Measurements of cross sections for Higgs boson production and forward jet calibration with the ATLAS detector

This thesis presents measurements of inclusive and differential cross sections for Higgs boson production in the fiducial and total phase space regions, as well as a calibration of the ATLAS calorimeter response to jets. The fiducial $pp \rightarrow H \rightarrow \gamma\gamma$ cross section for $m_{H}=125.4$ GeV was measured at ${\sqrt{s}=8}$ TeV with 20.3 fb$^{-1}$ of proton-proton collision data collected by the ATLAS detector. The $H \rightarrow \gamma\gamma$ signal was extracted from the background with a fit to the diphoton invariant mass spectrum, within the fiducial phase space defined by two isolated photons with $|\eta|<2.37$ and fractional transverse momentum greater than 0.35 and 0.25 relative to the diphoton invariant mass. The signal yields were corrected for inefficiency and resolution of the detector. The $pp \rightarrow H \rightarrow \gamma\gamma$ cross section is measured to be $\sigma_{\rm fid} = 43.2 \pm 9.4 \, ({\rm stat.}) \, {}^{+3.2}_{-2.9} \, ({\rm syst.}) \pm 1.2 \, ({\rm lumi}) \, \, {\rm fb}$. A variety of differential cross sections were also measured for observables that are sensitive to different properties of the Higgs boson. The distributions are statistically limited but are in reasonable agreement with the Standard Model predictions. The inclusive and differential cross sections were measured in the total phase space combining the $H \rightarrow \gamma\gamma$ and $H \rightarrow ZZ^{*} \rightarrow 4\ell$ decay channels, corrected for fiducial acceptance and decay branching fractions. The total cross section for $pp \rightarrow H$ was measured to be $\sigma = 33.0 \pm 5.3\text{(stat)} \pm 1.6\text{(sys)}~{\rm pb}$. The measured differential and normalised differential cross sections are found to be in reasonable agreement with the predictions. The centre-of-mass energy dependence of the Higgs boson production cross section was also tested with additional measurements using 4.5 fb$^{-1}$ at $\sqrt{s} = 7$ TeV and 3.2 fb$^{-1}$ at $\sqrt{s} = 13$ TeV. The theoretical predictions and the data are in fair agreement. The calibration of the jet energy scale in the forward region of the ATLAS calorimeter was performed using dijet events in 72 pb$^{-1}$ of $\sqrt{s}=13$ TeV proton-proton collision data collected in 2015. A significant $\eta$-dependent mismodelling in the response of jets in MC simulation was found, and residual calibration factors were derived. A new dijet balance method was also studied to probe the relative response of jets as a function of $\phi$ in the forward region.