Search for the Higgs Boson in its decay to two muons

The Standard Model (SM) describes a wide range of particle physics phenomena with high precision. In the SM, the Brout-Englert-Higgs (BEH) mechanism spontaneously breaks the electroweak (EW) gauge symmetry and generates masses for the W and Z gauge bosons as well as for the charged fermions via Yukawa couplings. In searches for the Higgs boson predicted by the BEH mechanism. A Higgs-like boson with a mass of about 125.5 GeV is now experimentally well established by the ATLAS and the CMS experiments at the Large Hadron Collider (LHC). So far, decays to pairs of photons, W+-, and Z boson pairs and tau lepton pairs have been observed. The rates of these decays are found to be consistent with the Standard Model (SM) of particle physics. The data is also consistent with a spin parity assignment of Higgs Boson of 0+. Other important properties are the couplings to other elementary particles such as the beauty quark, charm quark, muon and electron, with their SM branching ratios proportional to the squares of the fermion masses. The SM branching ratio for the H -> mu+mu- is 2.2x10-4 for a Higgs boson mass (mH ) of 125 GeV. Some new physics extensions of the SM like the Minimal Supersymmetric Standard Model (MSSM) modify these predictions. The H -> mu+mu- decay has a clean final state signature that allows a measurement of the Higgs boson coupling to second-generation fermions. The dominant irreducible background is the Z/gamma -> mu+mu- process, which has an approximately three orders of magnitude higher production rate compared to that of the expected signal. This thesis will describe the search for H -> mu+mu- decays using data collected between 2015 and 2018 with the ATLAS detector at the LHC. The dataset will be large enough to rule out certain new physics extensions of the SM. This search is for the presence of a narrow H -> mu+mu- resonance, with a signal width determined by experimental resolution. It is performed by fitting the invariant mass distribution in the 110-160 GeV region. This range allows determination of the background shape and normalization and setting a limit on the dimuon decay of the SM Higgs boson with a mass of 125.5 GeV. Chapter 1 of the thesis will be a theoretical introduction to the Standard Model and the Higgs Boson. Chapter 2 & 3 will provide a description of each of the subsystems of the ATALS experiment and the trigger and data acquisition and summarizes the data sample and Monte Carlo (MC) simulation samples used to model the signal process and to develop an analytical model for the background processes. Chapter 4 & 5 will describe the event selections and categorization, the software used to reconstruct muons, data driven determinations of muon efficiency, Monte Carlo determination of the acceptance, and analytical models used to describe invariant mass distributions for signal and background processes. Chapter 6 will describe in detail the methodology and evaluation of the systematic uncertainties. Finally, in Chapter 7 the results will be presented and compared to (a) previous upper limits from ATLAS and CMS and from the competing measurement (or upper limit) from CMS (b) to theoretical predictions.