Investigation of Higgs Boson Decaying to Di-muon, Dark Matter Produced in Association with a Higgs Boson Decaying to $b$-quarks and Unbinned Profiled Unfolding

Abstract

The discovery of the Standard Model (SM) Higgs boson by ATLAS and CMS at the LHC in 2012 marked a major milestone in particle physics. However, many questions remain unanswered, which has led to an active research program to search for either rare SM phenomena or Beyond Standard Model (BSM) physics that involve the Higgs boson. In this dissertation, I present two example searches involving the Higgs boson, using proton-proton ($pp$) collision data collected by the ATLAS detector. The first search tackles the problem of how the SM Higgs couples to the second-generation fermions. It searches for the dimuon decay of the SM Higgs boson using data corresponding to an integrated luminosity of 139 fb$^{-1}$ collected by the ATLAS detector in $pp$ collisions at $\sqrt{s} = 13\ \mathrm{TeV}$ at the LHC. In the second search, we look for Dark Matter produced in association with a Higgs boson decaying to $b$-quarks. This search uses the same dataset as the $H\toμμ$ search and targets events that contain large missing transverse momentum and either two $b$-tagged small-radius jets or a single large-radius jet associated with two $b$-tagged subjets. In addition to the two physics analyses, unfolding is a key procedure in the high energy experiments, which corrects data for the detector effect. I present a new unfolding method that allows to unfold data without having any artificial binning and is also able to profile nuisance parameters simultaneously. This new method provides much higher flexibility and increases the reusability for different downstream tasks compared to the traditional approach. It will benifit any future analyses including Higgs physics and Dark Matter searches.