Diphoton measurements with the ATLAS detector at the LHC: search for new resonances and study of diphoton production in association with jets.

This thesis studies pp collisions at $\sqrt{s}=8$ TeV with pairs of photons in the final state, as collected by the ATLAS experiment at the LHC during 2012. The diphoton final state has proven to be crucial to the discovery of the Higgs boson. The motivation for searching for additional resonances decaying into two photons is very strong due to many models predicting an extended Higgs sector. At the same time, detailed measurements of diphoton cross sections are necessary to establish the quality of the theoretical predictions currently available for these processes. The measurement of photons by ATLAS relies on an excellent calibration of the electromagnetic calorimeter response. For this reason, studies related to the calibration of the photon response in the ATLAS Liquid Argon (LAr) calorimeter are initially performed, including a measurement of the calorimeter layer energy scale relative calibration using photons, and the potential need of an inter-calibration of the photon energy response as a function of the azimuthal angle $\phi$. A study of noise performance of the calorimeter in high pile-up conditions is also presented, resulting in recommendations for the LAr pulse sampling configurations for Run2. A search for an additional resonance decaying into two photons is performed, leading to no significant excess being observed. A limit on the fiducial cross section times branching fraction of a new resonance decaying into two photons is therefore computed. The measurement of the production cross section of isolated prompt diphotons in association with jets is performed as a function of the diphoton invariant mass, the transverse momentum of the diphoton system, the azimuthal angle difference of the two photons and the coplanarity angle theta, for jet multiplicity categories and as a function of various jet variables. The results are compared to Sherpa and Pythia MC predictions. Such a measurement has not been previously published at the LHC.