Prospects for the measurement of the rare Higgs boson decay $H\to\mu\mu$ with 3000 fb$^{-1}$ of $pp$ collisions collected at $\sqrt{s} = 14$ TeV by the ATLAS experiment

This note presents a study of the prospects for the measurement of the rare Higgs boson decay $H\to \mu\mu$ using 3000 fb$^{-1}$ of proton-proton collisions at $\sqrt{s}=14$ TeV recorded with the ATLAS detector at the high-luminosity LHC. The studies assume an average number of interactions per bunch crossing $\langle\mu\rangle=200$. The $H\to\mu\mu$ signal from gluon fusion and vector-boson fusion is expected to be observed with a significance of more than 9 standard deviations. The uncertainty on the Higgs production cross section times the branching ratio to dimuons normalised by the Standard Model prediction is expected to be around 13%.