Search for a light charged Higgs boson in t → H±b decays, with H± → cb, in the lepton+jets final state in proton-proton collisions at √s = 13 TeV with the ATLAS detector

Abstract

<p>A search for a charged Higgs boson, <em>H</em>^<em>±</em>, produced in top-quark decays, <em>t</em> → <em>H</em>^<em>±</em><em>b</em>, is presented. The search targets <em>H</em>^<em>±</em> decays into a bottom and a charm quark, <em>H</em>^<em>±</em> → <em>cb</em>. The analysis focuses on a selection enriched in top-quark pair production, where one top quark decays into a leptonically decaying <em>W</em> boson and a bottom quark, and the other top quark decays into a charged Higgs boson and a bottom quark. This topology leads to a lepton-plus-jets final state, characterised by an isolated electron or muon and at least four jets. The search exploits the high multiplicity of jets containing <em>b</em>-hadrons, and deploys a neural network classifier that uses the kinematic differences between the signal and the background. The search uses a dataset of proton-proton collisions collected at a centre-of-mass energy � = 13 TeV between 2015 and 2018 with the ATLAS detector at CERN’s Large Hadron Collider, amounting to an integrated luminosity of 139 fb^{<em>−</em>1}. Observed (expected) 95% confidence-level upper limits between 0.15% (0.09%) and 0.42% (0.25%) are derived for the product of branching fractions �(<em>t</em> → <em>H</em>^<em>±</em><em>b</em>) × <em>B</em>(<em>H</em>^<em>±</em> → <em>cb</em>) for charged Higgs boson masses between 60 and 160 GeV, assuming the SM production of the top-quark pairs.</p>