Search for charged Higgs bosons through the violation of lepton universality in ttbar events using pp collision data at sqrt(s) = 7 TeV with the ATLAS experiment

TL;DR

This paper searches for charged Higgs bosons in top quark decays by analyzing lepton universality violation in ttbar events using 7 TeV pp collision data from ATLAS, setting upper limits on decay branching fractions.

Contribution

It introduces a ratio-based method to detect charged Higgs bosons via lepton universality violation, reducing systematic uncertainties and combining results from multiple decay channels.

Findings

No significant deviation from the Standard Model observed.

Upper limits on B(t to bH+) range from 0.8% to 4.4%.

Constraints on charged Higgs boson masses between 90 and 160 GeV.

Abstract

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.