Physics with photons at the ATLAS experiment

ATLAS is a general-purpose detector due to start operation next year at the Large Hadron Collider (LHC). The LHC will collide pairs of protons at a centre-of-mass energy of 14 TeV, with a bunch-crossing frequency of 40 MHz, and luminosities up to L = 10^34 cm^-2s^-1. The identification of photons is crucial for the study of a number of physics channels, including the search for a Higgs boson decaying to photon pairs, and measurements of direct production of single photons and photon pairs. Events containing true high-p_T photons must be selected with high efficiency, while rejecting the bulk of high-p_T jet events produced with enormously larger rate through QCD processes. The photon--photon and photon--jet channels are interesting in their own right, allowing the study of QCD at high energy. It is also essential to understand these proceses as the dominant background in the search for certain new physics processes, notably the production and decay of Higgs bosons to photon pairs. There are large uncertaintines in model predictions at LHC energies, and experimental determination of the differential cross-sections is mandatory. The photon-identification strategy (including the trigger and the offline event selection) in ATLAS will be presented along with estimates of signal efficiencies and background rates. Emphasis will be placed on the perfomance for identifying photons from direct-photon production in the early running (with instantaneous luminosities starting around L = 10^31 cm^-2s^-1), and on the search for Higgs to two-photon decays at higher luminosities (L = 10^33 cm^-2s^-1 and above). Techniques for determining identification efficiencies from the data, minimizing model dependence on the cross-section determination, will also be addressed.