Search for a light charged Higgs boson in $$t \rightarrow H^{\pm } b$$ decays, with $$H^{\pm } \rightarrow cs$$, in $$pp$$ collisions at $$\sqrt{s}={13}\hbox { TeV}$$ 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 t¯ t → WbH±b, with W → ν ( = e,μ), 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¯ t background. The data analysed correspond to 140 fb−1 of pp collisions at √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) = 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.