangles the results of the "minimum bias" analysis.

As a general remark to NA50 results, it can be observed that the crucial feature of this experiment is the capability of studying physics observables {J/4> production in this case) in small steps of centrality. Thanks to this fact it has been possible to observe the two-step pattern of Fig. 3, which is in a sense a model-independent indication of the occurrence of the phase transition. However, the peripheral Pb-Pb collisions should be explored in more detail in order to give a really convincing evidence for the occurrence of the drop at ET ~ 40 GeV. The set-up for the year 2000 run will in fact be optimized in view of such measurement. Finally, the observation of the onset of the anomalous J/V> suppression in collisions induced by intermediate mass projectiles (Ag, In) would also be of great interest. 2.3

Strangeness

enhancement

Strangeness enhancement has been proposed long ago as a signature of the transition to QGP. Gluon abundances in the plasma phase, (partial) restora-

67

tion of chiral symmetry and Pauli blocking of u and d quark states rise the strangeness content of the measured final state. The most spectacular manifestation of this signature at the SPS has been seen in the study of the production of multistrange hyperons close to midrapidity. Very crudely, if what happens at SPS is the statistical hadronization of a strangeness-enhanced deconfined phase, with global enhancement factor Ea, one expects hadrons containing N strange quarks to be produced with a rate E% times higher than in an environment with no strangeness enhancement, such as p-A collisions. This implies EQ > Ez > E\, where Ei is the enhancement of the particle i with respect to p-A interactions. On the contrary, models based on hadron rescattering give an opposite kind of behaviour, due to high production thresholds (~3 GeV for the process TCTT —> ClCl) and low cross sections. Hyperon production has been experimentally studied in Pb-Pb collisions by WA97. The detection of multistrange particles in nucleus-nucleus interactions poses non-negligible experimental problems, since their decay products have to be identified in a very high multiplicity hadron environment. In Fig.4 the measured multiplicity of strange particles per participant is shown, using p-Be results as a reference. The hyerarchy of enhancements foreseen in case of QGP formation is clearly observed, with an D, enhancement factor of about 17 3 .

1

ff+n*

'.

_+_-*-*•• H"

-*--*-•++

-++*" ;

A

- K h f T I"'

1

1 pBe 1

pPb 10

PbPb 102

pBe 10?

-+-++t A

t

:

pPb 10

PbPb 10 2

10°

Figure 4: Negative hadrons and hyperon yields per participant, normalized to p-Be results, measured by WA97.

The other remarkable result is that the for Npart >100 a saturation of the enhancement is observed. It would of course be very interesting to determine the onset point of such enhancement, to see if it occurs in a limited Npart range, and, if this would be the case, if its position on the Npart axis agrees with the onset of the anomalous J/ip suppression observed by NA50. Fortunately NA57

68

covers the region of peripheral Pb-Pb interactions down to Npart ~50, and its first results are expected to appear very soon. The other possible approach to the study of strangeness is the measurement in a large kinematical region. In this way a "global" strangeness enhancement factor can be extracted. The NA49 experiment, using its large acceptance spectrometer, has studied in detail if-meson production (kaons carry about 75% of the global strangeness content of the final state). A global enhancement factor of about 2 has been found for central Pb-Pb collisions relatively to p-A (see Fig. 5) 4 . An enhancement of the same size is also visible in central S-S and S-Ag collisions. The same study has also been performed as a function of the collision centrality. As a function of Npart (Fig. 6), it is possible to see that in Pb-Pb collisions the kaon production smoothly increases from peripheral to central reactions, with a possible flattening only for Npart >300. P b + P b , NA4-9 P r e l i m i n a r y

NA4(i preliminary

A K

0.16

S+S

V_ 0 . 2

0.14

* •

s+s

A v: V

Pb+Pb

*

'+=0.15

S °-1 s. „ 0.08

•

1

S+AQ

0.1

•a 0.12 0.06

K

1 + +

50

-&-

,&.

100 150 200 250 300 350 400 Number of participants

Figure 6: Phase space integrated K/ir (and p/tr) multiplicities as a function of Npart, for p-p, S-S and Pb-Pb collisions.

It must also be noted that the S-S point is not compatible with the Pb-Pb point corresponding to the same Npart. The results on kaons pose a certain number of questions from the point of view of the comparison with the results of the other experiments. In fact, if kaon and hyperon production can be explained in the frame of a common statistical hadronization mechanism, one would expect in Pb-Pb the same kind of centrality dependence for the yields. Experimentally it is found that the mulitplicity per participant is flat for multistrange hyperons and smoothly increasing for kaons. Finally, it must also be stressed that the " pattern" of global strangeness enhancement is rather different from the one seen by NA38/NA50 for the charmonium suppression; the last one, in fact, does not exhibit any "anomaly" for S-induced collisions,

69

while global strangeness is already enhanced there. 2.4

The evolution of the final state

Thanks to the combined progress of theory and experiment, our knowledge of the late stages of the collision is today much deeper than a few years ago. The essential parameters needed to characterize the evolution towards the final state have been fixed with good precision. Very briefly, the chemical freeze-out temperature and baryon chemical potential have been determined in the frame of statistical models of the hadronization process. The availability of hadron data in large phase space region, as the ones produced by NA49, has allowed to have accurate fits for the free parameters. Several different version of these models exist, but they agree in indicating T ~170 MeV for the chemical freezeout temperature, and /XB ~270 MeV. Since such a temperature is very close to the one predicted for the phase transition to QGP, it is quite probable that the system crosses the phase boundary shortly before the chemical freeze-out point. Furthermore, the almost complete strangeness saturation indicated by these models, shows that the observed strangeness enhancement comes essentially from the partonic phase. The following step in the history of the collision, namely the expansion of the chemically equilibrated hadronic system, has been investigated using as tools the study of the transverse mass spectra of identified particles and the results from HBT analysis. The study of the transverse mass spectra has been carried out by several experiments (NA49, NA44, WA97). It was soon realized that the inverse slope T of the transverse mass distribution, which should be connected with the temperature of the system at decoupling time, increases steadily with the mass of the considered particle. This can be seen in Fig. 7 5 . This fact can be explained as arising from the relativistic superposition of the thermal motion of the expanding system plus a radial ordered collective flow, caused by the explosion of the fireball. Since we observe this "Little Bang" from the outside, we see a blue-shift of the temperature, conceptually similar to the red-shift we observe for the background radiation from the Big Bang. The departure of the fi from the systematics can be due to an early freeze-out of such particle. In fact the fi, having zero isospin, cannot form resonances with the pions and therefore decouples very soon from the expanding hadron gas. By the way this is a further indication that the enhancement of the O, does not originate in the late stages of the reaction but it is rather due to the pre-hadronic phase. Different ways are in principle available to disentangle thermal motion and radial flow. The most accurate of them combines the information from

70

Figure 7: Dependence of the m y spectra inverse slope T on the particle mass m for Pb-Pb collisions.

Figure 8: Allowed region of freeze-out temperature vs. radial flow velocity for central Pb-Pb collisions.

rriT spectra with the ones from Bose-Einstein pion correlations. Very essentially, by fitting the dependence of the HBT transverse radius on the transverse momentum of the pion pair, it is possible to extract, in a model dependent way, the ratio 0^/T between the radial flow velocity and the freeze-out temperature of the system. The remaining ambiguity, as shown in Fig. 8, can be eliminated using the information from h~ and d transverse spectra. This finally gives T = (120 ± 12) GeV and pT = 0.55 ± 0.12 (WA98 has obtained with a similar analysis slightly lower T values). This means that the fireball explodes at half the speed of light, justifying therefore the term "Little Bang" used to describe the expansion of the hadronic phase. 3

CONCLUSIONS

In summary at the CERN SPS in the lead-lead collisions a new state of matter has been created and a "little bang" reproduced in laboratory. Looking at the results on charmonium suppression and multistrange hyperons production this state cannot be explained by conventional models of hadronic interactions. 1. 2. 3. 4. 5.

F. Karsch, hep-lat/9909006. M.C. Abreu et al. (NA50 collaboration), Phys. Lett. B477 (2000) 28. E. Andersen et al. (WA97 collaboration), Phys. Lett. B449 (1999) 401. S. Margetis et al. (NA49 collaboration), J. Phys. G25 (1999) 189. F. Antinori et al. (WA97 collaboration), Eur. Phys. Journ. C14 (2000) 633.

T H E SEARCH FOR T H E QGP: A CRITICAL A P P R A I S A L HELMUT SATZ University of Bielefeld, Faculty for Physics, P.O.Box 10 01 31, D-33501 Bielefeld, Germany E-mail: [email protected]

1

Abstract

Over the past 15 years, an extensive program of high energy nuclear collisions at BNL and CERN has been devoted to the experimental search for the quarkgluon plasma predicted by QCD. The start of RHIC this year will increase the highest available collision energy by a factor 10. This seems a good time for a critical assessment: what have we learned so far and what can we hope to learn in the coming years?

Contents: 1. Expecting the Unexpected 2. Charmonium Suppression 3. In-Medium Hadron Modifications 4. Strangeness and Thermalization 5. Catching the Elusive

An extended version of this was presented in a plenary talk at Lattice 2000: XVIII International Symposium on Lattice Field Theory, August 17-22, 2000, Bangalore/India, proceedings to be published in Nucl. Phys. B. It is also accessible on the Server under hep-ph/0009099. 71

STRANGE BARYON SIGNALS OF A NEW STATE OF MATTER IN LEAD-LEAD COLLISIONS AT THE CERN SPS Presented by: FEDERICO ANTINORI INFN, Sezione di Padova, via Marzolo 8, 1-35131 Padova, Italy E-mail: federico. antinori @padova. infn. it on behalf of the WA97 Collaboration:

F.ANTINORIj, W.BEUSCH f, I.J.BLOODWORTH d, R.CALJANDROa, N.CARRERf, D.DI BARI a , S.DI LJBERTO', D.ELIAa, D.EVANS d, K.FANEBUST b, F.FAYAZZADEH \ R.A.FINIa, J.FTA* NIK g, B.GHIDINIa, G.GRELLA m, M.GUIJNO e , H.HELSTRUPc, M.HENRIQUEZ \ A.K.HOLME \ D.HUSS h, A. JACHOLKOWSKIa, G.T.JONES d, J.B.KINSON d, K.KNUDSON f, I.KRALIK g, V.LENTIa, R.LIETAVAf, R.A.LOCONSOLE a , G.L0VH0IDEN\ V.MANZARI a, M.A.MAZZONI', F.MEDDIf4, A.MICHALON n, M.E.MICHALON-MENTZER n, M.MORANDO', P.I.NORMAN d, B.PASTIR' AK 8 , E.QUERCIGH fj, D.ROHRICH b, G.ROMANO m, K.SAFA* IK f , L.§ANDOR fg , G.SEGATOj, P.STAROBAk, M.THOMPSON d, J.URBAN g, T.VK 4 , O.VILLALOBOS BAILLIE d, T.VIRGILIm, M.F.VOTRUBA d and P.ZAVADA k. Dipartimento La. di Fisica dell'Universita e del Politecnico di Bari and Sezione INFN, Ban, Italy Fysisk Institutt, Universitetet i Bergen, Bergen, Norway H0gskolen i Bergen, Bergen, Norway School of Physics and Astronomy, University of Birmingham, Birmingham U.K. Unversitil di Catania and INFN, Catania, Italy CERN, European Laboratory for Particle Physics, Geneva, Switzerland g Institute of Experimental Physics, Slovak Academy of Sciences, KoSice, Slovakia GRPHE, Universite de Haute Alsace, Mulhouse, France Fysisk Institutt, Universitetet i Oslo, Oslo, Norway J Dipartimento di Fisica "G.Galilei" dell'Universita and Sezione INFN, Padua, Italy Institute of Physics, Academy of Sciences of the Czech Republic, Prague, Czech Republic Dipartimento di Fisica dell'Universita "La Sapienza" and Sezione INFN, Rome, Italy Dipartimento di Scienze Fisiche "E.R.Caianiello " dell 'Universita and INFN, Salerno, Italy Institut de Recherches Subatomiques, IN2P3/ULP, Strasbourg, France The evidence for the formation of a deconfined state of matter in Pb-Pb collisions at the CERN SPS coming from the study of the production of strange and multi-strange particles in p-Be, p-Pb and Pb-Pb collisions in Experiment WA97 is reviewed and discussed.

73

74

1

Introduction

It was suggested almost 20 years ago [1,2] that strange particles would be a sensitive probe in the search for the colour deconfinement phase transition in ultrarelativistic nucleus-nucleus collisions. Deconfinement is expected to be accompanied by a partial restoration of the chiral symmetry: as partons are liberated from the confining effects of strong interaction, the masses of the quarks would decrease from the effective value they take on inside hadrons to their current values. While the effective mass of a strange quark inside a hadron is of the order of 500 MeV, the current value of the strange quark mass, about 150 MeV, is of the same order of the critical deconfinement temperature. Therefore, one expects, in the deconfined phase, to have a rapid and copious production of strange quarkantiquark pairs, mostly by gluon-gluon fusion. If such an enhancement of strangeness production survives hadronization, it will result in enhanced yields of strange particles, and especially of particles containing multiple strange quarks: the increase in the relative abundance of the various species of hyperons should be larger and larger as one goes from lsl=l (A) to lsl=2 (2) to lsl=3 (Q). Some enhancement in the abundance of strange particles when going from protonnucleus to nucleus-nucleus collisions could be present, due to inelastic rescattering among the reaction products, even if the reaction proceeds through purely hadronic stages, with no deconfinement. However, if the production of A and K via inelastic rescattering among the hundreds of hadrons produced in the nucleus-nucleus collision, may be relatively easy (throughror—»KK,TIN-»KA) , it is more and more difficult (i.e. slow) to produce in this way baryons with higher and higher strangeness content, as the thresholds and the number of required multiple interactions increase. This crucial difference between the hadronic and deconfined scenarios makes the study of the production of particles with lsl>l of particular interest in this context. 2

The WA97 Experiment

Experiment WA97 at the CERN-SPS has been designed to study the production of lsl=l,2,3 particles at central rapidity in Pb-Pb collisions and, as a reference, in p-A collisions at a beam momentum of 158 GeV/c per nucleon. The layout of the experiment is sketched in figure 1 and is described in [3,4,5]. In order to cope with the large density of tracks emerging from the nucleus-nucleus collisions, tracking is performed in a high granularity Pixel Tracking Chamber (PTC). The PTC, with a cross section of 5x5cm2 and a total length of 90cm, is placed above the beam line and is inclined with its lower edge pointing back to the

75

target. Strange particles are detected by reconstructing in the PTC the tracks emerging from their weak decays. At the chosen inclination setting (40mrad), the telescope is sensitive to K°s, A, 5" and UT produced over about one unit of rapidity around mid-rapidity, with a transverse momentum larger than a few hundreds MeV. The target, the multiplicity detectors (see below) and the PTC are placed inside the 1.8T magnetic field of the OMEGA magnet, which is followed by a system of pad-cathode multiwire proportional chambers used as lever-arm detectors pad chambers

, beam PTC silicon 6 '• * telescope; somip^/ 0.5111 channels 5A

scintillatorA _ 7 , /t petals / V j Pb t a r g e t > £ V

mufti

ta

P|icity

det6Ct

°

rS

Figure 1. Sketch of the layout of the WA97 experiment «* 600

1.3 +

1.35

Ajt"+Ajr mass, GeV

1.65

1.7

1.75

AK*+AK* mass, GeV

Figure 2. 5" and QT signalsfromthe WA97 Pb-Pb sample

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Scintillator petals placed behind the target provide an interaction trigger selecting approximately the most central 40% of the inelastic Pb-Pb cross section. The petals are followed by two stations of MicroStrip Detectors (MSD) which sample the charged multiplicity at central rapidity providing information on the event centrality (i.e. number of wounded nucleons) which is used for the off-line study of the centrality dependence of the particle yields. The Pb-Pb data sample is divided into four MSD multiplicity classes and the average number of wounded nucleons for each class is determined from a Wounded Nucleon Model [6] fit to the charged multiplicity distribution da/dNch. This analysis is discussed in detail in [7,8]. In the proton beam reference runs, Be and Pb targets were used. In these runs, a trigger was applied demanding at least two tracks in the telescope, as required to find V°s (two-track trigger). Data were also collected requiring at least one track in the telescope (one-track trigger). These data were used for the study of the production of negative particles. The numbers of wounded nucleons corresponding to p-Be and p-Pb collisions have been determined as an average value for inelastic collisions in the framework of the Glauber model [9]. The details of the analysis, i.e. the reconstruction procedure, the extraction of the signals and the event weighting procedures are described in [4,10,11]. As an example of the quality of the data, the 5 " and Q invariant mass spectra are shown in figure 2. 3

Hyperons' transverse mass spectra

The analysis of the transverse mass distributions is described in detail in [11]. The ntj- = -y/m2 + pT the form: dN dm?

, oc nij- exp(

distributions of negatives and strange particles are fitted to

m,. ) T

The results of the fits for hyperons are shown in figure 3. The values of the fitted transverse mass slope parameter T are plotted in figure 4 as a function of the rest mass of the particles, along with results from other CERN heavy-ion experiments (see e.g. [11] and references therein). The presence of strong radial flow in Pb-Pb collisions at the SPS was deduced from the systematics of experimental data on non-strange and singly-strange hadron, which suggests a linear increase of the

77

inverse slope with the particle mass [12,13], as indicated by the line in figure 4. From figures 3 and 4 we note that the slopes for all singly- and multiply-strange baryons transverse mass spectra are instead remarkably similar. Such a behaviour can be expected both in a collective expansion scenario [14] as due to an early freeze-out of fi and E [15,16] or in a scenario where a fireball of deconfined partons suddenly hadronizes [17,18]. In both cases, the £2 and 5 abundances would be frozen in at the point of hadron formation.

mT (GeV/c2) Figure 3. Transverse mass distributions for hyperons in Pb-Pb collisions.

78

0.5

.

0

O

o NA49 • NA44 • WA97

0.4

0.3

/ \ *

-

f a'+a*

ir+WK0 0.2

K

'• fX

J% 0.1

0.5

2 m(GeV)

1.5

Figure 4. Dependence of the transverse mass inverse slope on the particle rest mass (see text).

W3

-

2

i

2

.810 i 10 i

»-»•

_- "

t

1 i i -2

—°-

™

-1:

10

-= :

s

• » 0 •

10 -1

a

»f

•4

10 10

-

pBe pPb '~1,HIIII|

PbPb

pBe pPb

10

3

101

10

:

PbPb 1

I

2

10

1

; 1 ; 1 ; 1

O

10 i -3:

—t

^ \ -*- n+nS

" " " I

10

' '

'1

10

(N rari > Figure S. Yields per unit rapidity at central rapidity as a function of the number of wounded nucleons (Nput). See text for details.

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4

Particle yields

The yields per unit of rapidity at central rapidity for negative particles and for A, 5", QT and their antiparticles are plotted in figure 5 as a function of the average number of wounded nucleons for p-Be and p-Pb collisions and for the four centrality classes in Pb-Pb collisions. All yields are extrapolated to a common phase space window covering full pT and one unit of rapidity centered at midrapidity. The horizontal error bars of the Pb-Pb points represent the FWHM of the Npart distributions in the four multiplicity classes [7]. In figure 5, as well as in the following figures, particles are divided in two classes: those with no valence quark in common with the nucleon are plotted on the right, the others are shown on the left. The two groups have been kept separate since it is known that they may exhibit different production features (for instance, A and A have different rapidity spectra both in p-S and S-S collisions [19]). Figure 6 shows the yields relative to those measured in p-Be collisions. The straight line indicates the expectation in case the yields per participant (wounded) nucleon did not change from p-Be collisions all the way to central Pb-Pb collisions. As can be seen, while the p-Pb points are compatible with such an expectation, there is a clear excess for all particle species in Pb-Pb collisions. The effect is more and more pronounced as the strangeness content of the particles increases. Figure 7 displays the yields per participant relative to the p-Be ones. The constant-yieldper-participant expectation now lays parallel to the horizontal axis. The yields per participant appear to be rather constant within the centrality range covered by WA97. The enhancement factors can be read off the vertical axis. The largest effect is observed for the triply-strange ft", which are enhanced by a factor between 15 and 20. This enhancement a pattern, as discussed above, is naturally expected in a deconfined scenario, while it is not explained by any known hadronic microscopic model [5,20,21].

80 0>

jj-n+n + '

+" + - t i

rfE" "*>A

•8

-t- -«»A

•aio 2 VI

'C

"3 P

'

U > :

-I :

A

1 -

~s

PbPb

pBepPb

li.l.n

pBe pPb

I l .mill

2

10

10

J 1

i

i

|

i

' 3

10

1

10

1

PbPb 1 J " " " ' 1 '_'•' 2 3

10

10

Figure 6. Yields per unit rapidity (same as in figure 5) relative to the p-Be yields.

3 0.

* £1+n*

10 -

1

• + * *

-H#^ ;

-+-"%A

I

-H+rTh"

J 1

.«t

f pBe pPb

PbPb

10

1

:

pBe pPb • ' -

10

-f-f-^x ' 1

10f

1

'

PbPb 1

10

'

2

10

3

10 is doubled to (tjj, ipc) with ipc = CT/>T. 6 For large chemical potential fi, the antiparticles decouple: the particle/hole energies are eq « =F(