Measurement of Higgs couplings using the diphoton channel and EFT interpretation with the ATLAS experiment using the full Run 2 data of LHC. Calibration of the electromagnetic calorimeter.

The discovery of the Higgs boson in 2012 opened an era of precise measurements in the Higgs sector. With the full Run 2 data, the precision on the measurements of the Higgs couplings reaches a level of ten per cent for the most precisely determined ones. In this thesis, an STXS analysis of the $H\to\gamma\gamma$ decay channel with full Run 2 data is made, which is one of the most sensitive channels. Results are presented in various granularities, from the inclusive signal strength to the STXS stage 1.2 measurement. A combination of STXS measurement from various Higgs channels is interpreted in an EFT, with the top-scheme symmetry of the SMEFT framework, and in 2HDM and MSSM models. For the specific case of $H\to\gamma\gamma$ and $H\to4\ell$ combination, the constraining power on EFT models from STXS inputs is compared to the one from differential cross-sections. SMEFT measurements are also performed in a PCA basis, selecting the most sensitive fit directions. This thesis also presents an alternative method for the in-situ calibration of the ATLAS electromagnetic calorimeter using the distribution of $E_T/p_T$ variable, complementary to the classical $m_{ee}$ lineshape method