Search for Higgs boson decays into a pair of pseudoscalar particles in the $bb\mu \mu $ final state with the ATLAS detector in $pp$ collisions at $\sqrt{s}=13\text{}\text{}\mathrm{TeV}$

This paper presents a search for decays of the Higgs boson with a mass of 125 GeV into a pair of new pseudoscalar particles, $H\to aa$, where one $a$-boson decays into a $b$-quark pair and the other into a muon pair. The search uses $139\text{}\text{}{\mathrm{fb}}^{-1}$ of proton-proton collision data at a center-of-mass energy of $\sqrt{s}=13\text{}\text{}\mathrm{TeV}$ recorded between 2015 and 2018 by the ATLAS experiment at the LHC. A narrow dimuon resonance is searched for in the invariant mass spectrum between 16 GeV and 62 GeV. The largest excess of events above the Standard Model backgrounds is observed at a dimuon invariant mass of 52 GeV and corresponds to a local (global) significance of $3.3\sigma $ ($1.7\sigma $). Upper limits at 95% confidence level are placed on the branching ratio of the Higgs boson to the $bb\mu \mu $ final state, $B\left(H\to aa\to bb\mu \mu \right)$, and are in the range $0.2–4.0×{10}^{-4}$, depending on the signal mass hypothesis.