UC Irvine

The production of two prompt J/ψ$$J/\psi $$ mesons, each with transverse momenta pT>8.5$$p_{\mathrm {T}}>8.5$$ GeV and rapidity |y|<2.1$$|y| < 2.1$$, is studied using a sample of proton-proton collisions at s=8$$\sqrt{s} = 8$$ TeV, corresponding to an integrated luminosity of 11.4 fb-1$$^{-1}$$ collected in 2012 with the ATLAS detector at the LHC. The differential cross-section, assuming unpolarised J/ψ$$J/\psi $$ production, is measured as a function of the transverse momentum of the lower-pT$$p_{\mathrm {T}}$$J/ψ$$J/\psi $$ meson, di-J/ψ$$J/\psi $$pT$$p_{\mathrm {T}}$$ and mass, the difference in rapidity between the two J/ψ$$J/\psi $$ mesons, and the azimuthal angle between the two J/ψ$$J/\psi $$ mesons. The fraction of prompt pair events due to double parton scattering is determined by studying kinematic correlations between the two J/ψ$$J/\psi $$ 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 σeff=6.3±1.6(stat)±1.0(syst)$$\sigma _{\mathrm {eff}} = 6.3 \pm 1.6 \mathrm {(stat)} \pm 1.0 \mathrm {(syst)}$$ mb.