Measurement of the inclusive isolated prompt photon cross section in $pp$ collisions at $sqrt{s} = 7$ TeV with the ATLAS detector using 35 pb$^{−1}$

A measurement of the cross section for the inclusive production of isolated prompt photons in $pp$ collisions at a center-of-mass energy $\sqrt{s}$ = 7 TeV is presented. The measurement covers the pseudorapidity ranges $|\eta|<$ 1.37 and 1.52 $\leq|\eta|<$ 2.37, in the transverse energy range 45 $\leq E_T <$ 400 GeV. The results are based on an integrated luminosity of 35 pb$^{−1}$, collected with the ATLAS detector at the LHC. Photon candidates are identified by exploiting 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 of the inner detector, which allows the reconstruction of photons that convert to electron-positron pairs. Residual background in the selected sample is estimated using data-driven techniques based on the observed distribution of the isolation energy in a narrow cone around the photon candidate. The results are compared with next-to-leading order perturbative QCD calculations.