Diphoton lineshape of the BEH boson using the ATLAS detector at the LHC: calibration, mass, width and interferences

Since the data collected during the first run of the LHC by the ATLAS and CMS collaborations allowed for the discovery of the BEH boson, a huge effort has been done toward the detailed studies of its properties. This thesis is particularly oriented toward the understanding of the BEH boson lineshape in its diphoton decay channel, using the data collected by the ATLAS detector in 2011 and 2012. The electromagnetic calibration of the ATLAS detector is described in details, and the precise measurement of the BEH boson mass it allowed for, at $m_h = 125.98 \pm 0.50$ GeV, is summarized. A first upper limit on the BEH boson decay width, that gave a limit at $\Gamma_h < 5.3$ GeV at 95% C.L., is presented in details). The last part of this thesis presents a study of quantum interferences between signal and background processes in the $gg \rightarrow \gamma\gamma$ channel, which are expected to distord the diphoton lineshape and create a shift of the measured BEH boson mass, that is estimated to be of 35 MeV, which is small but not negligible.