Search for New Resonances in the Diphoton Final State with the ATLAS Detector

This thesis presents the search for new resonances in the diphoton final state with proton-proton collision data collected by the ATLAS detector at the LHC at a centre-of-mass energy of $\sqrt{s}=13$ TeV. Search for a low-mass spin-0 resonance in the diphoton invariant mass range from 65 to 110 GeV is performed using 80 $fb^{-1}$ data collected in 2015, 2016 and 2017. Selected events are split into three categories depending on the conversion state of the two photons, in order to increase the sensitivity of the search. In the high-mass region above 160 GeV, two kinds of signal are searched for using 139 $fb^{-1}$ data collected in 2015-2018: a spin-0 model-independent resonant state, and a spin-2 graviton excitation state predicted by the Randall-Sundrum model with one warped extra dimension. Analysis selections are optimized and harmonized for both spin-0 and spin-2 searches. Functional decomposition method is applied for the first time in the background modeling procedure, in order to reduce the corresponding systematic uncertainty. For both low-mass and high-mass searches, there is no significant excess observed with respect to the Standard Model expectation. Upper limits are set on the fiducial (total) production cross section times branching ratio as a function of the signal mass for the spin-0 (spin-2) resonances. In addition, a study on the photon-specific energy calibration systematic uncertainty from electromagnetic shower leakage mismodeling is also presented. This uncertainty is quantified as the difference between the lateral energy leakage mismodeling for photons and electrons using $Z\to ee$ events. Results obtained with diphoton events are also shown for the first time as a cross check in a larger kinetic region.