UC Santa Cruz

Presented in this dissertation is a study of Higgs boson production with high transverse momentum and decaying to a pair of bottom quarks. The analysis uses the pp-collision data collected by the ATLAS Detector at the Large Hadron Collider between 2015 and 2018, corresponding to a total integrated luminosity of 136 fb^{−1} . The decay products of the Higgs boson candidate are reconstructed into a single large-radius jet, using the experimental signature of two subjets identified as coming from b-hadrons. The Z → bb̄ cross section is extracted in the same kinematical regions and compared to Standard Model predictions to validate the analysis methods. The large-radius jets receive a mass and momentum correction to account for losses due to heavy hadron decays to muons and neutrinos by adding the muon four-momenta to that of the jet. The template normalizations of the top-quark-related contributions in the signal region are determined by a simultaneous fit to data in a control region rich in semileptonic tt̄ events. In the log likelihood fit used to extract the signal strengths, an external constraint, derived from tt̄ events with a resolved Wb pair from a hadronically decaying top quark in a dedicated control region, is applied to the jet mass resolution systematic uncertainty nuisance parameter. This external constraint serves to stabilize the fit response and reduce the correlation between the extracted Z signal strength and jet mass resolution parameter. For Higgs boson production at transverse momentum above 450 GeV, the production cross section is found to be 13 ± 57 (stat.) ± 22 (syst.) ± 3 (theo.) fb. The differential cross section 95% confidence level upper limits, as a function of the Higgs boson transverse momentum, are σ_{H} (300 < p_{T}^{H} < 450 GeV) < 2.9 pb, σ_{H} (450 < p_{T}^{H} < 650 GeV) < 89 fb, σ_{H} (650 < p_{T}^{H} < 1000 GeV) < 39 fb, and σ_{H} (p_{T}^{H} > 1 TeV) < 9.6 fb. All results are consistent with Standard Model predictions.