Searches for Higgs Boson Decays to Muon Pairs in the Standard Model and in its Minimal Supersymmetric Extension with the ATLAS Detector

Two studies of Higgs boson decays to two oppositely charged muons in the Standard Model and in its minimal supersymmetric extension with the ATLAS experiment at the Large Hadron Collider (LHC) at CERN are presented. The search for the decays of the neutral Higgs bosons, $h$, $A$ and $H$, of the Minimal Supersymmetric Extension of the Standard Model (MSSM) to $\mu^{+}\mu^{-}$ is performed in proton-proton collisions at a centre-of-mass energy of 7 TeV based on the data recorded in 2011 which correspond to an integrated luminosity of 4.8 fb$^{-1}$. The branching fraction for the $h/A/H\rightarrow\mu^{+}\mu^{-}$ decay is about 300 times smaller compared to the most sensitive $h/A/H\rightarrow\tau^{+}\tau^{-}$ decay channel. But due to the very high experimental $\mu^{+}\mu^{-}$ mass resolution, the signal appears as narrow resonances in the $\mu^{+}\mu^{-}$ invariant mass distribution on top of a continuous background. Motivated by the two Higgs boson production modes, via gluon fusion and in association with b quarks, the search is performed in two complementary event categories either without or with at least one reconstructed b jet in the selected $\mu^{+}\mu^{-}$ events. The two event categories have different signal-to-background ratios and their statistical combination improves the sensitivity of the search. Signal and background are parametrised by analytic functions. The total background contribution is estimated directly from the data. No significant excess of events above the Standard Model background is observed. Exclusion limits are derived for the MSSM $m_h^\text{max}$ benchmark scenario with positive Higgs mixing parameter as a function of the free parameters $m_A$ and $\tan\beta$. In addition, less model-dependent exclusion limits are set on the production cross section times branching fraction of a generic Higgs boson, $\phi$, produced in either gluon fusion or in association with b quarks as a function its mass, $m_\phi$. The second study evaluates the physics potential of the ATLAS experiment for the search for the rare decay of the Standard Model Higgs boson with a mass of 125 GeV to $\mu^{+}\mu^{-}$. The study uses Monte Carlo simulations of the proton-proton collisions at a centre-of-mass energy of 14 TeV with simplified description of the detector response. The sensitivity is evaluated for two benchmark scenarios with integrated luminosities of 300 fb$^{-1}$ and 3000 fb$^{-1}$ which are expected to be reached after two stages of luminosity upgrades of the LHC machine. The prospects are expressed in terms of expected exclusion limits and signal significances and contribute to the combined Higgs boson coupling measurement. With the proposed data set of 3000 fb$^{-1}$, decays of the Standard Model Higgs boson with mass of 125 GeV to $\mu^{+}\mu^{-}$ are expected to be discovered with a significance of more than 6 standard deviations. Higgs boson couplings to the second-generation leptons can be determined with an accuracy of about 25 % relative to the third-generation lepton couplings.