Measurement of the inclusive prompt photon cross section and preparation of the search of the Higgs boson decaying into two photons with the ATLAS detector at the LHC

This thesis presents the measurements with the ATLAS experiment of the cross section for inclusive production of isolated prompt photons in proton-proton collisions at a centre-of-mass energy sqrt(s) = 7 TeV. The first results are based on an integrated luminosity of 880 nb−1 and the later results are based on 36 pb−1 , collected with the ATLAS detector at the Large Hadron Collider. The measurements performed with 880 nb−1 and 36 pb−1 cover the transverse energy range 15 ≤ ET < 100 GeV and 45 ≤ ET < 400 GeV, respectively. The results are compared to predictions from next-to-leading order perturbative QCD calculations. In addition, are also presented the first studies for the search for the Higgs boson in the decay channel into two photons with 38 pb−1 data collected by the ATLAS experiment in 2010 (and more recently with 210 pb−1 collected in 2011). Observed exclusion limits are quoted as a function of the Higgs mass in the range 110-140 GeV.