Evidence for prompt photon production in p p collisions at √s = 7 TeV with the ATLAS detector

Photon identification is important for many physics signatures at the LHC. Prompt photon identification in ATLAS relies on the fine granularity of the electromagnetic calorimeter, which provides event by event rejection of the dominant background from photons produced by $\pi^0$ decays, and on the inner detector, which allows the reconstruction of photon conversions to electron-positron pairs. This note describes the extraction of the isolated prompt photon signal above the background for data from pp collisions at $\sqrt{s} = 7$TeV, corresponding to an integrated luminosity 15.8 nb$^{-1}$, collected with the ATLAS detector at the LHC. Using a 2-dimensional sideband background subtraction technique, a statistically significant signal of prompt photons is observed, with a yield $N =(618 \pm 72)$ and a purity $P=(72\pm 7)%$ for transverse energies above 20 GeV.