Calibration du calorimètre électromagnétique a l'aide des évenements $Z \to g$ et recherches de bosons de Higgs additionnels dans le canal $H \to gg$ dans l'experience CMS au LHC

The parameters of the Standard Model of particle physics have been verified experimen- tally with a very high accuracy. With the Higgs mechanism, this model explains the origin of the mass of the W and Z bosons, while keeping the photon massless, and thus breaks the gauge symmetry of the electroweak interaction. This mechanism is associated with a particle, the Higgs boson, which is studied in the experiments of the Large Hadron Collider at CERN. In July 2012, ATLAS and CMS collaborations have jointly announced the discovery of a new boson with a mass of 125 GeV, consistent with the predictions of the Standard Model Higgs boson. The CMS electromagnetic calorimeter, which has a very good energy resolution, has been specially designed to study the decay of the Higgs boson into two photons. By its clear final state in hadron environment, this channel is particularly interesting and allows the reconstruction of a narrow resonance in the invariant mass spectrum of diphotons events. Although the Standard Model is one of the scientific theories that have experienced the greatest number of successes to date, it is imperfect. The inability of this model to describe certain phenomena or the ad hoc choice of some parameters seems to suggest that it is only an approximation of a more general theory. Models beyond the Standard Model, such as 2HDM or NMSSM for example, can compensate some of its limitations and postulate the existence of additional Higgs bosons. Thus, in addition to the study of the photon energy in order to calibrate in situ the CMS electromagnetic calorimeter, my thesis also includes the search for additional Higgs bosons decaying into two photons. The first part of my work focuses on the study of decays Z ! , one of the only sources of real high-energy photons of the Standard Model selectable with a high purity. These events, in spite of their low cross section, are particularly suitable for the calibration of the CMS electromagnetic calorimeter. With these ones, we have extracted the energy scale of photons for 7 and 8 TeV data, have developed a method for measuring the energy resolution of the ECAL and have studied an algorithm designed to reduce the loss of resolution due to the increase of pileup. The second part of my work deals with the search for additional Higgs bosons decaying into a pair of photons with an invariant mass below 125 GeV. Our analysis is based on the Standard Model H ! study, which has been essential for the discovery of the new boson at 125 GeV. After the reoptimization of the analysis to improve efficiency on the signal and correctly take into account the Drell-Yan background, which becomes very important near the Z peak, we have been able to extract limits on the cross section of an additional Higgs boson between 90 and 115 GeV.