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

Abstract

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 $20.3\mathrm{f}{\mathrm{b}}^{-1}$ of data collected with the ATLAS experiment at the LHC in $pp$ collisions at $\sqrt{s}=8\mathrm{TeV}$, the first measurement of associated $Z+J/\psi$ 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 $J/\psi (\to {\mu }^{+}{\mu }^{-})$ 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 $J/\psi$ 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.

Published version, 15 pages + author list + cover page (30 pages total), 6 figures, 5 tables, All figures including auxiliary figures are available at https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/BPHY-2014-01/

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