Measurement of the production cross section of prompt $J/\psi$ mesons in association with a $W^{\pm}$ boson in $pp$ collisions at $\sqrt{s}=7$ TeV

The process $pp \to W^{\pm} J/\psi$ 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 4.6 $fb^{-1}$ of $pp$ collisions at $\sqrt{s} = 7$ TeV, the first observation is made of the production of $W^{\pm}$ + prompt $J/\psi$ events in hadronic collisions where the $W^{\pm}$ boson and the $J/\psi$ both decay to muons. A total of $29.2^{+7.5} _{-6.5} $ $W^{\pm}$ + prompt $J/\psi$ events are observed, with a statistical significance of $5.3\sigma$. The production rate as a ratio to the inclusive $W^{\pm}$ boson production rate is measured, and the double parton scattering contribution to the cross section is estimated.