This paper reports a search by the CMS experiment for a light charged Higgs boson (H±) produced in top quark pair events. The team used proton–proton collision data recorded at a center‑of‑mass energy of 13 TeV, corresponding to 138 inverse femtobarns of data (the full 2016–2018 run). They looked for cases where one top quark decays to H± plus a bottom quark and the other top decays normally to a W boson and a bottom quark. The data agree with the expectations from the Standard Model, so no new particle was found and upper limits were set on how often top quarks could decay to H± and a bottom quark.

Why look for a charged Higgs? Extensions of the known Higgs sector, such as two‑Higgs‑doublet models, predict extra Higgs particles including charged ones. In some of these models the charged Higgs can decay into a charm quark and a strange quark (written H±→cs). If the charged Higgs is lighter than the top quark, it can appear in top decays. The analysis probes H± masses between 40 and 160 GeV, a range chosen so that the charged Higgs could be produced in top decays but still be consistent with the top quark mass.

How the search was done: the analysis selects events with one charged lepton (an electron or muon), missing transverse momentum from a neutrino, and four jets. Two of those jets come from bottom quarks and two are “light” jets (defined here as jets that do not pass the bottom‑quark tagging requirement). The mass of the two light jets (their invariant mass) is used to look for a peak that would signal H±→cs. The team used two analysis strategies: a traditional set of selection cuts and a multivariate approach based on boosted decision trees to improve sensitivity. Simulation tools such as MADGRAPH5_aMC@NLO and POWHEG were used to model signal and background, and the dominant background is ordinary top‑quark pair production.