  The process pp--> W + J/psi provides a powerful probe of the production
mechanism of charmonium in hadronic collisions, and is also sensitive to
multiple parton interactions in the colliding protons. Using the 2011 ATLAS
dataset of 4.5 fb-1 of sqrt{s} = 7 TeV pp collisions at the LHC, the first
observation is made of the production of W + prompt J/psi events in hadronic
collisions, using W-->mu+nu and J/psi-->mu+mu. A yield of 27.4+7.5-6.5 W +
prompt J/psi events is observed, with a statistical significance of 5.1 sigma.
The production rate as a ratio to the inclusive W boson production rate is
measured, and the double parton scattering contribution to the cross section is
estimated.
