PHYSICS WITH PHOTONS WITH THE ATLAS RUN 2 DATA: CALIBRATION AND IDENTIFICATION, MEASUREMENT OF THE HIGGS BOSON MASS AND SEARCH FOR SUPERSYMMETRY IN DI-PHOTON FINAL STATE.

The work presented in this manuscript is based on the proton-proton collision data from the Large Hadron Collider at a centre-of-mass energy of 13 TeV recorded by the ATLAS detector in 2015 and 2016. The research program of the ATLAS experiment includes the precise measurement of the parameters of the Standard Model (SM) and the search for signals of physics beyond the SM. Both these approaches are pursued in this thesis, which presents two different analyses. The first one is the measurement of the Higgs boson mass in the di-photon decay channel. The measured value of the mass is (125.11 ± 0.42) GeV. Its combination with a similar measurement in the four lepton Higgs boson decay final state is presented. The value of the Higgs boson mass obtained from the combined measurement is (124.98 ± 0.28) GeV. The second one is the search for production of supersymmetric particles (gluinos, squarks or winos) in a final state containing two photons and missing transverse momentum. No significant excess with respect to the SM background is observed and lower limits on the gluino, squark and wino masses are set in the context of a generalised model of gauge-mediated supersymmetry breaking with a bino-like next-to-lightest supersymmetric particle. Finally, ATLAS detector performance studies (electron and photon energy calibration and measurement of the photon->electron misidentification probability), which are key ingredients for the two analyses presented before, are also performed and described.