Abstract
A search for singly produced vector-like Q quarks, where Q can be either a T quark with charge +2/3 or a Y quark with charge -4/3, is performed in proton–proton collisions recorded with the ATLAS detector at the LHC. The dataset corresponds to an integrated luminosity of 20.3 fb^{-1} and was produced with a centre-of-mass energy of sqrt{s}=8 TeV. This analysis targets Q rightarrow Wb decays where the W boson decays leptonically. A veto on massive large-radius jets is used to reject the dominant tbar{t} background. The reconstructed Q-candidate mass, ranging from 0.4 to 1.2 TeV, is used in the search to discriminate signal from background processes. No significant deviation from the Standard Model expectation is observed, and limits are set on the Q rightarrow Wb cross-section times branching ratio. The results are also interpreted as limits on the QWb coupling and the mixing with the Standard Model sector for a singlet T quark or a Y quark from a doublet. T quarks with masses below 0.95 TeV are excluded at 95 % confidence level, assuming a unit coupling and a BR(Trightarrow Wb) = 0.5, whereas the expected limit is 1.10 TeV.
Highlights
Background estimationThe multijet background is obtained from data using a matrix method [69] which predicts the shape and normalisation of the background process
The expected yields for signal masses of 0.7 and 0.9 TeV are shown alongside the background prediction, which includes the normalisation of the ttand W +jets event yields obtained in the fit control region (FitCR) and the number of events observed in data
The data used in this search correspond√to 20.3 fb−1of pp collisions at a centre-of-mass energy of s = 8 TeV
Summary
The multijet background is obtained from data using a matrix method [69] which predicts the shape and normalisation of the background process. This method relies on differences between the probability of a “real” (prompt) lepton and that of a “fake” (non-prompt or misidentified) lepton to fulfil certain selection criteria. The “fake” lepton efficiencies are measured in data using background-enriched control regions and are parameterised for different values of pT and η of the charged lepton candidate. 3. A fit control region (FitCR) is defined in order to estimate the normalisation of the ttbackground and of the W +jets background from data. Two additional W +jets-enriched CRs are defined to validate the modelling (W1CR and W2CR)
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