First evidence of prompt J/$\psi $ production in association with a W boson and measurement of prompt J/$\psi +$W cross-section with 4.7 fb$-$1 of data at $\surd $s$=$ 7 TeV at the Large Hadron Collider

The process pp $\to $W$+$/- J/$\psi $ provides a powerful probe of the production mechanism of charmonium in hadronic collisions. Using the full 2011 ATLAS dataset corresponding to an integrated luminosity of 4.7 fb$-$1, we have observed the first evidence of the production of W$+$/- $+$ prompt J/$\psi $ events in proton-proton collisions in the W $+$ J/$\psi \to $ $\mu +$/- $\mu +\mu -$ channel, and measured the differential cross-section for this process.