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 $\sqrt{s}=13$ TeV

TL;DR

This paper searches for Higgs boson decays into a pair of pseudoscalar particles, with one decaying into b-quarks and the other into muons, using ATLAS data at 13 TeV, finding a slight excess at 52 GeV.

Contribution

It reports the first search for Higgs decays into pseudoscalars with this specific final state using the full Run 2 ATLAS dataset, setting new upper limits.

Findings

A 3.3 sigma local excess at 52 GeV in the dimuon spectrum.

Upper limits on the branching ratio range from 0.2 to 4.0 times 10^{-4}.

No definitive discovery of Higgs to pseudoscalar decays.

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, $H\rightarrow aa$, 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 $3.3 \sigma$ ($1.7 \sigma$). Upper limits at 95% confidence level are placed on the branching ratio of the Higgs boson to the $bb\mu\mu$ final state, $\mathcal{B}(H\rightarrow aa\rightarrow bb\mu\mu)$, and are in the range $\text{(0.2-4.0)} \times 10^{-4}$, depending on the signal mass hypothesis.