Search for pairs of muons with small displacements in $pp$ collisions at $\sqrt{s} = 13$ TeV with the ATLAS detector

A search new phenomena giving rise to pairs of opposite electrically chargedmuons with impact parameters in the millimetre range is presented, using 139fb$^{-1}$ of $\sqrt{s} = 13$ TeV $pp$ collision data from the ATLAS detector atthe LHC. The search targets the gap in coverage between existing searchestargeting final states with leptons with large displacement and prompt leptons.No significant excess over the background expectation is observed and exclusionlimits are set on the mass of long-lived scalar supersymmetric muon-partners(smuons) with much lower lifetimes than previously targeted by displaced muonsearches. Smuon lifetimes down to 1 ps are excluded for a smuon mass of 100GeV, and smuon masses up to 520 GeV are excluded for a proper lifetime of 10ps, at 95% confidence level. Finally, model-independent limits are set on thecontribution from new phenomena to the signal-region yields.