Luca Franco – Phd’s defense – jan. 13, 2022

The discovery of the Higgs boson in 2012 represented a true milestone in the historyof particle physics: the last missing piece of the Standard Model (SM) was finallyobserved at the Large Hadron Collider (LHC) by the ATLAS and CMS experiments.Since then, physicists around the world have questioned whether this particle is in factthe Higgs boson or just another particle very similar to it. The only way to answer thisquestion is studying its properties with the highest accuracy and comparing with thetheoretical predictions provided by the SM. If any significant deviation was present inthe measurements, 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 photonsrepresents one of the most interesting channel to study Higgs properties: it profits froma large signal over background ratio, thanks to the clean experimental signature of onlytwo energetic photons; it is possible to take advantage of the very high experimentalphoton energy resolution achieved by the ATLAS detector. The studies presented inthis manuscript exploit proton-proton collisions at a center-of-mass energy of 13 TeVrecorded by ATLAS at the LHC, corresponding to the full Run 2 dataset of 139 fb^{-1}.These include the measurements of the Higgs production cross sections and SimplifiedTemplate Cross Sections (STXS), as well as an analysis which aims to constrain theHiggs decay width. All of them are carried out entirely using diphoton decay events. Inorder to achieve a high level of accuracy, a correct calibration of the photon energy isnecessary. In this context, a detailed study of the non-linearities in the electronics of theATLAS electromagnetic calorimeter is also presented, which significantly contributesto enhance the precision of the upcoming Higgs mass measurement in the diphotondecay channel.