Search for a light charged Higgs boson in the decay channel $H^+ \to c\bar{s}$ in $t\bar{t}$ events using pp collisions at $\sqrt{s}$ = 7 TeV with the ATLAS detector

A search for a charged Higgs boson ($H^+$) in $t\bar{t}$ decays is presented, where one of the top quarks decays via $t \to H^+b$, followed by $H^+ \to$ two jets ($c\bar{s}$). The other top quark decays to Wb, where the W boson then decays into a lepton ($e/\mu$) 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 $B(H^+ \to c\bar{s})$=100%.