  A search for a massive $W'$ gauge boson decaying to a top quark and a bottom
quark is performed with the ATLAS detector in $pp$ collisions at the LHC. The
dataset was taken at a centre-of-mass energy of $\sqrt{s} = 8$ TeV and
corresponds to 20.3 fb$^{-1}$ of integrated luminosity. This analysis is done
in the hadronic decay mode of the top quark, where novel jet substructure
techniques are used to identify jets from high-momentum top quarks. This allows
for a search for high-mass $W'$ bosons in the range $1.5 - 3.0$ TeV.
$b$-tagging is used to identify jets originating from $b$-quarks. The data are
consistent with Standard Model background-only expectations, and upper limits
at 95% confidence level are set on the $W' \rightarrow tb$ cross section times
branching ratio ranging from $0.16$ pb to $0.33$ pb for left-handed $W'$
bosons, and ranging from $0.10$ pb to $0.21$ pb for $W'$ bosons with purely
right-handed couplings. Upper limits at 95% confidence level are set on the
$W'$-boson coupling to $tb$ as a function of the $W'$ mass using an effective
field theory approach, which is independent of details of particular models
predicting a $W'$ boson.
