Observation and measurements of the production of prompt and non-prompt $J/\psi$ mesons in association with a $Z$ boson in $pp$ collisions at $\sqrt{s}$ = 8 TeV with the ATLAS detector

The 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.