Property Measurements of the Higgs Boson and Search for High Mass Resonances in Four-Lepton Final State with the ATLAS Detector at the LHC

[摘要] The discovery of a Higgs boson by the ATLAS and CMS experiments at the Large Hadron Collider (LHC) at CERN in 2012 marked the beginning of a new era of experimental particle physics research through studies of the properties of this new particle. It is extremely important to determine the new particle;;s the quantum numbers - spin and parity, and to measure its couplings to fermions and vector bosons to confirm if the newly discovered particle is the Higgs boson predicted almost 50 years ago by the Standard Model (SM) of particle physics. In the SM, the Higgs boson is a scalar particle with positive parity. It is responsible for generating masses for all the massive particles in our universe through the electroweak symmetry breaking mechanism.This dissertation presents the measurements of the quantum numbers and couplings of the newly discovered boson using data collected in proton-proton collisions at the center-of-mass energies of 7, 8 and 13 TeV at the LHC by the ATLAS experiment during 2011-2012 and 2015-2016. The measurements are conducted with the four-lepton final state, where leptons are electrons and muons, which are produced from the cascade decay chain of the Higgs boson to the Z boson pair.In the determination of the new particle;;s spin and parity, data is compared with different theoretical hypotheses using multivariate analysis approach. The SM Higgs boson prediction is tested against non-SM spin and parity hypotheses, including spin-0 and spin-2 models with universal and non-universal couplings to fermions and vector bosons. Data is in favor of the SM Higgs boson predicted quantum numbers, and all tested alternative models are excluded at more than 99.9% confidence level (CL) combining diboson decay channels using datacollected at the center-of-mass energies of 7 and 8 TeV with a total integrated luminosity of 25 inverse-fb.The Higgs boson coupling measurement is made by selecting inclusive four-lepton events from Higgs boson decays. These events are further divided into different categories based on the associated jet production and lepton kinematic distributions of the events. Each category is designed to have good sensitivity to a certain Higgs production mechanism. The Boosted-Decision-Tree multivariate analysis technique has been developed and used in event categorization. The likelihood fit technique is used to compare kinematic distributions of the selected four-lepton events with different coupling scenarios to determine the coupling strength and compared to the SM predictions. Using data collected at the center-of-mass energy of 13 TeV with a total integrated luminosity of 36.1 inverse-fb, the Higgs to vector boson and fermion couplings are measured with precision of 15% and 40%, respectively.Many theoretical models beyond the SM (BSM), such as the two-Higgs-doublet model and the electroweak singlet model, predicted the existence of additional Higgs bosons at higher mass range. A search for BSM high mass Higgs boson has been performed using the data collected at the center-of-mass energy of 13 TeV with a total integrated luminosity of 3.2 inverse-fb, and no evidence of an additional Higgs boson has been observed in the four-lepton mass spectrum. Upper limits at the 95% CL are set on the production cross section times branching ratio of a heavy scalar boson decaying to four leptons. This search results will also be presented in this dissertation.

[发布机构] University of Michigan

[效力级别] Atlas Experiment

[关键词] Measurements of Higgs boson properties;Atlas Experiment;Physics;Science;Physics