Abstract
In studies of the dense medium produced in ultra-relativistic heavy ion collisions, photons are important hard probes, since they are not expected to be modified by the medium. The measurement of isolated prompt photon production in Pb–Pb collisions provides a test of perturbative quantum chromodynamics and the information to constrain the nuclear parton distribution functions. CMS has shown photon purity measurement capabilities in pp collisions using the shower shape templates. In Pb–Pb collisions at CMS, this technique was applied for the first time in heavy ion collisions. We report the first measurement of the transverse momentum spectra of isolated photons with pT from 20 to 80 GeV/c in Pb–Pb collisions at = 2.76 TeV. The centrality dependence of the nuclear modification factor is also reported by comparing the result to the photon spectrum of pp reference which is computed from NLO calculations.
Highlights
High transverse energy (ET) prompt photons in nucleus-nucleus collisions are produced directly from the hard scattering of two partons
3 Results the MinBias sample using the total sum of energy signals from both positive and negative HF
The Island algorithm [7] is used for the electromagnetic calorimeter (ECAL) energy clustering for photon reconstruction, and the energy is corrected taking into account the material in front of the ECAL and the electromagnetic shower containment
Summary
High transverse energy (ET) prompt photons in nucleus-nucleus collisions are produced directly from the hard scattering of two partons. Photons traverse the produced hot and dense medium without interacting strongly, they provide a direct test of perturbative QCD (pQCD) and the nuclear parton√densities [1]. In this analysis, we report the isolated photon production in PbPb collisions at sNN = 2.76 TeV as a function of the event centrality with the CMS detector [2]
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