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Production of isolated photons in pp and PbPb collisions at √s_NN = 5.02 TeV with the CMS detector at the LHC

  • 주제(키워드) Nuclear physics , Heavy ion collisions , Isolated photons , CMS
  • 발행기관 고려대학교 대학원
  • 지도교수 홍병식
  • 발행년도 2020
  • 학위수여년월 2020. 8
  • 학위구분 박사
  • 학과 대학원 물리학과
  • 세부전공 원자핵물리학
  • 원문페이지 156 p
  • UCI I804:11009-000000231906
  • DOI 10.23186/korea.000000231906.11009.0001164
  • 본문언어 영어
  • 제출원본 000046045925

초록/요약

Prompt photon production in hadronic collisions is one of the valuable tools for test- ing perturbative quantum chromodynamics and parton distribution functions (PDFs), especially, of the gluon. The prompt photons with high transverse energy (ETγ ) are pro- duced in early phase of collisions via hard parton scattering, and experience the whole evolution of heavy ion collisions. As a colorless particle, high-ETγ photons do not interact with the quark-gluon plasma, a hot and dense QCD matter created in ultra-relativistic heavy ion collisions. Therefore, high-E_T photons can provide the information about the initial state effect. The E_T spectra of isolated prompt photons are measured in proton- proton (pp) and lead-lead (PbPb) collisions with the CMS detector at the CERN LHC at √s = 5.02 TeV. The integrated luminosities are 27.4 pb−1 and 404 μb−1 for pp NN and PbPb data, respectively. Photons with 25 < E_T < 200GeV in the pseudorapidity range |η| < 1.44 are used for the centrality dependence in PbPb collisions. The nuclear modification factor RAA is also reported as a function of ET in various centrality ranges. The results are compared with the predictions from the next-to-leading-order jetphox generator for different PDFs and nuclear PDFs (nPDFs). The comparisons will help to constrain the nPDFs global fits.

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목차

1 Introduction 1
2 Theoretical framework 5
2.1 Quantum chromodynamics .......................... 5
2.1.1 Standard model............................. 5
2.1.2 Asymptotic freedom and color confinement . . . . . . . . . . . . . 6
2.1.3 QCD phase diagram and Quark Gluon Plasma . . . . . . . . . . . 9
2.1.4 Nuclear parton distribution function ................. 9
2.2 Photon production ............................... 14
2.2.1 Isolated photons ............................ 17
2.3 Relativistic heavy ion collisions ........................ 18
2.3.1 Nuclear modification factor ...................... 22
3 Experimental setup 25
3.1 The Large Hadron Collider .......................... 25
3.2 The Compact Muon Solenoid ......................... 28
3.2.1 Subdetectors .............................. 28
3.2.2 Trigger system ............................. 34
4 Datasets and event selection 35
4.1 Photon triggers................................. 35
4.2 Offline collision event selection ........................ 36
4.3 Centrality determination............................ 40
5 Monte Carlo simulation 43
5.1 Generation of PYTHIA events ......................... 43
5.2 Embedding of PYTHIA into HYDJET for PbPb simulation . . . . . . . . . . 44
5.3 Reweighting................................... 44
6 Analysis procedure 49
6.1 Photon reconstruction ............................. 49
6.2 Photon selection ................................ 51
6.2.1 Photon identification variables .................... 51
6.2.2 Photon isolation variables....................... 53
6.2.3 Electron rejection............................ 55
6.2.4 Summary of the photon selection................... 56
6.3 Photon energy correction ........................... 59
6.3.1 Photon energy scale .......................... 59
6.3.2 Validation of photon energy scale................... 63
6.3.3 Photon energy resolution ....................... 65
6.4 Efficiency .................................... 69
6.4.1 Reconstruction efficiency........................ 69
6.4.2 Identification and isolation efficiency................. 69
6.4.3 Trigger efficiency............................ 70
6.4.4 Tag and Probe ............................. 73
6.4.5 Total efficiency ............................. 75
6.5 Purity...................................... 77
6.6 Unfolding .................................... 86
6.7 Total corrections ................................ 88
7 Systematic uncertainty 89
7.1 Systematic uncertainty from photon energy scale . . . . . . . . . . . . . . 90
7.2 Systematic uncertainty from photon energy resolution . . . . . . . . . . . 90
7.3 Systematic uncertainty from efficiency .................... 90
7.4 Systematic uncertainty from purity...................... 96
7.5 Systematic uncertainty from electron rejection. . . . . . . . . . . . . . . . 100
7.6 Systematic uncertainty from unfolding .................... 103
7.7 Systematic uncertainty from pileup...................... 103
7.8 Systematic uncertainty from global numbers. . . . . . . . . . . . . . . . . 103
7.9 Total systematic uncertainty ......................... 106
8 Results 111
8.1 JETPHOX predictions.............................. 111
8.2 Differential cross section............................ 112
8.3 Nuclear modification factors.......................... 114
9 Conclusions 119
Bibliography 121

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