  The results of a search for charged Higgs bosons decaying to a $\tau$ lepton
and a neutrino, $H^{\pm} \rightarrow \tau^{\pm} \nu$, are presented. The
analysis is based on 19.5 fb$^{-1}$ of proton-proton collision data at
$\sqrt{s}=8$ TeV collected by the ATLAS experiment at the Large Hadron
Collider. Charged Higgs bosons are searched for in events consistent with
top-quark pair production or in associated production with a top quark. The
final state is characterised by the presence of a hadronic $\tau$ decay,
missing transverse momentum, $b$-tagged jets, a hadronically decaying $W$
boson, and the absence of any isolated electrons or muons with high transverse
momenta. The data are consistent with the expected background from Standard
Model processes. A statistical analysis leads to 95\% confidence-level upper
limits on the product of branching ratios $ {\cal B}(t\rightarrow bH^\pm)
\times {\cal B} (H^\pm\rightarrow \tau^{\pm}\nu) $, between 0.23% and 1.3% for
charged Higgs boson masses in the range 80-160 GeV. It also leads to 95%
confidence-level upper limits on the production cross section times branching
ratio, $\sigma(pp \rightarrow tH^{\pm} + X) \times {\cal B}(H^{\pm} \rightarrow
\tau^{\pm} \nu)$, between 0.76 pb and 4.5 fb, for charged Higgs boson masses
ranging from 180 GeV to 1000 GeV. In the context of different scenarios of the
Minimal Supersymmetric Standard Model, these results exclude nearly all values
of $\tan\beta$ above one for charged Higgs boson masses between 80 GeV and 160
GeV, and exclude a region of parameter space with high $\tan\beta$ for
$H^{\pm}$ masses between 200 GeV and 250 GeV.
