A search is presented for pair production of a new heavy quark ($Q$) that decays into a $W$ boson and a light quark ($q$) in the final state where one $W$ boson decays leptonically (to an electron or muon plus a neutrino) and the other $W$ boson decays hadronically. The analysis is performed using an integrated luminosity of $20.3{\mathrm{fb}}^{-1}$ of $pp$ collisions at $\sqrt{s}=8\mathrm{TeV}$ collected by the ATLAS detector at the LHC. No evidence of $Q\overline{Q}$ production is observed. New chiral quarks with masses below 690 GeV are excluded at 95% confidence level, assuming $\mathrm{BR}(Q\to Wq)=1$. Results are also interpreted in the context of vectorlike quark models, resulting in the limits on the mass of a vectorlike quark in the two-dimensional plane of $\mathrm{BR}(Q\to Wq)$ versus $\mathrm{BR}(Q\to Hq)$.

• Received 14 September 2015

DOI:https://doi.org/10.1103/PhysRevD.92.112007

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© 2015 CERN, for the ATLAS Collaboration