Direct gamma and gamma+jet measurement capability of ATLAS @ LHC: Quark Matter 2009 talk

Direct photon and photon-jet correlations are ideal tools for tomographic studies of the dense medium created 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 excellent capabilities to make these measurements. In particular, the electromagnetic calorimeter, covering the full azimuth for |eta| < 4.9, has longitudinal segmentation and fine transverse segmentation along eta in the range |eta| < 2.4. This combination of fine granularity, longitudinal segmentation and large acceptance is unique among the LHC detectors. We show how this will provide an optimal capability to distinguish direct photon clusters from neutral meson clusters based on their shower profile over a wide acceptance in eta, phi out to 200 GeV in pT . This opens up the possibility for studying various final state photons, including those from jet fragmentation, in-medium gluon conversion and medium-induced bremsstrahlung. Combined with a photon isolation cut, we show that ATLAS should be able to measure a relatively background-free direct photon yield from 10-200 GeV along with the corresponding gamma-jet correlations for one nominal LHC Pb+Pb year. These high pT photons allow a clean and statistically significant measure of the gamma-jet correlation and t he fragmentation function for photon-tagged jets.