Observation of a Higgs boson and measurement of its mass in the diphoton decay channel with the ATLAS detector at the LHC

The Standard Model of particle physics predicts the existence of a massive scalar boson, referred to as the Higgs boson, resulting from the introduction of a doublet of complex scalar fields and the Spontaneous Symmetry Breaking mechanism, needed to generate the mass of the particles. The Higgs boson, whose mass is theoretically undetermined, has been searched for experimentally since almost half a century by various experiments. The search for the Higgs boson is one of the goals of the LHC physics program. One of the most important decay channels at the LHC is the diphoton channel, because the final state can be completely reconstructed with high precision. In this thesis, a detailed study of the photon energy response, using the ATLAS electromagnetic calorimeter has been performed. In particular, the stability and uniformity of the energy response has been tested. This study has provided a better understanding of the photon energy resolution and scale, which are very important for the determination of the systematic uncertainties on the mass and production rate in the diphoton channel. This channel had a prominent role in the discovery of a new particle compatible with the Standard Model Higgs boson by the ATLAS and CMS experiments. Using this channel as well as the improved understanding of the photon energy response, a measurement of the mass of this particle is proposed in this thesis, with the data collected in 2011 and 2012 at a center-of-mass energy of 7 TeV and 8 TeV. A mass of $126.8 ~\pm ~0.2 ~\rm{(stat)} ~\pm~ 0.7~ \rm{(syst)~} \GeV/\rm{c}^2$ is found. The calibration of the photon energy measurement with the calorimeter is the source of the largest systematic uncertainty on this measurement. Strategies to reduce this systematic error are discussed. Among them, a method to measure the amount of material upstream of the calorimeter, which provides the largest contribution to the uncertainty on the energy scale, has been developed. The energy scale measurement of the different layers of the electromagnetic calorimeter, that is also a source of uncertainty for the global energy scale, is presented.