ATLAS collaboration (2818 authors)Aguilar-Saavedra, Juan AntonioAmor Dos Santos, Susana PatriciaAnjos, NunoAraque, Juan PedroCantrill, RobertCarvalho, JoãoCastro, Nuno FilipeConde Muiño, PatriciaDa Cunha Sargedas De Sousa, Mario JoseFiolhais, MiguelGalhardo, BrunoGomes, AgostinhoGonçalo, RicardoJorge, PedroLopes, LourencoMachado Miguens, JoanaMaio, AméliaManeira, JoséOnofre, AntónioPalma, AlbertoPedro, RutePina, João AntonioPinto, BelmiroSantos, HelenaSaraiva, JoãoSilva, JoséTavares Delgado, AdemarVeloso, FilipeWolters, Helmut2019-02-042019-02-042015-05-27http://dx.doi.org/10.1140/epjc/s10052-015-3406-9http://hdl.handle.net/10400.26/26749The production of a $Z$ boson in association with a $J/\psi $ meson in proton–proton collisions probes the production mechanisms of quarkonium and heavy flavour in association with vector bosons, and allows studies of multiple parton scattering. Using $20.3\,\text {fb}^{-1}$ of data collected with the ATLAS experiment at the LHC in $pp$ collisions at $\sqrt{s}=8\,\text {TeV}$ , the first measurement of associated $Z\, +\, J/\psi $ production is presented for both prompt and non-prompt $J/\psi $ production, with both signatures having a significance in excess of $5\sigma $ . The inclusive production cross-sections for $Z$ boson production (analysed in $\mu ^+\mu ^-$ or $e^+e^-$ decay modes) in association with prompt and non-prompt $J/\psi (\rightarrow \mu ^+\mu ^-)$ are measured relative to the inclusive production rate of $Z$ bosons in the same fiducial volume to be $(36.8\pm 6.7\pm 2.5)\, \times \, 10^{-7}$ and $(65.8\pm 9.2\pm 4.2)\, \times \, 10^{-7}$ respectively. Normalised differential production cross-section ratios are also determined as a function of the $\text {J}\uppsi $ transverse momentum. The fraction of signal events arising from single and double parton scattering is estimated, and a lower limit of $5.3\ (3.7)\,\text {mb}$ at $68\ (95) \%$ confidence level is placed on the effective cross-section regulating double parton interactions.engjournal article2019-02-04