Search For A New Gauge Boson Z’ And Di-higgs Production With The ATLAS Detector At The LHC

This dissertation presents two physics analyses of searches with the ATLAS experiment at the LHC using proton-proton collision data collected at(?)=13 TeV,corresponding to an integrated luminosity of 139 fb-1.The first search targets a new vector gauge boson Z’,predicted by Lμ-Lτ models to address observed phenomena beyond the Standard Model of particle physics.If the predicted Z’ exists in the mass range we are searching for,the signature would show up as a peak in the di-muon mass spectrum,where di-muon pairs are selected from the four-muon(4μ)final state.A deep learning neural network classifier is utilized to increase the significance of the Z’ signal signature.No significant excess of events is observed over the predicted background,and the corresponding common coupling strengths,gZ’,of the Z’ boson to the second and third generation leptons above 0.003-0.2 have been excluded at 95%confidence level.The second analysis is searching for the Standard Model Higgs boson pair production,which is crucial to understand the structure of the Higgs field potential that breaks the electroweak symmetry to generate the masses of the elementary particles.The analysis focuses on the final state containing bb l+l-+v’s(l=e,μ),where one of the Higgs bosons decays to a b-quark pair(bb)and the other to WW*,ZZ*,or τ+τ-,which further decay to l+l-+v’s.The experimental signature is the 2b+2l+ETmiss events.The rate of the di-Higgs production is very low.Multivariate analysis is used to classify the small signal over large background.The statistic model is used to extract the signal from data and an upper limit on the cross-section for Higgs boson pair production is determined at 95%confidence level to be 11.3 times the predicted cross-section of the Standard Model.The expected intervals for the tri-Higgs self-coupling and the di-Higgs interaction with di-boson coupling modifiers are set to be-6.74<κλ<14.5 and-0.69<κ2V<2.76 at 95%confidence level,respectively.A combination study including both the di-Higgs and single-Higgs production channels is also performed,providing results with fewer assumptions by adding more coupling modifiers in the fit to account for the Higgs boson interactions with other Standard Model particles.With the assumption that new physics affects only the Higgs boson self-coupling,values outside the interval-0.4<κλ<6.3 are excluded at 95%confidence level.In addition to the physics analyses,this dissertation also summarizes the major detector work carried out by the author on the construction and commissioning of the small-diameter monitored drift-tube(sMDT)detectors at the University of Michigan and CERN,which is a contribution to the ATLAS muon spectrometer upgrade for the high-luminosity LHC at CERN.Based on the measurements of the first 40(including 2 full-size prototypes)constructed chambers,the wire position deviations from the designed positions are all within the specification of 20 μm.The cosmic ray tests on all the reconstructed sMDT chambers demonstrate that the measurement of the tracking trajectory can achieve a spatial resolution of 100±7 μm for a single wire detection while maintaining an average detection efficiency of over 99%.Commissioning of a small portion of the sMDT chambers installed in the ATLAS Muon Spectrometer during the Phase 1 upgrade is a pilot project of the Phase 2 upgrade.This task is carried out with cosmic ray tests to validate the functionalities of the chambers and to resolve hardware issues promptly.