Centrality, rapidity and p_T dependence of isolated prompt photon production in lead–lead collisions at √(s_NN)=2.76 TeV with the ATLAS detector at the LHC

The ATLAS experiment at the LHC has measured prompt photon production in √(s_NN)=2.76 TeV Pb + Pb collisions using data collected in 2011 with an integrated luminosity of 0.14 nb⁻¹. The measurement is performed with a hermetic, longitudinally segmented calorimeter, which gives excellent spatial and energy resolution, and detailed information about the shower shape of each measured photon. Using a selection based on shower shapes and an isolation criterion gives measured purities between approximately 50–90%, depending on p_T, η and centrality. Photon yields, scaled by the mean nuclear thickness function, are determined as a function of collision centrality, pseudorapidity (in two intervals |η|<1.37 and 1.52<|η|<2.37) and transverse momentum (22T<280 GeV). The scaled yields are shown compared to expectations from JETPHOX (perturbative QCD calculations at next to leading order), as are the ratios of the forward yields to those near mid-rapidity (for 22T<140 GeV). The observed photon yields agree well with the predictions for proton–proton within statistical and systematic uncertainties, but are unable to distinguish between these calculations and others including isospin and nuclear PDF effects