The production of isolated photons in PbPb and pp collisions at s NN $$ \sqrt{s_{\mathrm{NN}}} $$ = 5 . 02 TeV
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Springer
Received: March 28, 2020 Accepted: June 1, 2020 Published: July 17, 2020
The CMS collaboration E-mail: [email protected] γ Abstract: The transverse energy (ET ) spectra of photons isolated from other particles are measured using proton-proton (pp) and lead-lead (PbPb) collisions at the LHC at √ sNN = 5.02 TeV with integrated luminosities of 27.4 pb−1 and 404 µb−1 for pp and PbPb γ data, respectively. The results are presented for photons with 25 < ET < 200 GeV in the pseudorapidity range |η| < 1.44, and for different centrality intervals for PbPb collisions. Photon production in PbPb collisions is consistent with that in pp collisions scaled by the number of binary nucleon-nucleon collisions, demonstrating that photons do not interact with the quark-gluon plasma. Therefore, isolated photons can provide information about the initial energy of the associated parton in photon+jet measurements. The results are compared with predictions from the next-to-leading-order jetphox generator for different parton distribution functions (PDFs) and nuclear PDFs (nPDFs). The comparisons can help to constrain the nPDFs global fits.
Keywords: Hadron-Hadron scattering (experiments), Heavy-ion collision, Photon production ArXiv ePrint: 2003.12797
Open Access, Copyright CERN, for the benefit of the CMS Collaboration. 3 Article funded by SCOAP .
https://doi.org/10.1007/JHEP07(2020)116
JHEP07(2020)116
The production of isolated photons in PbPb and pp √ collisions at sNN = 5.02 TeV
Contents 1
2 The CMS detector
3
3 Analysis procedure 3.1 Monte Carlo simulation 3.2 Event selection 3.3 Photon reconstruction and identification 3.4 Signal extraction 3.5 Efficiency corrections 3.6 Unfolding 3.7 Systematic uncertainties
3 3 4 4 6 6 7 8
4 Results 4.1 Differential cross section in pp and PbPb collisions 4.2 Nuclear modification factors
10 10 11
5 Summary
12
The CMS collaboration
19
1
Introduction
One of the most important reasons for studying relativistic heavy ion collisions is understanding the deconfined state of matter, so called quark-gluon plasma (QGP), which is predicted by the theory of strong interactions, quantum chromodynamics (QCD), to exist at high temperatures and energy density [1–4]. In heavy ion collisions, the expectation is that high transverse momentum (pT ) photons do not strongly interact with the QGP and thus provide a direct way to test perturbative QCD (pQCD). Comparing photon production in proton-proton (pp) and heavy ion collisions is important to both establish that we understand the production of photons in collisions of nuclei and that the photons are not affected by the medium through which they pass. In contrast to photons, partons lose energy in the medium and their production is significantly modified compared to pp collisions [5–7]. The production of photons paired back-to-back with jets from fragmented partons has been studied at the CERN LHC [8–11] to test energy loss in the strongly interacting medium produced in heavy ion collisions. Prompt photons are def
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