Search for a light charged Higgs boson in the decay channel H+ -> csbar in ttbar events using pp collisions at sqrt(s) = 7 TeV with the ATLAS detector

A search for a charged Higgs boson (H +) in tt¯ decays is presented, where one of the top quarks decays via t -> H + b, followed by H + -> two jets ( cs¯ ). The other top quark decays to Wb, where the W boson then decays into a lepton (e/µ) and a neutrino. The data were recorded in pp collisions at sqrt(s)=7 TeV by the ATLAS detector at the LHC in 2011, and correspond to an integrated luminosity of 4.7 fb-1. With no observation of a signal, 95 % confidence level (CL) upper limits are set on the decay branching ratio of top quarks to charged Higgs bosons varying between 5 % and 1 % for H + masses between 90 GeV and 150 GeV, assuming B(H+ -> cs¯)=100 % .