Dissociative associated J/$ψ$ and dimuon production in Ap ultraperipheral collisions via double parton scattering

TL;DR

This work addresses DPS in ultraperipheral $Ap$ collisions involving photons and gluons to produce a dissociative $J/\psi$ alongside a dimuon. It develops a DPS pocket formula for the $\gamma\gamma\gamma g$ initial state and introduces an energy-dependent $\sigma_{\text{eff}}(\xi_1,\xi_2,\xi_3)$, highlighting a nontrivial dependence on photon momentum fractions. The dissociative $J/\psi$ production is modeled with the Color Evaporation Model and fitted to HERA and ALICE data, while exclusive dimuon production is treated via the EPA and validated against ALICE measurements. Predictions for differential cross sections at $\sqrt{s}=8.16$ TeV (LHC) and $62.8$ TeV (FCC) indicate a sizable DPS contribution with distinctive rapidity distributions, offering a new probe of the proton’s photon density and its correlation with the gluon density in UPCs.

Abstract

We study the dissociative associated production of a J/$ψ$ meson and a dimuon via double parton scattering (DPS) in nucleus--proton ultraperipheral collisions. This new channel, characterized by a rapidity gap, is sensitive to the photon and gluon distributions of the proton. We derive a DPS pocket formula for this process, together with a corresponding expression for the effective cross section. Furthermore, we demonstrate the kinematic dependence of the effective cross section and present predictions for the differential DPS cross section at LHC and FCC energies.

Figures (9)

• Figure 1: Illustration of the proposed process: DPS in $Ap$ UPC with proton dissociation. The initial state consists of three photons and one gluon, producing a $\mu^-\mu^+$ pair and a charm quark pair $c\bar{c}$ that subsequently hadronizes into a $J/\psi$ meson. The projectile nucleus $A$ remains intact after the collision.
• Figure 2: Photon distributions of the proton on the impact-parameter $b_{\gamma}$ plane for two values of the momentum fraction $\xi$. On the left, the distribution for $\xi = 0.1$ is more concentrated, while on the right, for $\xi = 0.001$, it is more spread out. In both cases, the distributions peak at $b_{\gamma} \sim 0.5$ fm, showing that the dominant photon contribution originates inside the proton.
• Figure 3: Differential cross section for dimuon ($\mu^- \mu^+$) production in $Pbp$ UPCs at the LHC energy of $\sqrt{s} = 8.16$ TeV as a function of the invariant mass, with the dimuon rapidity $Y_{\mu\mu}$ integrated over. Our theoretical calculations, labeled as "EPA", are compared with experimental data from the ALICE Collaboration ALICE:2023mfc, showing good agreement. We also include a comparison with results obtained using the "CT18qed_elastic" parton distribution function.
• Figure 4: Dissociative and inelastic $J/\psi$ production cross sections in $\gamma p$ collisions as a function of the $\gamma p$ c.m. energy $W$. The results are obtained by fitting the CEM to data from ALICE ALICE:2023mfc, ZEUS ZEUS:2012qog, and H1 H1:2013okqH1:2010udv. The CT18LO Yan:2022pzl and NNLO PDF4LHC21 PDF4LHCWorkingGroup:2022cjn gluon PDFs were used.
• Figure 5: SPS dissociative $J/\psi$ production cross section in $Pbp$ UPCs at the LHC ($\sqrt{s} = 8.16$ TeV) as a function of vector meson rapidity $Y_{J/\psi}$, compared with ALICE data ALICE:2023mfc.