A search for the dimuon decay of the Standard Model Higgs boson with the ATLAS detector

A search for the dimuon decay of the Standard Model (SM) Higgs boson is performed using data corresponding to an integrated luminosity of $139{\text{fb}}^{-1}$ collected with the ATLAS detector in Run 2 pp collisions at $\sqrt{s}=13$ TeV at the Large Hadron Collider. The observed (expected) significance over the background-only hypothesis for a Higgs boson with a mass of 125.09 GeV is 2.0σ (1.7σ). The observed upper limit on the cross section times branching ratio for $pp\to H\to \mu \mu $ is 2.2 times the SM prediction at 95% confidence level, while the expected limit on a $H\to \mu \mu $ signal assuming the absence (presence) of a SM signal is 1.1 (2.0). The best-fit value of the signal strength parameter, defined as the ratio of the observed signal yield to the one expected in the SM, is $\mu =1.2±0.6$.