Abstract
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 → bH +, in addition to the Standard Model decay t → bW. Since W bosons decay to the three lepton generations equally, while H + maypredominantlydecayinto τν, 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−1 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 μ) and/or a hadronically decaying τ (τ had) are used. Event yield ratios between e + τ had and e + μ, as well as between μ + τ had andμ + 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 $ \mathcal{B} $ (H + → τν) is 100%, upper limits in the range 3.2%–4.4% can be placed on the branching fraction $ \mathcal{B} $ (t → 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 $ t\overline{t} $ decays using the τ had + jets final state, upper limits on $ \mathcal{B} $ (t → bH +) can be set in the range 0.8%–3.4%, for m H+ in the range 90-160 GeV.
Highlights
H+ may predominantly decay into τ ν, charged Higgs bosons can be searched for using the violation of lepton universality in 4.6 fb−1 of proton-proton collision top quar√k decays
For each of the four final states considered here (e + τhad, e + μ, μ + τhad and μ + e), the OS-SS event yield N can be split into two contributions: from ttevents and from all other SM processes except tt → bbW +W −
The event yields in the τhad+lepton and dilepton final states are summarised in table 2 for the background-only hypothesis, as well as in the presence of a 130 GeV charged Higgs boson in the top quark decay
Summary
At least 75% of the tracks associated to a jet (weighted by their transverse momenta) must point to the primary vertex, corresponding to the hardest interaction. This requirement on the “Jet Vertex Fraction” [15] allows the identification of jets originating from the hard-scatter interaction. In order to reconstruct hadronically decaying τ leptons, anti-kt jets with either one or three associated tracks, depositing ET > 10 GeV in the calorimeter, are considered as τ candidates [17]. The working point chosen for this study corresponds to an efficiency of about 30% for hadronically decaying τ leptons with pτT > 20 GeV in Z → τ τ events, leading to a rejection factor of about 100–1000 for jets. All generated events are propagated through a detailed GEANT4 simulation [23, 24], and they are reconstructed using the same algorithms as the data
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