A search for $R$-parity violating supersymmetric decays of the top squark to a $b$-jet and a lepton in $\sqrt{s}$ = 13 TeV $pp$ collisions with the ATLAS detector

A search is presented for direct pair production of the stop, the supersymmetric partner of the top quark, in a decay through an $R$-parity violating coupling to a charged lepton and a $b$-quark. The dataset corresponds to an integrated luminosity of 140 fb$^{-1}$ of proton-proton collisions at a center-of-mass energy of $\sqrt{s}$ = 13 TeV collected between 2015 and 2018 by the ATLAS detector at the LHC. The final state has two charged leptons (electrons or muons) and two $b$-jets. The results of the search are interpreted in the context of a Minimal Supersymmetric Standard Model with an additional $B-L$ gauge symmetry that is spontaneously broken. No significant excess is observed over the Standard Model background and exclusion limits on stop pair production are set at 95% confidence level. The corresponding lower limits on the stop mass for 100% branching ratios to a $b$-quark and an electron, muon, or tau-lepton are 1.9 TeV, 1.8 TeV and 800 GeV, respectively, extending the reach of previous LHC searches.