In several extensions of the Standard Model, the top-quark can decay to a bottom-quark and a light charged Higgs boson ($m_{H^+}<m_{\text{top}}$) which is assumed to decay only into $\tau\nu$. Discriminating variables were identified, which allow the distinction between the leptons produced in \tau \rightarrow l+ \nu\mbox{s} (arising in decays of $W$ or charged Higgs bosons) and the leptons coming directly from $W$ boson decays. This paper aims at validating these variables, in single-lepton and dilepton $t\bar{t}$ events, through a comparison between Monte Carlo simulations and 35 pb$^{-1}$ of ATLAS data collected in 7 TeV proton-proton collisions at the LHC, in 2010. We find that Monte Carlo simulations agree well with the ATLAS data. The impact of systematic uncertainties is also discussed.

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