Search for Higgs boson decays into a pair of light bosons in the Final State in Collision at with the ATLAS detector

A search for decays of the Higgs boson into a pair of new spin-zero particles, $H\to aa$, where the a-bosons decay into a b-quark pair and a muon pair, is presented. The search uses 36.1 ${\text{fb}}^{-1}$ of proton–proton collision data at $\sqrt{s}=13$ TeV recorded by the ATLAS experiment at the LHC in 2015 and 2016. No significant deviation from the Standard Model prediction is observed. Upper limits at 95% confidence level are placed on the branching ratio $\left({\sigma }_{H}/{\sigma }_{\mathrm{SM}}\right)×B\left(H\to aa\to bb\mu \mu \right)$, ranging from $1.2×{10}^{-4}$ to $8.4×{10}^{-4}$ in the a-boson mass range of 20–60 GeV. Model-independent limits are set on the visible production cross-section times the branching ratio to the $bb\mu \mu $ final state for new physics, ${\sigma }_{\mathrm{vis}}\left(X\right)×B\left(X\to bb\mu \mu \right)$, ranging from 0.1 fb to 0.73 fb for ${m}_{\mu \mu }$ between 18 and 62 GeV.