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

In several extensions of the Standard Model, the top quark can decay into a bottom quark and a light charged Higgs boson $H^+$, $t \rightarrow bH^+$, in addition to the Standard Model decay $t \rightarrow bW$. 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 \LUMI of proton–proton collision data at $\sqrt{s} = 7~\mbox{TeV}$ collected by the ATLAS experiment at the Large Hadron Collider. Signatures containing leptons ($e$ or $\mu$) and/or a hadronically decaying $\tau$ ($\tau_{\text{had}}$) are used. Event yield ratios between $e+\tau_{\text{had}}$ and $e+\mu$, as well as between $\mu+\tau_{\text{had}}$ 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 ${\cal B}(H^+ \rightarrow \tau\nu)$ is 100%, upper limits in the range 3.2%–4.4% can be placed on the branching fraction ${\cal B}(t\rightarrow bH^+)$ for charged Higgs boson masses $m_{H^+}$ in the range 90–140GeV. After combination with results from a search for charged Higgs bosons in $t\bar{t}$ decays using the $\tau_{\text{had}}$+jets final state, upper limits on ${\cal B}(t\rightarrow bH^+)$ can be set in the range 0.8%–3.4%, for $m_{H^+}$ in the range 90–160GeV.