  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 fb$^{-1}$ of $pp$ collisions at $\sqrt{s} = 8$
TeV collected by the ATLAS detector at the LHC. No evidence of $Q\bar{Q}$
production is observed. New chiral quarks with masses below 690 GeV are
excluded at 95% confidence level, assuming 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 BR$(Q\to Wq)$
versus BR$(Q\to Hq)$.
