Abstract
A
bstract
A search for a charged Higgs boson,
H
±
, produced in top-quark decays,
t
→
H
±
b
, is presented. The search targets
H
±
decays into a bottom and a charm quark,
H
±
→
cb
. The analysis focuses on a selection enriched in top-quark pair production, where one top quark decays into a leptonically decaying
W
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
b
-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
s
= 13 TeV between 2015 and 2018 with the ATLAS detector at CERN’s Large Hadron Collider, amounting to an integrated luminosity of 139 fb
−
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
B
(
t
→
H
±
b
) ×
B
(
H
±
→
cb
) for charged Higgs boson masses between 60 and 160 GeV, assuming the SM production of the top-quark pairs.