Search for light charged Higgs bosons decaying to charm and strange quarks in t t-bar events in proton-proton collisions at sqrt(s) = 13 TeV

A search is presented for a light charged Higgs boson H± in top quark pair production (t¯t), where one of the top quarks decays to an H± and a bottom quark, while the other decays to a W∓ boson and a bottom quark. The H± is assumed to decay into a charm and a strange quark, whereas the W∓ boson decays into a charged lepton (electron or muon) and a neutrino. Results are reported based on proton-proton collision data at √s = 13 TeV, corresponding to an integrated luminosity of 138 fb−1. The analysis probes H± masses in the range 40 to 160 GeV using the invariant mass spectrum of the two light jets, where light jets are defined as jets that do not satisfy the bottom quark tagging requirement. The observed yield is found to be consistent with standard model predictions. Upper limits are set on the branching fraction B(t → H±b), with values in the range of 0.07–1.12% (0.12–1.20% expected) at 95% confidence level, under the assumption that B(H± → cs) = 100%. These are the first direct limits on charged Higgs bosons produced in top quark decays for masses between 40 and 50 GeV, and the most stringent limits to date in the 70–110 GeV range.