Search for charged Higgs bosons through the violation of lepton universality in $t\bar{t}$ events using $pp$ collision data at $\sqrt{s}=7$ TeV with the ATLAS experiment

In several extensions of the Standard Model, the top quark can decay into a bottom quark and a light charged Higgs boson $H^+, t \to bH^+$, in addition to the Standard Model decay $t \to bW$. Since W bosons decay to the three lepton generations equally, while $H^+$ may predominantly decay into $\tau\nu$, charged Higgs bosons can be searched for using the violation of lepton universality in top quark decays. The analysis in this paper is based on 4.6/fb of proton-proton collision data at $\sqrt{s}$ = 7 TeV collected by the ATLAS experiment at the Large Hadron Collider. Signatures containing leptons (e or $\mu$) and/or a hadronically decaying $\tau (\tau_{had})$ are used. Event yield ratios between $e+\tau_{had}$ and $e+\mu$, as well as between $\mu+\tau_{had}$ and $\mu+e$, final states are measured in the data and compared to predictions from simulations. This ratio-based method reduces the impact of systematic uncertainties in the analysis. No significant deviation from the Standard Model predictions is observed. With the assumption that the branching fraction $B(H^+ \to \tau\nu)$ is 100%, upper limits in the range 3.2%-4.4% can be placed on the branching fraction $B(t \to bH^+)$ for charged Higgs boson masses $m_{H+}$ in the range 90-140 GeV. After combination with results from a search for charged Higgs bosons in ttbar decays using the $\tau_{had}$+jets final state, upper limits on $B(t \to bH^+)$ can be set in the range 0.8%-3.4%, for $m_{H+}$ in the range 90-160 GeV.