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

TL;DR

This paper reports a search for a light charged Higgs boson in top quark decays using 13 TeV proton-proton collision data from the ATLAS detector, setting upper limits on the decay branching fraction.

Contribution

The study introduces a novel search strategy employing $b$- and $c$-quark identification and multivariate techniques to improve sensitivity to $t o H^\pm b$ decays with $H^\pm o cs$.

Findings

Set upper limits on $ ext{Br}(t o H^\pm b)$ between 0.066% and 3.6%.

Analyzed 140 fb$^{-1}$ of data from 2015-2018.

Excluded certain charged Higgs mass ranges at 95% confidence level.

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 $\mathscr{B}(t\to H^\pm b)$, assuming $\mathscr{B}(t\to Wb) + \mathscr{B}(t \to H^\pm (\to cs)b)=1.0$, 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.