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Nuclear modification factor of excited Υ states in PbPb collisions at √sNN = 5.02 TeV

초록/요약

Bottomonium, a bound state of a bottom quark and its antiquark, serves as an important probe for studying the quark-gluon plasma (QGP) formed in heavy-ion collisions. Previous measurements have demonstrated suppres- sion of bottomonium states in such environments, but have been statistically limited to the ground and first excited S-wave states, Υ(1S) and Υ(2S). This thesis presents a measurement of the nuclear modification factor (RAA) for excited Υ mesons, with particular focus on the observation of Υ(3S) state. The analysis uses data collected by the CMS detector during the 2018 LHC PbPb run at √sNN = 5.02 TeV, along with the corresponding pp reference data recorded in 2017 at the same center-of-mass energy. The integrated luminosities of the PbPb and pp datasets are 1.61 nb−1 and 300 pb−1, re- spectively. The measured RAA values integrated over transverse momentum and cen- trality are 0.115 ± 0.008 (stat) ± 0.007 (syst) for the Υ(2S) state and 0.080 ± 0.014 (stat) ± 0.012 (syst) for the Υ(3S) state. The RAA is further presented i as a function of transverse momentum and collision centrality. The centrality- dependent measurements confirm the sequential suppression pattern among the Υ states, consistent with expectations from color screening in a deconfined medium. Additionally, double ratios of RAA values are reported to quantify the relative suppression between the excited states. A stronger suppression of the Υ(3S) compared to the Υ(2S) is observed, reinforcing the picture of binding energy–dependent quarkonium dissociation in the quark-gluon plasma.

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목차

1. Introduction 1
1.1 The Standard Model 2
1.2 The Quantum Chromodynamics 3
1.2.1 The strong coupling constant and asymptotic freedom 5
1.2.2 Confinement 6
1.2.3 Hadronization 6
1.2.4 Hadron production in high-energy proton collision 8
1.3 Heavy quark and Quarkonia 10
1.3.1 Production of heavy flavor 14
1.4 The Quark Gluon Plasma 18
1.4.1 The QCD Matter Phase Diagram and Phase Transition 20
1.4.2 Ultra relativistic heavy ion collision 21
1.4.3 Collision centrality 23
1.5 Quarkonia suppression 26
1.5.1 Cold nuclear matter effects 26
1.5.2 Hot QCD Medium Effects 28
1.6 Motivation of the analysis 33
1.6.1 Previous measurements on bottomonium suppression 33
1.6.2 Goal of this analysis 34
2 Experiment Setup 36
2.1 The Large Hadron Collider 37
2.2 The Compact Muon Solenoid (CMS) 39
2.2.1 Overview 39
2.2.2 Coordinates 39
2.2.3 Silicon strip and pixel tracker 41
2.2.4 Electromagnetic Calorimeter 43
2.2.5 Hadron Calorimeter 44
2.2.6 Muon detector system 45
2.2.7 Drift Tube Chambers 46
2.2.8 Cathode Strip Chambers 47
2.2.9 Resistive Plate Chambers 48
2.2.10 Measuring beam pickup timing 48
2.3 Trigger and data acquisition 50
2.3.1 The level-1 trigger 50
2.3.2 The high level trigger 51
2.3.3 Data acquisition system 52
3 Data analysis 55
3.0.1 Analysis procedure 55
3.1 Offline reconstruction 57
3.1.1 CMS Particle Flow 57
3.1.2 Track reconstruction 57
3.1.3 Muon reconstruction 60
4 Measurement of Υ in pp and PbPb 66
4.1 Monte Carlo simulation 66
4.2 Analysis data selection 67
4.2.1 Event selection 67
4.2.2 Acceptance 68
4.2.3 Candidate selection 68
4.3 Signal extraction 75
4.3.1 Data model 75
4.3.2 Fit result 85
4.4 Corrections 99
4.4.1 Reweight of MC for transverse momentum distribution 100
4.4.2 Acceptance 100
4.4.3 Efficiency 104
4.5 Systematic uncertainty 109
4.5.1 Uncertainties from signal extraction 109
4.5.2 Uncertainty from varying BDT score 115
4.5.3 Uncertainty from variation of centrality calibration 115
4.5.4 Uncertainties from acceptance correction 117
4.5.5 Uncertainties from efficiency correction 117
4.5.6 Correction uncertainty in double ratio calculation 118
4.5.7 Global uncertainties 118
4.5.8 Summary of systematic uncertainty 119
5 Results and discussion 120
5.1 The nuclear modification factor RAA 120
5.2 Double ratio of Υ(3S) and Υ(2S) 122
5.3 Theoretical predictions 123
5.4 Summary 135
6 Bibliography 136
A Appendix 156
A.1 BDT parameter plots 156
A.1.1 BDT side band region variable distribution 156
A.1.2 BDT signal region variable distribution 161

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