Isolated photon production in $\sqrt{s_{NN}}$ = 2.76 TeV PbPb collisions as a function of transverse energy and reaction centrality

TL;DR

This paper reports the first measurement of isolated photon production in PbPb collisions at 2.76 TeV, testing pQCD predictions and nuclear PDFs by analyzing photon spectra and centrality dependence.

Contribution

It introduces the first application of shower shape template techniques for photon purity measurement in heavy ion collisions at CMS.

Findings

Photon spectra measured from 20 to 80 GeV/c

Centrality dependence of nuclear modification factor analyzed

Results consistent with pQCD predictions

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 PbPb collisions provides a test of perturbative quantum chromodynamics (pQCD) 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 PbPb co llisions 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 GeV/c to 80 GeV/c in PbPb collisions at $\sqrt{s_{NN}}$ =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.