Associated quarkonium production at ATLAS

The associated production of a vector boson with quarkonia is a key new observable for understanding the quarkonium production mechanisms, including the separation of single and double parton scattering components. Using the 2011 ATLAS dataset of 4.5 $\mathrm{fb}^{-1}$ of $\sqrt{s} = $7 TeV $pp$ collisions at the LHC, the first observations were made of the production of $W^\pm$ + prompt $J/\psi$ events in hadronic collisions, and using 20.3 $\mathrm{fb}^{-1}$ of data collected with the ATLAS experiment, in $pp$ collisions at $\sqrt{s}=$ 8 TeV, the first measurements of associated $Z$ + $J/\psi$ production were made for both prompt and non-prompt $J/\psi$ production mechanisms. The cross section ratio of prompt $J/\psi$ production in association with a vector boson ($W^\pm$ and $Z^0$) to the inclusive vector boson production is measured, and the double parton scattering contribution to the cross section is estimated for both analyses. Normalised differential production cross-section ratios are also determined as functions of $J/\psi$ transverse momentum.