  This paper describes a measurement of fiducial and differential cross
sections of gluon-fusion Higgs boson production in the
$H{\rightarrow\,}WW^{\ast}{\rightarrow\,}e\nu\mu\nu$ channel, using 20.3
fb$^{-1}$ of proton-proton collision data. The data were produced at a
centre-of-mass energy of $\sqrt{s} = 8$ TeV at the CERN Large Hadron Collider
and recorded by the ATLAS detector in 2012. Cross sections are measured from
the observed $H{\rightarrow\,}WW^{\ast}{\rightarrow\,}e\nu\mu\nu$ signal yield
in categories distinguished by the number of associated jets. The total cross
section is measured in a fiducial region defined by the kinematic properties of
the charged leptons and neutrinos. Differential cross sections are reported as
a function of the number of jets, the Higgs boson transverse momentum, the
dilepton rapidity, and the transverse momentum of the leading jet. The jet-veto
efficiency, or fraction of events with no jets above a given transverse
momentum threshold, is also reported. All measurements are compared to QCD
predictions from Monte Carlo generators and fixed-order calculations, and are
in agreement with the Standard Model predictions.
