Search for a light charged Higgs boson in decays, with , in collisions at with the ATLAS detector

A search for a light charged Higgs boson produced in decays of the top quark, with , is presented. This search targets the production of top-quark pairs , with ( ), resulting in a lepton-plus-jets final state characterised by an isolated electron or muon and at least four jets. The search exploits $\textit{b}$-quark and $\textit{c}$-quark identification techniques as well as multivariate methods to suppress the dominant background. The data analysed correspond to of collisions at recorded with the ATLAS detector at the LHC between 2015 and 2018. Observed (expected) 95% confidence-level upper limits on the branching fraction , assuming , 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.