Search for an additional low mass Higgs boson in the diphoton channel within the CMS experiment in the LHC and interpretations

This thesis presents the search for a new particle within the CMS experiment in the LHC (CERN). The search analysis for an additional low mass (lower than 125 GeV) Higgs boson decaying into two photons is shown with some phenomenological interpretations. The very existence of this new Higgs boson is, indeed, motivated by a lot of beyond the Standard Model theories, some of which contain low mass Higgs bosons and are presented here. The CMS detector is described so that one can understand the physical aspects of the search analysis. It is also essential that the way photons are reconstructed and identified, within the CMS detector, is studied because of the diphoton decay channel that is considered in this thesis. Especially, the reconstructed photon energy needs to be corrected to account for energy leakage within the CMS electromagnetic calorimeter and a dedicated method, that enables the validation of such corrections on photons, is presented here. With this in mind, the search analysis methods, widely using multivariate variable tools, which goal is to find a new low mass Higgs boson are shown together with the derived results using 2017 and 2018 data years. Following this step, some elements and discussions for analysis improvements, especially concerning the modeling of the Drell-Yan background, will be studied. Finally, constraints on some theoretical models are derived and the possibility of discovering other Higgs bosons at the LHC and in future colliders is discussed thanks to phenomenological works. (author)