Observation of light-by-light scattering in heavy-ion collisions with the ATLAS detector at the LHC

Observation of light-by-light scattering in heavy-ion collisions with the ATLAS detector at the LHC is carried out with data collected by ATLAS detector in 2015 and 2018 corresponding to a total luminosity of 2.2$\mathrm{nb}^{-1}$. The number of event observed is given and the fiducial and differential cross section are calculated. The measured diphoton invariant mass distribution is used to search for axion-like particles and set new exclusion limits on their production assuming 100\% decay to photons. The technical part of this thesis is dedicated to the tuning of argon emulation in the simulation of Transition Radiation Tracker (TRT) which was carried out by testing multiple parameters to decide the best one to be used for emulation argon from xenon without carrying out a full simulation which is time, CPU and disk space consuming.