Search for charged Higgs bosons through the violation of lepton universality in $tart$ events using $pp$ collision data at $sqrts=7$ TeV with the ATLAS experiment

Abstract

In several extensions of the Standard Model, the top quark can decay into a bottom quark and a light charged Higgs boson H_s16, t to bH_s16, in addition to the Standard Model decay t to bW. Since W bosons decay to the three lepton generations equally, while H_s16 may predominantly decay into tau_s16nu, 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_s16tau_had and e_s16mu, as well as between mu_s16tau_had and mu_s16e, 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_s16 to tau_s16nu) is 100%, upper limits in the range 3.2%-4.4% can be placed on the branching fraction B(t to bH_s16) for charged Higgs boson masses m(H_s16) in the range 90-140 GeV. After combination with results from a search for charged Higgs bosons in ttbar decays using the tau_had_s16jets final state, upper limits on B(t to bH_s16) can be set in the range 0.8%-3.4%, for m(H_s16) in the range 90-160 GeV.