Higgs boson cross section measurements in the diphoton decay channel in proton-proton collisions at $\sqrt{s} = 13$ TeV using the ATLAS detector at the LHC

This dissertation presents ATLAS measurements of the total and differential fiducial cross sections for the process of the Higgs boson production and subsequent decay to two photons. The analyzed dataset of proton-proton collisions at $\sqrt{s} = 13$ TeV has an integrated luminosity of $139~\text{fb}^{-1}$ and was collected during the Run 2 of the Large Hadron Collider in 2015–2018. The cross sections were measured in a fiducial phase space closely matching the experimental selection, and compared to the Standard Model predictions from the state-of-the-art calculations. The differential cross sections were measured with respect to an array of observables, allowing us to probe the kinematics, jet activity, and spin and CP properties of the Higgs boson interactions. The data were analyzed iteratively, with yearly results from partial datasets presented at conferences and in publications. The inclusive fiducial cross section obtained from the analysis of the full Run 2 dataset is $65.2 \pm 4.5\,(\text{stat.}) \pm 5.6\,(\text{exp.}) \pm 0.3\,(\text{theory})$ fb, which is in excellent agreement with the SM prediction of $63.5\pm 3.3$ fb. The results provide the state-of-the-art experimental measurements of the Higgs boson production cross sections, and extend our confidence in the SM in the Higgs sector and QCD, as well as in the computational techniques used to obtain theoretical predictions.