Search for resonances in light-by-light scattering in 14.6 fb$^{-1}$ of $pp$ collisions at $\sqrt{s}=13$ TeV

This thesis reports a search for resonances in light-by-light scattering using 14.6 fb$^{-1}$ of $\sqrt{s}=13$ TeV $pp$ collision data collected by the ATLAS experiment at the LHC. The light-by-light scattering mediates a scattering of the beam protons. The scattered proton is detected by the ATLAS Forward Proton (AFP) spectrometer, while the ATLAS detector reconstructs the photons. At least one of the outgoing protons is required to match the kinematics of the final state diphoton to reduce the background events. Diphoton resonances in the mass range from 150 GeV to 1600 GeV are searched using signal and background distribution models with unbinned fitting. The maximum observed local significance is 2.51 at $m_X=454$ GeV, and there is no excess from the standard model. Upper limits are placed on the production cross section as a function of the resonance mass. The results are interpreted as the upper limits of the ALP coupling constant to two photons. All the results are obtained assuming the branching ratio to two photons is 100%. The observed upper limits of the coupling constant are about 0.04 TeV$^{-1}$ in the range $m_X=200$ to 1000 GeV.