Paper detail

Observation and measurements of the production of prompt and non-prompt $J/ψ$ 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 $J/ψ$ 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 $20.3\, \mathrm{fb^{-1}}$ of data collected with the ATLAS experiment at the LHC in $pp$ collisions at $\sqrt{s}=8\, \mathrm{TeV}$, the first measurement of associated $Z + J/ψ$ production is presented for both prompt and non-prompt $J/ψ$ production, with both signatures having a significance in excess of $5σ$. The inclusive production cross-sections for $Z$ boson production (analysed in $μ^+μ^-$ or $e^+e^-$ decay modes) in association with prompt and non-prompt $J/ψ(\toμ^+μ^-)$ 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 $J/ψ$ transverse momentum. The fraction of signal events arising from single and double parton scattering is estimated, and a lower limit of $5.3\ (3.7)\, \mathrm{mb}$ at $68\ (95)$% confidence level is placed on the effective cross-section regulating double parton interactions.