Measurement of the prompt J/$\psi$ pair production cross-section in pp collisions at $\sqrt{s}$ = 8 TeV with the ATLAS detector

The production of two prompt J/$\psi$ mesons with transverse momenta $p_{\mathrm{T}}>8.5$~GeV and rapidity $|y| < 2.1$ is studied using a sample of proton-proton collisions at $\sqrt{s} = 8$~TeV, corresponding to an integrated luminosity of 11.4 fb$^{-1}$ collected in 2012 with the ATLAS detector at the LHC. The differential cross-section is measured as a function of the transverse momentum of the lower-$p_{\mathrm{T}}$ J/$\psi$ meson, di-J/$\psi$ $p_{\mathrm{T}}$, and di-J/$\psi$ mass, the difference in the rapidity between the two J/$\psi$ mesons, and the difference in the azimuthal angle between the two J/$\psi$ mesons. The total and double parton cross-sections are compared with predictions. The fraction of prompt pair events due to double parton scattering is determined by studying kinematic correlations between the two J/$\psi$ mesons. The effective cross-section of double parton scattering is measured to be $\sigma_{\mathrm{eff}} = 8.7 \pm 1.1 \mathrm{(stat)} \pm 1.4 \mathrm{(syst)} \pm 0.1 \mathrm{(BF)} \pm 0.3 \mathrm{(lumi)}\, \mathrm{mb}$.