Background Modelling in the ATLAS $H\rightarrow\gamma\gamma$ Differential Cross Section Analysis

Measurements of fiducial integrated and differential cross sections of the Higgs boson decaying into a pair of photons are presented with an emphasis on discussing the background modelling strategy. The measurements use the full ATLAS dataset corresponding to 139 fb$^{-1}$ of proton-proton collision data recorded during the 2015 - 2018 Run-2 of the Large Hadron Collider. Several diphoton and jet distributions related to the event kinematics as well as the multiplicity of the jets produced in association with the Higgs boson are measured and show good agreement with the Standard Model prediction. All presented results are unfolded to particle level. The integrated fiducial production cross section is measured to be $65.2\pm7.1$ fb, which is in good agreement with the Standard Model prediction of $63.6\pm3.3$ fb.