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A Study of Cascade and Strange Baryon Production in Sulphur-Sulphur Interactions at 200 Gev/C Per Nucleon S

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A Study of Cascade and Strange Baryon Production in Sulphur-Sulphur Interactions at 200 Gev/C Per Nucleon S Fysisk institutt NO9800037 NEI-NO--895 Strange and non-strange baryon and antibaryon production in sulphur-tungsten and sulphur-sulphur interactions at 200 A GeV/c by Alv Kjetil Holme November 1995 Universitetet i Bergen Bergen, Norway Fysisk institutt Strange and non-strange baryon and antibaryon production in sulphur-tungsten and sulphur-sulphur interactions at 200 A GeV/c by Alv Kjetil Holme a thesis submitted to Fysisk institutt, Universitetet i Bergen, in partial fulfilment of the requirements for the degree of Doctor Scientiarum November 1995 Universitetet i Bergen Bergen, Norway And since these things are so, it is necessary to think that in all the objects that are compound there existed many things of all sorts, and germs of all objects, having all sorts of forms and colours and flavours. - Anaxagoras Abstract A new phase of strongly interacting matter, the quark-gluon plasma, is predicted to exist at extreme energy densities. In this plasma, quarks and gluons are no longer confined inside hadrons. A relativistic heavy-ion collision is one likely way to produce the required energy density in a sufficiently large volume. The production of strange baryons and antibaryons is considered to be an important diagnostic tool for understanding the dynamics of matter under these extreme conditions. The WA85 collaboration has measured the production of strange particles (A, E~, Q~, and kaons) and antiparticles at central rapidities and high transverse momentum in central sulphur-tungsten collisions at 200 AGeV/c. The main results are the high E+/A ratio and the first observation of the Q~ and the Q+ in heavy-ion collisions. The WA94 collaboration has extended the strange particle studies of WA85 to sulphur- sulphur collisions. In addition, a Ring Imaging Cerenkov detector has been used to identify charged particles in order to determine the production of protons and antiprotons, and charged kaons and pions. The results on As and Es are very similar to the WA85 results. In particular, the E+/A ratio is still high. The particle identification in the RICH worked well, but the low efficiency of other detectors has prevented the extraction of physical results. The WA97 collaboration measures production of strange particles in lead-lead colli- sions at 160 A GeV/c. It is the first experiment to use silicon pixel detectors. The first lead-beam data were taken in the autumn of 1994. The analysis is not finished, but some preliminary results are available. NEXT PAGE(S) toft BLANK Acknowledgements I should like to thank the administration of Fysisk Institutt, Universitetet i Bergen, for providing an office and computer facilities, and the people of the nuclear physics group for creating a stimulating research environment. First and foremost I would like to thank my supervisor, Professor Gunnar L0vh0iden, and my group leader, Professor Tor Fredrik Thorsteinsen, for their support and advice, for their patience, and for all the help they provided when I was fed up with writing on my thesis. I would also like to thank Drs. Erling Andersen and Havard Helstrup for interesting discussions about physics and other subjects and for several nice walks in the Juras. I am grateful to Erling for help on computer problems and for reading a draft of my thesis and suggesting valuable improvements. I have highly appreciated the help from Havard in communicating with French speaking people and I have ignored his frequent hints about the nominal time needed to write a thesis. I am grateful to Dr. Larisa Bravina for several suggestions on theoretical papers I ought to read. It is a pleasure to thank the students in the nuclear physics groups in Bergen and Oslo, for enjoyable Student Seminars and for lots of fun during the Nordic conferences at Graftavallen and in Ronneby. I have appreciated all the questions from Kristin Fanebust, Tryggve Storas, J0rgen Lien, and Per Sennels, because they gave me lots of excuses to work in "interesting problems" instead of writing my thesis. I have enjoyed working in the WA85, WA94, and WA97 collaborations. I am grateful to the members of the collaborations for the work they have done in order to make these experiments successful. However, I will not mention everybody by name, since there were over 120 authors on the latest WA97 paper. I began my work in the collaborations as a summer student way back in 1989. I would like to thank my summer student supervisor, Dr. Emanuele Quercigh, for his continued support and valuable suggestions and for his ability to invent money. I was the first person from Bergen to work in WA85, and I would like to thank all the people who helped us join the collaboration in a smooth manner. I am grateful to the members of the group from the University of Birmingham, in particular, Professor John Kinson and Drs. Orlando Villalobos Baillie and David Evans, who introduced me to the world of detector hardware and on-line software, and arranged some entertaining barbecue parties. I gratefully acknowledge the help I have received from Drs. Federico Antinori, Domenico Elia, Adam Jacholkowski, Vassili Katchanov, Jean-Claude Lassalle, and Raymond Sene, during data taking and analysis, and when writing this thesis. I would also like to thank Dr. Kai Martens of the WA89 collaboration for teaching me how the RICH worked and how to set it up, and for some nice evenings in Geneva. iii This work was supported by Norges Allmennvitenskapelige Forskningsrad (grant 422.91 /013), Norges Forskningsrad (grant 100129/432), and Universitetet i Bergen ("avslut- ningsstipend" 94/3204). Additional financial support has been received from CERN, NATO, NorFA/Nordplus, Universitetet i Bergen, and Universitetet i Oslo. Finally, I am greatly indebted to my family and especially my parents and brothers for their continual love, support, and encouragement. Bergen, November 1995 Alv Kjetil Holme IV Contents Abstract i Acknowledgements iii Introduction 1 1 The Quark-Gluon Plasma and Relativistic Heavy-Ion Collisions 3 1.1 Quarks and Gluons 3 1.2 The QCD Phase Transition and the Quark-Gluon Plasma 5 1.3 Heavy-Ion Collisions 6 1.3.1 The language of high energy heavy-ion collisions 8 1.4 Signatures 9 1.4.1 Thermqdynamic variables 10 1.4.2 Chiral symmetry restoration 11 1.4.3 Colour response function 12 1.4.4 Electromagnetic signals 13 1.4.5 Exotica 14 1.5 Strangeness 14 1.5.1 Strangeness production 14 1.5.2 Particle ratios in a thermal model 16 1.6 Overview of Accelerators and Experiments 18 2 The WA85 Experiment 21 2.1 Experimental Setup 21 2.1.1 The Omega magnet 23 2.1.2 The beam 23 2.1.3 The beam counters 23 2.1.4 The target 23 2.1.5 The multiplicity microstrips 24 2.1.6 The A chambers 24 2.1.7 The hodoscopes 25 2.1.8 The Cerenkov detector 25 2.1.9 The calorimeter 26 2.1.10 The trigger 26 2.2 Data Analysis 26 2.2.1 Track finding 26 2.2.2 Finding V°s 27 2.2.3 Finding As 28 2.2.4 Finding charged Es 29 2.2.5 Finding fi" . : 31 2.2.6 Summary of raw particle yields 35 2.3 Corrections 36 2.4 Results 38 2.4.1 Transverse mass distributions 38 2.4.2 Particle ratios 39 2.5 Summary 40 The WA94 Experiment 41 3.1 Experimental Setup 41 3.1.1 The beam counters 41 3.1.2 The target 43 3.1.3 The multiplicity counters 43 3.1.4 The multiplicity microstrips 43 3.1.5 The A chambers in the 1991 run 43 3.1.6 The hadron calorimeter 43 3.1.7 The trigger in the 1991 run 45 3.1.8 The silicon telescope in the 1992 run 45 3.1.9 The iron wall in the 1992 run 46 3.1.10 The RICH in the 1992 run 46 3.1.11 The B chambers in the 1992 run 46 3.1.12 The trigger in the 1992 run 46 3.2 Data Analysis 47 3.3 Results 48 3.3.1 Transverse mass distributions 48 3.3.2 Particle ratios 48 3.4 Interpretation of the WA85 and WA94 Results 50 3.5 Summary 52 Particle Identification in the Omega RICH 53 4.1 The Ring Imaging Cerenkov Detector 53 4.1.1 Cerenkov radiation 53 4.1.2 Ring imaging 55 4.1.3 The Omega RICH 56 4.2 Track finding 61 4.2.1 Track finding in the silicon telescope 61 4.2.2 Track finding in the B chambers 61 4.2.3 Track matching 62 4.3 Analysis of the RICH Data 62 4.3.1 The RICH coordinate system 62 4.3.2 Alignment and calibration 63 4.3.3 Background pattern recognition 66 4.3.4 Ring finding and fitting 66 vi 4.3.5 Particle identification 66 4.4 Preliminary Results 72 4.5 Summary 72 5 The WA97 Experiment 75 5.1 Experimental Setup 75 5.2 Data Analysis and Preliminary Results 78 5.3 Summary 81 6 Summary and Outlook 83 A Published Papers 85 B The Butterfly Geometry 105 C Decay Kinematics 107 C.I Transverse momentum distribution of daughter particles from A decay . 107 C.2 The distance between cowboy and sailor crossings in a constant magnetic field 107 C.2.1 V° decay 107 C.2.2 Cascade decay 109 C.3 Podolanski-Armenteros plot 110 C.3.1 V° decay 110 C.3.2 Cascade, decay 112 D The Ring Imaging Geometry 115 D.I The deviation from the ideal impact point 116 References 119 NEXT PAGE(S) toft BLANK List of Figures 1.1 Effective quark-antiquark potential in QCD 4 1.2 Energy density and pressure as function of temperature in lattice QCD.
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