  A search for charged leptons with large impact parameters using 139 fb$^{-1}$
of $\sqrt{s} = 13$ TeV $pp$ collision data from the ATLAS detector at the LHC
is presented, addressing a long-standing gap in coverage of possible new
physics signatures. Results are consistent with the background prediction. This
search provides unique sensitivity to long-lived scalar supersymmetric
lepton-partners (sleptons). For lifetimes of 0.1 ns, selectron, smuon and stau
masses up to 720 GeV, 680 GeV, and 340 GeV are respectively excluded at 95%
confidence level, drastically improving on the previous best limits from LEP.
