Search for a light charged Higgs boson in the $H^{+} \to c\bar{s}$ decay with the CMS detector

A search for a light charged Higgs boson, originating from the decay of a top quark and subsequently decaying into a charm and an antistrange quark, is presented. The analysed data correspond to an integrated luminosity of 19.7 $\rm fb^{-1}$ recorded in proton-proton collisions at a center-of-mass energy of $\sqrt{s} =8 $ TeV by the CMS experiment at the LHC. The search is conducted in the decays of the top-antitop quark pairs $t\bar{t} \to W^{\pm}bH^{\mp}b$ with the $W$ decaying to a lepton (electron or muon) and a neutrino and the $H$ boson decaying to $c\bar{s}$ . The decay final state comprises an isolated lepton, at least four jets and large missing transverse energy. No significant deviation is found with respect to standard model predictions, and model-independent upper limits are set on the branching fraction ${\cal B}(t \to H^{+}b)$ in the range of 1.2-6.5% for a charged Higgs boson mass between 90 and 160 GeV, under the assumption that ${\cal B}(H^{+} \to c\bar{s})$=100%.