  A measurement of the production of a prompt $J/\psi$ meson in association
with a $W^\pm$ boson with $W^\pm \rightarrow \mu\nu$ and $J/\psi \rightarrow
\mu^+ \mu^-$ is presented for $J/\psi$ transverse momenta in the range 8.5-150
GeV and rapidity $|y_{J/\psi}|<2.1$ using ATLAS data recorded in 2012 at the
LHC. The data were taken at a proton-proton centre-of-mass energy of $\sqrt{s}$
= 8 TeV and correspond to an integrated luminosity of 20.3 fb$^{-1}$. The ratio
of the prompt $J/\psi$ plus $W^{\pm}$ cross-section to the inclusive $W^{\pm}$
cross-section is presented as a differential measurement as a function of
$J/\psi$ transverse momenta and compared with theoretical predictions using
different double-parton-scattering cross-sections.
