Measurement of Isolated Direct Photons in Lead–Lead Collisions at sNN=2.76TeV with the ATLAS Detector

AbstractDirect photons are a powerful tool to study heavy ion collisions. Their production rates provide access to the parton distribution functions, which are expected to be modified by nuclear effects. They also provide a mean to calibrate the energy of jets that are produced in the medium, and thus are a tool to probe the physics of jet quenching more precisely both through jet rates and fragmentation properties. The ATLAS detector measures photons with its hermetic, longitudinally segmented calorimeter, which gives excellent spatial and energy resolution, and detailed information about the shower shape of each measured photon. This gives powerful rejection against backgrounds from neutral pions produced in jets. Rejection against jet fragmentation products is further enhanced by isolation criteria, which can be based on energy flow in the calorimeter. First results on the rates of isolated direct photons from approximately 0.13nb−1 of lead–lead data will be shown, as a function of transverse momentum, pseudorapidity and centrality