New Search for Higgs Boson Decay to a Beyond the Standard Model Vector Boson with the ATLAS Detector

This dissertation presents the search for the beyond Standard Model process H -> ZZd -> 4l; the decay of a Higgs boson to four leptons via an intermediate Z boson and an intermediate beyond Standard Model dark vector boson Zd. The Zd is in this analysis presumed to exist in a mass range of 15 GeV to 55 GeV, and connect to the Standard Model via kinetic mixing with the Z boson. Only electrons and muons are considered in the leptonic final states. The analysis is based on 139 inverse femto barn of proton-proton collision with a center of mass energy of 13 TeV recorded with the ATLAS detector between 2015 and 2018. Expected distributions and yields for the main backgrounds are derived from Monte Carlo simulations of the relevant processes. The estimate of the reducible backgrounds is derived from a semi-data derived method, where the background distribution is estimated from Monte Carlo simulations while the yield is estimated from data. The simulation of the ZZ* -> 4l and reducible backgrounds are verified against data events in dedicated validation regions, whereas the yield of the H -> ZZ* -> 4l background is estimated in-situ along with the measurement of the H -> ZZd -> 4l cross section limits in the signal region. The H -> ZZd -> 4l cross section limits are calculated via a profile likelihood fit that includes a number of nuisance parameters to account for experimental and theoretical systematic uncertainties. This dissertation puts upper limits on the H -> ZZd -> 4l cross section between 0.17 inverse femto barn and 0.93 inverse femto barn at 95% confidence interval, depending on the presumed Zd mass. Slight excesses in the observed cross section limits above the expected ones present themselves around Zd masses of mZd = 20 GeV, and 40 GeV.