H+ -> Tau in Top quark decays

The search for the light charged Higgs boson ($m_{H^{+}}~< ~ m_{t}$) produced from the top quark decay $t \rightarrow H^{+}b$ and decaying into $\tau^{+} \nu$ has been performed using about 1 fb$^{-1}$ of data collected in proton-proton collisions at $\sqrt{s}$=7 TeV. The dominant source of top quarks at LHC is the $pp \rightarrow t \bar{t}$ process, therefore the charged Higgs boson has been searched for in the subsequent decay products of the top quark pairs: $t \bar{t} \rightarrow H^{\pm} W^{\mp} b \bar{b}$ and $t \bar{t} \rightarrow H^{\pm} H^{\mp} b \bar{b}$. Three types of final states with large missing transverse energy and b-tagged jets have been analyzed: with electron plus muon pair, with muon plus $\tau$ decaying hadronically and with no leptons and $\tau$ decaying hadronically. Combining the results of these three analyzes and assuming $BR(H^{+} \rightarrow \tau \nu)$=1 we extract upper limits on the $BR(t \rightarrow H^{+}b)<$ 4-5 \% depending on the Higgs boson mass fo the interval 80 $< m_{H^{+}} <$ 160 GeV/$c^2$.