  A search for a Higgs boson produced via vector-boson fusion and decaying into
invisible particles is presented, using 20.3 fb$^{-1}$ of proton--proton
collision data at a centre-of-mass energy of 8 TeV recorded by the ATLAS
detector at the LHC. For a Higgs boson with a mass of 125 GeV, assuming the
Standard Model production cross section, an upper bound of 0.28 is set on the
branching fraction of $H\to$ invisible at 95% confidence level, where the
expected upper limit is 0.31. The results are interpreted in models of
Higgs-portal dark matter where the branching fraction limit is converted into
upper bounds on the dark-matter--nucleon scattering cross section as a function
of the dark-matter particle mass, and compared to results from the direct
dark-matter detection experiments.
