Measurement of Higgs boson production cross sections in the diphoton channel with the full ATLAS Run-2 data and constraints on anomalous Higgs boson interactions

This thesis presents the measurement of the fiducial integrated and differential cross sections for the production of the Higgs boson decaying to two photons. The measurement is performed using 139 fb$^{-1}$ of proton–proton collision data at $\sqrt{s}=13~$TeV collected by the ATLAS experiment at the Large Hadron Collider. The cross section measurement in the two-photon final state is performed in a fiducial region closely matching the experimental selection and compared to Standard Model (SM) predictions. The measurement is performed for the inclusive fiducial region and differentially for the Higgs boson transverse momentum, the Higgs boson absolute rapidity, and various jet variables including the jet multiplicity, the transverse momentum of the leading jet, the invariant mass of the two leading jets and Azimuthal-angle separation of the two leading jets. These variables probe precisely the Higgs boson kinematic and CP properties. This resulted in a measurement of the inclusive $H\to\gamma\gamma$ fiducial cross section of $65.2\pm7.1$ fb which is in excellent agreement with the SM predictions of $63.6\pm3.3$ fb. The measured differential cross sections are used to probe the strength and tensor structure of the anomalous Higgs boson interactions using an effective Lagrangian, which introduces additional CP-even and CP-odd interactions.