Precision measurements of the Higgs Boson properties with the decay into two photons at the ATLAS experiment

The discovery of the Higgs boson in 2012 represented a milestone in the history of par- ticle physics: the last missing piece of the Standard Model (SM) was finally observed at the Large Hadron Collider (LHC) by the ATLAS and CMS collaborations. Since then, physicists around the world have questioned whether this particle is in fact the Higgs boson or another particle very similar to it. The only way to answer this question is studying its properties with the highest accuracy and comparing with the theoretical predictions provided by the SM. If any significant deviation was present in the mea- surements, it would be an important hint of physics Beyond the SM (BSM). Despite a very low branching ratio (about 0.2%), the Higgs decay into two photons represents one of the most interesting channel to study Higgs properties: it profits from a favor- able signal over background ratio, thanks to the clean experimental signature of only two energetic photons; it is possible to take advantage of the very high experimental photon energy resolution achieved by the ATLAS detector. The studies presented in this manuscript exploit proton-proton collisions at a center-of-mass energy of 13 TeV recorded by ATLAS at the LHC, corresponding to the full Run 2 dataset of 139 fb$^{−1}$ . These studies include the measurements of the Higgs production cross sections and Simplified Template Cross Sections (STXS), as well as an analysis which aims to con- strain the Higgs decay width. All of them are carried out entirely using diphoton decay events. In order to achieve a high level of accuracy, a correct calibration of the photon energy is necessary. In this context, a detailed study of the non-linearities in the elec- tronics of the ATLAS electromagnetic calorimeter is also presented, which significantly contributes to enhance the precision of the upcoming Higgs mass measurement in the diphoton decay channel.