Search For Exotic Decays of the 125 GeV Higgs Boson in the bb$\mu\mu$ Final State With the ATLAS Detector

Exotic decays of the 125 GeV Higgs boson provide a unique window for the dis- covery of new physics, as the Higgs may couple to hidden-sector states that do not interact under Standard Model (SM) gauge transformations. Models predicting ex- otic Higgs decays to additional light bosons appear in many extensions to the SM and can explain several unknowns in high energy particle physics, such as the na- ture of dark matter and the existence of supersymmetry. In this Dissertation, a search for the exotic decay of the 125 GeV Higgs boson is performed using proton- proton collisions from the Large Hadron Collider at $\sqrt{s}$ = 13 TeV with 36.1 fb$^{−1}$ of data collected by the ATLAS detector. In this search, we only consider the ex- otic decays to the final state H → bb$\mu\mu$. The experimental signature consists of two oppositely signed muons, two b-tagged jets, and a negligible amount of missing transverse energy. Further, the two b-jet and dimuon systems are required to have nearly equivalent masses and the four-object mass of these particles must be within 15 GeV of 125 GeV. These constraints are placed on the system via a kinematic like- lihood fit that adjusts the energy of the b-jets and assigns an event scores based on the compatibility of the hypothesis $m_{bb} \approx m_{\mu\mu}$. The main background contributions come from the Standard Model Drell-Yan + jets production and $t\bar{t}$decays with two muons; minor background contributions from W+jets, single-top, and $t\bar{t}$ + X processes are also considered. Monte Carlo simulation samples are used to estimate each background contributions, except for the Drell-Yan background, which is estimated using a data-driven template method. Experimental and theoretical uncertainties from a variety of sources are assessed for the signal and background estimations.As no significant excesses are found in the signal region selections, upper limits on the production cross-section times branching ratio $\frac{\sigma_{H}}{\sigma_{\textrm{SM}}} \times \mathcal{B}(H\rightarrow aa \rightarrow bb\mu\mu)$ which range from $1.2\times10^{-4}$ to $8.4\times10^{-4}$ in the a-boson mass range, $20 \leq ma \leq 60$ GeV. Additional model-independent limits, where no signal model is considered, are set on the visible production cross-section times the branching ratio of a new physics particle X to the bb$\mu\mu$ final state, $\sigma_{vis}(X) \times B(X \rightarrow bb\mu\mu)$, ranging from 0.1 fb to 0.73 fb in the dimuon mass range $18 \leq ma \leq 62$ GeV. This analysis provides stringent limits on $H \rightarrow aa$ decays with comparable or greater sensitivity to other search channels performed on LHC data. Finally, we discuss the limiting factors of the 36.1 fb$^{−1}$ analysis and present new experimental avenues needed to establish a greater chance for discovery potential using the full LHC Run-2 dataset.