Recherche du boson de Higgs dans le canal de désintégration en deux photons avec le détecteur ATLAS au LHC

This document comprises of three parts related to different aspects in the search for the Higgs boson in the $gammagamma$ channel with the ATLAS detector. A study of the intrinsic uniformity of the EM barrel calorimeter of ATLAS using the drift time measurement of ionization electrons is performed. About 500 000 cosmic muon pulses have been recorded and their shape predicted using the extit{First Principle Method}. The drift time uniformity in the Middle compartment of the barrel is derived per region of size $0.1 imes 0.1$ in the ($eta,phi$) plane. Its impact on the calorimeter response uniformity is found to be of 0.29 % at nominal voltage. With the lead thickness variation expected to contribute by $sim$ 0.18%, this leads to an intrinsic calorimeter uniformity in the barrel of 0.34%. The drift velocity of electrons is measured in the different layers of the barrel and amounts to v(E = 1 kV/mm) = 4.58 $pm$ 0.07 mm.$mu s^{-1}$ which is in good agreement with previous measurements at the operating temperature of 88.5 K. A study of the measurement of the material the beam pipe (BP) using photon conversions is presented. The BP is a well isolated and well known material to normalize the conversion rates in the upstream material of the EM calorimeter. Given that a large fraction of the photons originate from $pi^{0}$ mesons, the initial flux of photons arising in the conversion rate is normalized to the $pi^{0}$ Dalitz decays. The BP conversion rate is expressed as a function of the ratio of the reconstructed BP conversions to that of Dalitz decays that is measured in data recorded at 7 TeV centre-of-mass energy. The BP material estimated in the data well agrees with prediction from both the Monte-Carlo (MC) simulation and layout design: $X/X_{0} = 0.669 pm 0.033(stat) ^{0.013}_{-0.080}(syst)%$, $X/X_{0} = 0.709 pm 0.028(syst) %$, and $X/X_{0} = 0.709 pm 0.028(syst) %$ respectively. The normalizations of the signal and background and the discriminating potential in the $H ightarrow gamgam$ inclusive analysis are studied. Different programs and MC simulations are used to estimate a Next-to-Leading Order normalization of the signal and irreducible background ($gamgam$) and predict their differential cross sections. There systematic uncertainties amount respectively to 16% and 26%. The discriminating power of the transverse momentum of the diphoton system and the $cos heta^{*}$ is reappraised. In particular a strong correlation is observed between the $cos heta^{*}$ and the diphoton invariant mass. The semi-reducible background ($gam$-jet) is normalized at NLO with 26% of systematic error. A refined normalization that is applied after the photon identification is proposed. It accounts for the different rejection of jets originating from quarks or gluons and amounts to $1.9 pm 0.6$.