Associated $Z$ + $J/\ddot{I}ˆ$ production as a probe of multiparton interactions in the forward region

Abstract

This letter reports a study of associated $Z$ boson and prompt $J/\ddot{I}ˆ$ production in proton-proton collisions at $\sqrt{s}=13$ TeV with the LHCb detector in the forward rapidity region, using a data sample taken during 2016 to 2018 corresponding to an integrated luminosity of $5.1{\mathrm{fb}}^{-1}$. The measured fiducial cross-section is $5.5\pm 1.5$ pb, significantly exceeding the single-parton scattering expectation of $0.10\pm 0.08$ pb, indicating that multiparton interactions dominate this process in the explored phase space. Interpreted within the standard double-parton scattering framework, the data yields an effective cross-section $\ddot{I}{ƒ}_{\mathrm{eff}}=16.6\pm 4.7$ mb. This provides a direct experimental constraint on the transverse spatial structure of the proton in a kinematic regime characterized simultaneously by small Bjorken-$x$ and an electroweak hard scale set by the $Z$ boson mass.

14 pages, 6 figures. Submitted to Phys. Rev. Lett. All figures and tables, along with any supplementary material and additional information, are available at https://lbfence.cern.ch/alcm/public/analysis/full-details/4415 (LHCb public pages)