DEPARTMENT OF PHYSICS UNIVERSITY OF JYVÄSKYLÄ RESEARCH REPORT No. 7/2017 JET FRAGMENTATION TRANSVERSE MOMENTUM FROM h–h CORRELATIONS IN pp AND p–Pb COLLISIONS BY JUSSI VIINIKAINEN Academic Dissertation for the Degree of Doctor of Philosophy To be presented, by permission of the Faculty of Mathematics and Science of the University of Jyväskylä, for public examination in Auditorium FYS1 of the University of Jyväskylä on December 15, 2017 at 12 o’clock noon Jyväskylä, Finland December 2017 PERSONAL CONTRIBUTION Viinikainen, Jussi Jet fragmentation transverse momentum from h–h correlations in pp and p–Pb collisions Jyväskylä: University of Jyväskylä, 2017, 114 p. Department of Physics Research Report No. 7/2017 ISSN 0075-465X; 7/2017 ISBN 978-951-39-7288-2 (paper version) ISBN 978-951-39-7289-9 (electronic version) Diss. Keywords: Jet, fragmentation, transverse momentum, ALICE, correlations, proton, lead, CERN, LHC The main contributions of the author to the research presented in this thesis are listed below. – Commissioning and maintenance of the level-0 trigger of the electromagnetic calorimeter at the ALICE experiment. (Section 2.3.6) p = – Jet fragmentationp transverse momentum (jT) analysis for s 7 TeV pp data and sNN = 5.02 TeV p–Pb data. – Implementation for the observable jT (Section 4.2). – Implementation for the background of the jT distribution (Section 4.2). – Deriving the acceptance correction for the analysis (Section 4.4). – Validating analysis assumptions using PYTHIA 8 simulation (Section 4.5). – Fitting the measured jT distributions (Section 4.6). – Deriving the final results from the fit parameters (Section 4.7). – Systematic uncertainty analysis (Chapter 5). – Interpretation of the results and comparing data to models (Chapter 6). Author Jussi Viinikainen University of Jyväskylä Helsinki Institute of Physics Finland Supervisor Professor Jan Rak University of Jyväskylä Helsinki Institute of Physics Finland Reviewers Professor Olga Evdokimov University of Illinois at Chicago USA Doctor Nathan Grau Augustana University USA Opponent Professor Gunther Roland Massachusetts Institute of Technology USA PREFACE FIGURE 1 Cartoon drawn by Zach Wienersmith. ACKNOWLEDGEMENTS The work presented in this thesis has been carried out between 2012 and 2017 at the University of Jyväskylä and at CERN. Here, in the beginning of my thesis, I would like to acknowledge the help I have received from several other people that has made my graduation possible. First, I would like to than my supervisor Jan Rak. Under his guidance I have learned how to become an independent researcher, which is a quality that is for sure helpful in the future. From our research group, I would also like to highlight Sami Räsänen and Dong Jo Kim. The support I have received from Sami has been invaluable. We have had numerous discussions on physics and practical issues during the PhD work, and Sami also provided a lot of feedback from my thesis. With Dong Jo we have also had physics discussions and his pieces of advice on how to write analysis code have been helpful in improving the analysis techniques. Regarding the experimental work at CERN, I would like to thank JiˇríKrál for teaching me all I know about the level-0 trigger system of the electromagnetic calorimeter in ALICE. I would also like to thank Martin Poghosyan for his help and advice regarding the practical work with the detector. I would like to thank Olga Evdokimov and Nathan Grau for reviewing my thesis and providing useful comments and Gunther Roland for promising to be my Opponent. For providing me things to do also outside physics, I would like to thank my friends both in Jyväskylä and in CERN. This aspect should not be underrated. A special thanks goes also to my family, from which I can always get uncon- ditional love and support. I would also like to acknowledge the financial support I have received from the University of Jyväskylä, the Magnus Ehrnrooth foundation and the Helsinki Institute of Physics. You cannot do science with an empty stomach. CONTENTS PERSONAL CONTRIBUTION PREFACE ACKNOWLEDGEMENTS CONTENTS 1 INTRODUCTION ............................................................................ 1 1.1 Quantum chromodynamics ....................................................... 2 1.1.1 Asymptotic freedom ...................................................... 3 1.2 Heavy ion physics .................................................................... 5 1.2.1 Nuclear phase diagram .................................................. 5 1.2.2 Developments on quark-gluon plasma signatures............. 6 1.2.3 Cold nuclear matter ....................................................... 10 1.3 Jet fragmentation...................................................................... 11 1.3.1 Parton shower evolution ................................................ 11 1.3.2 Hadronization ............................................................... 15 1.3.3 Experimental techniques ................................................ 15 1.4 Monte Carlo implementations of jet fragmentation ...................... 16 1.4.1 PYTHIA ......................................................................... 16 1.4.2 Herwig ......................................................................... 20 2 EXPERIMENTAL DETAILS............................................................... 23 2.1 CERN...................................................................................... 23 2.2 Large Hadron Collider .............................................................. 25 2.2.1 Heavy ion experiment: ALICE ........................................ 26 2.2.2 Multipurpose experiments: ATLAS and CMS................... 27 2.2.3 Matter-antimatter asymmetry: LHCb .............................. 27 2.2.4 Smaller experiments: LHCf, TOTEM and MoEDAL .......... 27 2.3 ALICE..................................................................................... 27 2.3.1 Tracking detectors.......................................................... 28 2.3.2 Particle identification detectors ....................................... 30 2.3.3 Electromagnetic calorimetry ........................................... 31 2.3.4 Muon spectrometer........................................................ 31 2.3.5 Forward and trigger detectors......................................... 31 2.3.6 L0 trigger in EMCal ....................................................... 32 3 EVENT AND TRACK SELECTION.................................................... 39 3.1 Event selection ......................................................................... 39 3.2 Track selection.......................................................................... 39 4 ANALYSIS METHODS ..................................................................... 45 4.1 Two-particle correlations ........................................................... 45 4.2 Definition of jT distribution and background............................... 46 4.3 Efficiency correction.................................................................. 49 4.4 Acceptance correction ............................................................... 50 4.5 Di-gluon PYTHIA ...................................................................... 60 4.6 Fitting jT distribution ................................................................ 62 q 2 4.7 Extracting jT and per trigger yield ....................................... 66 5 SYSTEMATIC UNCERTAINTIES ....................................................... 69 5.1 Background ............................................................................. 69 5.2 Fitting methods ........................................................................ 70 5.3 Tracking .................................................................................. 72 5.4 Summary................................................................................. 72 6 RESULTS.........................................................................................q 75 2 6.1 Results for jT ..................................................................... 75 6.2 Results for per trigger jT yield.................................................... 78 6.3 Comparison to models .............................................................. 80 7 SUMMARY AND OUTLOOK ........................................................... 82 APPENDIX 1 TOY SIMULATION........................................................... 84 1.1 Simulation details..................................................................... 84 1.2 Results in xk and pTa bins .......................................................... 85 1.3 Breaking the trigger assumptions ............................................... 87 APPENDIX 2 STUDY ON hZi OF TRIGGER PARTICLES.......................... 89 APPENDIX 3 GLOSSARY OF USED SYMBOLS ....................................... 92 REFERENCES.......................................................................................... 93 1 INTRODUCTION The nature is governed by four basic interactions: electromagnetic, strong, weak and gravity. While the gravity is best modeled by the theory of relativity [1], all the other forces are very successfully and accurately described in the Standard Model of particle physics [2]. The Standard Model consists of the electroweak model [3–5] combining the electromagnetic and weak interactions, and quantum chromodynamics (QCD) [6] describing the strong interactions. This thesis con- centrates on studying QCD. The constituents of matter interacting via the