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

Abstract

A search for a light charged Higgs boson produced in decays of the top quark, $t\to H^{\pm }b$ with $H^{\pm }\to cs$, is presented. This search targets the production of top-quark pairs $t\overline{t}\to Wb{H}^{\pm }b$, 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 $t\overline{t}$ background. The data analysed correspond to $140{\text{fb}}^{-1}$ of $\mathit{pp}$ collisions at $\sqrt{s}=13\text{TeV}$ recorded with the ATLAS detector at the LHC between 2015 and 2018. Observed (expected) 95% confidence-level upper limits on the branching fraction $\mathcal{B}(t\to H^{\pm }b)$, assuming $\mathcal{B}(t\to Wb)+\mathcal{B}(t\to H^{\pm }(\to cs)b)=1.0$, 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.