Direct photon and photon-jet measurement capability of the ATLAS experiment at the LHC

Direct photon and photon-jet correlations are perfect tools for tomographic studies of the dense medium produced in heavy ion collisions at LHC energies. Due to their weak interactions with the medium, direct photons serve as standard candles for hard-scattering processes, providing a clean calibration of the momentum of the associated jets. The ATLAS detector has powerful opportunities to carry out these measurements. The combination of fine granularity, longitudinal segmentation and large acceptance is unique for the LHC detectors. We show how it will provide optimal capability to distinguish direct photon clusters from neutral meson clusters based on their shower profile over a wide acceptance in η−ϕ up to 200 GeV in pT. We show that combined with a photon isolation cut ATLAS would be able to measure a relatively background-free direct photon yield from 50–200 GeV along with the corresponding gamma-jet correlations in one nominal LHC Pb+Pb year. These high pT photons provide clean and statistically significant measurements of gamma-jet correlations and the fragmentation function for photon-tagged jets.