Measurement of the prompt J / ψ pair production cross-section in pp collisions at s = 8 …

The production of two prompt $J/\\psi$ mesons, each 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, assuming unpolarised $J/\\psi$ production, 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 mass, the difference in rapidity between the two $J/\\psi$ mesons, and the azimuthal angle between the two $J/\\psi$ mesons. The fraction of prompt pair events due to double parton scattering is determined by studying kinematic correlations between the two $J/\\psi$ mesons. The total and double parton scattering cross-sections are compared with predictions. The effective cross-section of double parton scattering is measured to be $\\sigma_{\\mathrm{eff}} = 6.3 \\pm 1.6 \\mathrm{(stat)} \\pm 1.0 \\mathrm{(syst)} \\pm 0.1 \\mathrm{(BF)} \\pm 0.1 \\mathrm{(lumi)}$ mb.