Search for a light charged Higgs boson in $t\to H^{\pm }b$ decays, with $H^{\pm }\to cs$, in $pp$ collisions at $\sqrt{s}=13$ TeV 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 $t\bar{t}\to Wb{H}^{\pm }b$, 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 $t\bar{t}$ background. The data analysed correspond to 140 ${\text{fb}}^{-1}$ of $pp$ collisions at $\sqrt{s}=13$ TeV recorded with the ATLAS detector at the LHC between 2015 and 2018. Observed (expected) 95% confidence-level upper limits on the branching fraction $\mathcal{B}(t\to H^{\pm }b)$, 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 GeV and 168 GeV.

49 pages in total, author list starting page 32, 6 figures, 4 tables, published in Eur. Phys. J. C 85 (2025) 153. All figures including auxiliary figures are available at https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/HDBS-2020-11

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