Search for a light charged Higgs boson in top quark decays in pp collisions at √s = 7\;TeV

Results are presented on a search for a light charged Higgs boson that can be produced in the decay of the top quark t → H+b and which, in turn, decays into τ +ν τ . The analysed data correspond to an integrated luminosity of about 2 fb−1 recorded in protonproton collisions at √s = 7\;TeV by the CMS experiment at the LHC. The search is sensitive to the decays of the top quark pairs tt̄ → H± W ∓ bb̄ and tt̄ → H±H∓ bb̄ . Various final states have been studied separately, all requiring presence of a τ lepton from H+ decays, missing transverse energy, and multiple jets. Upper limits on the branching fraction B(t → H+b) in the range of 2-4 % are established for charged Higgs boson masses between 80 and 160 GeV, under the assumption that B(H+ → τ +ν τ ) = 1.