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

Abstract

Abstract A search for a light charged Higgs boson produced in decays of the top quark, t → H ± b with H ± → c s , is presented. This search targets the production of top-quark pairs t t ¯ → W b H ± b , with W → ℓ ν ( ℓ = 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 → W b ) + B ( t → H ± ( → c s ) 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.