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JET TRANSVERSE MOMENTUM DISTRIBUTIONS from RECONSTRUCTED JETS in P–PB COLLISIONS at √Snn =5.02 Tev

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JET TRANSVERSE MOMENTUM DISTRIBUTIONS from RECONSTRUCTED JETS in P–PB COLLISIONS at √Snn =5.02 Tev JYU DISSERTATIONS 99 DEPARTMENT OF PHYSICS, UNIVERSITY OF JYVÄSKYLÄ RESEARCH REPORT No. ??/2019 JET TRANSVERSE MOMENTUM DISTRIBUTIONS FROM RECONSTRUCTED JETS IN P–PB COLLISIONS AT √sNN =5.02 TeV BY TOMAS SNELLMAN Academic Dissertation for the Degree of Doctor of Philosophy To be presented, by permission of the Faculty of Mathematics and Natural Sciences of the University of Jyväskylä, for public examination in Auditorium FYS 1 of the University of Jyväskylä on June 19th, 2019, at 12 o’clock noon Jyväskylä, Finland June 2019 Editors Kim Dong Jo Department of Physics, University of Jyväskylä Ville Korkiakangas Open Science Centre, University of Jyväskylä Copyright © 2019, by University of Jyväskylä Permanent link to this publication: http://urn.fi/URN:ISBN:978-951-39-7800-6 ISBN 978-951-39-7800-6 (PDF) URN:ISBN:978-951-39-7800-6 ISSN 2489-9003 Abstract Snellman, Tomas Jet transverse momentum distributions from reconstructed jets in p–Pb collisions p at sNN = 5:02 TeV Jyväskylä: University of Jyväskylä, 2019, 140 p. (JYU Dissertations ISSN 2489-9003; 99) ISBN 978-951-39-7800-6 (PDF) In this thesis we study the transverse structure of reconstructed jets via transverse fragmentation momentum, jT, distributions in proton-lead (p–Pb) collisions at the centre-of-mass energy per nucleon of 5.02 TeV. The data is measured with the ALICE experiment at the CERN LHC. In previous analysis that used two-particle correlations, it has been observed that the measured jT distributions can be factorised into two components, a narrow Gaussian component, and a wide non-Gaussian component. It was argued that these components can be linked to the non-perturbative hadronisation and to the perturbative showering process, respectively. We have observed the same factori- sation holds for jT distributions obtained using reconstructed jets. Furthermore although a direct quantitative comparison is not possible, our data is qualitatively compatible with jT distributions measured from two-particle correlations. Studies of collective flow in high multiplicity p–Pb collisions have seen hints of behaviour that in Pb–Pb collisions has been taken as indication of the creation of Quark Gluon Plasma (QGP), a deconfined state of QCD matter. However studies of jet observables have shown no modification in high multiplicity p–Pb collisions. As expected it has been observed in Pb–Pb collisions that jets traversing through QGP medium will lose energy from interactions with the medium. Thus it remains an open question whether QGP is created in a p–Pb collision. In this thesis we compare measured jT distributions between minimum bias and high multiplicity p–Pb collisions. Our results show no sign of modification within the current experimental capabilities. Keywords: jet, jet shape, jet fragmentation, jet reconstruction, heavy ion, p–Pb, transverse momentum, ALICE, CERN, LHC Personal Contribution The main contributions of the author to the research presented in this thesis are listed below p – Jet fragmentation transverse momentum (jT) analysis for sNN = 5:02 TeV p–Pb data – Implementation of the observable (jT) for jet analysis – Implementation and improvement of the perpendicular cone background method – Developing the random background method – Performing unfolding of detector effects on the data – Systematic uncertainty evaluation – Repetition of the analysis with Pythia and HERWIG Monte Carlo generators and comparison of model results and data – Results comparison between high multiplicity and minimum bias p–Pb collisions – Comparison of results to jT extracted from two-particle correlations – Quality assurance (QA) of the Gas Electron Multiplier (GEM) foils that are built into the Time Projection Chamber (TPC) detector of the ALICE experiment. The work was performed at the Helsinki Institute of Physics. – At CERN contributing to the maintenance of the level-0 trigger of the elec- tromagnetic calorimeter at the ALICE experiment Author Tomas Snellman University of Jyväskylä Finland Supervisors Dr. Kim Dong Jo University of Jyväskylä Finland Prof. Jan Rak University of Jyväskylä Finland Reviewers Prof. Stefan Bathe Baruch College, CUNY USA Prof. Marcin Chrząszcz Instytut Fizyki Jądrowej PAN Poland Opponent Prof. Jamie Nagle University of Colorado Boulder USA In memory of my father Acknowledgements The work presented in this thesis has been carried out between 2014 and 2019 at the University of Jyväskylä, at the University of Helsinki and at CERN. I would like to open my thesis by acknowledging the help I have received from several di- rections which has made my graduation possible. First, I thank my supervisors Dong Jo Kim and Jan Rak. Dong Jo for his contin- ued guidance and patience and Jan for giving me the opportunity to work in this field. Additionally from our research group I would like to highlight Sami Räsänen for his help in both practical and physics matters and providing feedback to my thesis. I would also like to thank all present and past members of the ALICE Jyväskylä group for providing a supportive working environment. For helping me get acquainted with the ALICE EMCal trigger maintenance work I am grateful to Jussi Viinikainen and Martin Poghoysan. I would also like to acknowledge the support to finalising my analysis from the Jet Physics Working Group, especially from Leticia Cunqueiro Mendez, Tatsuya Chujo and Marco van Leeuwen. I would like to thank the personnel of the Helsinki Institute of Physics detec- tor laboratory for a pleasant working environment during my time in Helsinki and especially Erik Brücken, Timo Hildén and Essi Kangasaho for guiding me through the GEM quality assurance responsibilities. I am grateful to Stefan Bathe and Marcin Chrząszcz for reviewing my thesis and providing useful comments and Jamie Nagle for agreeing to be my opponent. A special thanks goes to my family for all the support I have received both before and during my academic career. Without my parents none of this would have been possible. Finally I would like to acknowledge the financial support I have received from the University of Jyväskylä and the Helsinki Institute of Physics. Jyväskylä, June 2019 Tomas Snellman Contents Abstract Personal Contribution Acknowledgements Contents 1 Introduction 1 1.1 Quantum chromodynamics . .3 1.1.1 Foundation of QCD . .3 1.1.2 Asymptotic Freedom . .6 1.2 Heavy-ion physics . .9 1.3 Features of heavy-ion collisions . 11 1.3.1 Collision Geometry . 11 1.3.2 Collective motion . 15 1.4 Hard processes . 22 1.4.1 pQCD factorization . 22 1.4.2 Jet showering . 24 1.4.3 Soft gluon radiation and angular ordering . 25 1.4.4 Jet hadronisation . 27 1.4.5 Interactions between jet and medium . 31 1.4.6 New paradigm of jet Quenching . 35 1.5 QGP in Small systems . 40 1.5.1 Collective phenomena . 40 1.5.2 Absence of jet quenching . 41 1.5.3 Centrality determination in small systems . 44 2 Experimental Setup 47 2.1 CERN . 47 2.2 Large Hadron Collider . 49 2.2.1 LHC experiments . 50 2.3 ALICE . 50 2.3.1 Tracking detectors . 52 2.3.2 TPC . 53 2.3.3 Particle identification . 54 2.3.4 Electromagnetic Calorimeters . 55 2.3.5 Forward and trigger detectors . 57 2.3.6 Muon spectrometer . 58 2.3.7 Triggers . 59 2.4 TPC upgrade . 60 2.4.1 ALICE upgrade during LS2 . 60 2.4.2 TPC upgrade . 61 3 Event and track selection 64 3.1 Event selection . 64 3.2 Track reconstruction . 64 3.3 Cluster selection . 68 4 Analysis method 70 4.1 Jet Finding . 70 4.1.1 Anti kT algorithm . 70 4.2 Definition of jT ............................. 72 4.3 Unfolding detector effects . 74 4.3.1 Bayesian unfolding . 75 4.3.2 Pseudo-experiment Monte Carlo . 77 4.3.3 Pythia Response matrices . 78 4.3.4 Unfolding closure test . 79 4.4 Background . 83 4.4.1 Perpendicular cone background . 83 4.4.2 Random background . 85 4.5 Fitting . 86 5 Systematic uncertainties 88 5.1 Background . 88 5.2 Unfolding . 91 5.2.1 Effect of number of iterations . 91 5.2.2 Effect of different prior . 91 5.2.3 Effect of pT truncation . 91 5.3 Tracking . 94 5.4 EMCal clusters . 95 5.5 Summary of systematic uncertainties . 97 6 Results 98 6.1 High multiplicity events . 101 7 Discussions 103 7.1 Comparing dihadron and jet jT results . 103 7.1.1 Different R parameters . 105 7.1.2 Leading tracks versus jet as reference . 106 8 Summary 109 Appendices 111 A Commonly Used Abbreviations 112 B Additional graphs 114 Chapter 1 Introduction This thesis focuses on studying Quantum Chromodynamics (QCD) [1], a part of the standard model of particle physics [2], which is the theory describing the strong interactions. Strong interaction is the force responsible for interactions that holds the nucleus of an atom together. Fundamentally it describes the interactions be- tween quarks and gluons, the elementary constituents of the building blocks of the nucleus, protons and neutrons. Because of specifics of this interaction quarks and gluons, together dubbed partons, can never be seen free [3]. Under ordinary con- ditions they are confined into bound states called hadrons. In extreme conditions they can form a medium of asymptotically free quarks and gluons, guark-gluon plasma (QGP) [4]. Indirectly partons can be observed in high energy particle collisions as jets, collimated showers of particles [5]. The physics of these jets is the primary topic discussed in this thesis. Understanding jets is important when one is interested in the processes that produce the partons that eventually fragment into jets. By themselves jets can provide an insight into QCD when the fragmentation is studied. Jets can also be used as probes of the QGP medium. Experimentally jets can be studied with jet reconstruction algorithms which group observed tracks, in essence undoing the showering process, to find a rea- sonable estimate of the initial parton.
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