A Search for Decays of the 125 GeV Higgs Boson in the $bb\tau\tau$ Final State with the ATLAS Detector using $p$-$p$ Collisions at $\sqrt{s}$=13 TeV

This thesis presents a search for decays of the 125 GeV Higgs boson ($h$) to a pair of new (pseudo)scalar particles ($a$), $h\rightarrow aa$, where the $a$-bosons decay to a $b$-quark pair and tau lepton ($\tau$) pair. Adding a new (pseudo)scalar to the Standard Model is a simple extension which produces a wide range of phenomenology. The addition of a new (pseudo)scalar is also motivated by dark matter, which could couple to ordinary matter via the $a$-boson, and by recent experimental excesses in the muon anomalous magnetic moment and $W$ boson mass, which both are modulated by a new scalar. The search is performed in the mass range $12<m_a<60$ GeV in different analysis channels defined by the decay products of the tau pair. The search uses 139 fb$^{-1}$ of proton-proton collision data with $\sqrt{s}=13$ TeV recorded by the ATLAS experiment at the LHC between 2015 and 2018. The analysis searches in three decay channels of the $\tau\tau$ system and makes use of both leptonic ($e$ or $\mu$) and hadronic ($\tau_{\text{had}}$) tau lepton decays: $e\mu$, $\mu\tau_{\text{had}}$, and $e\tau_{\text{had}}$. A further categorization is performed by the experimental signature of the heavy-flavor jets, including a boosted double b-jet tagger. Background processes are modeled with Monte Carlo for event topologies with two prompt leptons or hadronic taus and a data-driven technique is used to estimate the contribution from non-prompt leptons and hadronic tau decays. Control regions are employed to constrain different background processes: high transverse mass for $t\bar{t}$, high $m_{\tau\tau}^{\text{vis}}$ for Drell-Yan, and same-sign visible tau decay products for non-prompt leptons and taus. A Parameterized Neural Network is employed to discriminate between signal and background in the low-mass ($m_{\tau\tau}^{\text{vis}}<60$ GeV) regime in which $h\rightarrow aa$ decays are kinematically allowed. Limits on the branching ratio for the $h\rightarrow aa\rightarrow bb\tau\tau$ process are set and no evidence of any beyond the Standard Model phenomena is observed. This thesis sets the most sensitive limits worldwide on exotic Higgs decays in the $bb\tau\tau$ final state. The branching ratio is limited to less than 1.5\% over the mass range from 12 to 60 GeV for the mass of the new psuedoscalar $m_a$. A measurement of the resolution, efficiency, and noise rate of drift tube chambers for the ATLAS Phase-II upgrade is also presented. Notably, a novel treatment of the multiple Coulomb scattering correction is included in the drift tube resolution measurement.