Search for a light charged Higgs boson in t→H±b decays, with H±→csˉ, in pp collisions at sqrt(s) = 13 TeV with the ATLAS detector

A search for a light charged Higgs boson produced in decays of the top quark, t -> H+- b with H+ -> cs, is presented. This search targets the production of top-quark pairs ttbar -> WbH+-b, with W->lv (l=e,mu), resulting in a lepton-plus-jets final state characterised by an isolated electron or muon and at least four jets. The search exploits b-quark and c-quark identification techniques as well as multivariate methods to suppress the dominant ttbar background. The data analysed correspond to 140 fb-1 of pp collisions at sqrt(s) =13 TeV recorded with the ATLAS detector at the LHC between 2015 and 2018. Observed (expected) 95% confidence-level upper limits on the branching fraction B(t->H+-b), assuming B(t->Wb) + B(t->H+-(->cs)b), are set between 0.066% (0.077%) and 3.6% (2.3%) for a charged Higgs boson with a mass between 60 and 168 GeV.