The Large Hadron Collider (LHC) at CERN (European Organisation for Nuclear Research) has been in operation since 2009 at the border between France and Switzerland. This particle accelerator has provided a significant quantity of proton-proton collisions, at center of mass energies of 7 TeV (2010 and 2011) and 8 TeV (since April 2012). One of the purposes of the Compact Muon Solenoid (CMS) experiment, which records and analyses those collisions, is the search for Higgs bosons. In the Standard Model, this particle would be the quantum of the Higgs field, thought to explain the electroweak symmetry-breaking mechanism. One of the most sensitive channels for the search for a light Higgs boson (i.e. for masses between 90 GeV=c2 and 150 GeV=c2) at the LHC is through its decay into two photons. Indeed, the production of two highly energetic and isolated photons in the final state is a clean signal in hadronic collisions, despite the small branching ratio. Moreover, at these masses, the width of the reconstructed invariant mass peak is dominated by the detector resolution. Hence, a precise understanding of the photon object as reconstructed by the experiment is required. The first part of the work presented here is the study of radiative decays Z$^0 \rightarrow \mu\mu\gamma$ these decays are the only Standard Model source of high purity certified photons, for which the transverse momentum of the photon is relatively similar to the one of the photons coming from a Higgs boson decay. The study of these decays enables the determination of the differential photon energy scale in the 7 TeV data. These decays have also been used in the 8 TeV data as a validation tool of photon cluster shape variables in the simulation, and also as a way to determine the electron rejection criteria efficiency. The second part of the work concerns the search for a Higgs boson decaying into two photons. This search is done in several event classes in order to increase the sensitivity of the analysis, the study of the class separation procedure is presented. Finally, a benchmark study for the search for a Higgs boson decaying into two photons via vector-like top-partner decays at 14 TeV has been performed.
Study of radiative decays Z$^0 \rightarrow \mu\mu\gamma$ and search for Higgs bosons in the H$\rightarrow \gamma\gamma$ channel, in the CMS experiment at LHC (CERN)