Associated production of $J/\psi$ pairs with the ATLAS detector

A recent measurement of prompt $J/\psi$ pair production is presented, using a sample of 11.4 fb$^{-1}$ of proton-proton collision data collected at $\sqrt{s} = 8$ TeV in the ATLAS detector. The differential cross-section is measured as a function of kinematic distributions for the lower-$p_T J/\psi$ meson and for the di-$J/\psi$ system. A data-driven approach is used to extract the fraction of prompt $J/\psi$ pair events due to double parton scattering and an effective cross-section of double parton scattering is measured to be $\sigma_{eff} = 6.3 \pm 1.6 (\text{stat}) \pm 1.0 (\text{syst}) \pm 0.1 (\text{BF}) \pm 0.1 (\text{lumi})$ mb.