Search for Higgs boson decays into a pair of pseudoscalar particles in the $bb\hat{I}¼\hat{I}¼$ final state with the ATLAS detector in $pp$ collisions at $\sqrt{s}=13$ TeV

Abstract

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 fb${}^{-1}$ of proton-proton collision data at a center-of-mass energy of $\sqrt{s}=13$ 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 $3.3\ddot{I}ƒ$ ($1.7\ddot{I}ƒ$). Upper limits at 95% confidence level are placed on the branching ratio of the Higgs boson to the $bb\hat{I}¼\hat{I}¼$ final state, $\mathcal{B}(H\to aa\to bb\hat{I}¼\hat{I}¼)$, and are in the range $\text{(0.2-4.0)}\times 10^{-4}$, depending on the signal mass hypothesis.

44 pages in total, author list starting page 28, 12 figures, 3 tables, published in PRD. All figures including auxiliary figures are available at https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/HDBS-2021-03/