Search for a light charged Higgs boson in $t\to H^{\pm} b$ decays, with $H^{\pm} \to cs$ 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, $t\to H^{\pm} b$ decays, with $H^{\pm} \to cs$, is presented. This search targets the production of top-quark pairs $t\bar{t} \to WbH^{\pm}b$, with $W \to \ell\nu (\ell = e, \mu)$, 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 $t\bar{t}$ 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 $\mathscr{B}(t \to H^{\pm}b)$, assuming $\mathscr{B}(t \to Wb) + \mathscr{B}(t \to H^{\pm}(\to cs)b) = 1.0$, 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.