Threshold $J/ψ$ Photoproduction as a Probe of Nuclear Gluon Structure

TL;DR

This work proposes a high-luminosity Hall D experiment using the GlueX detector and a 12 GeV electron beam to study incoherent J/psi photoproduction from $^4$He near and below the photoproduction threshold. By measuring the semi-inclusive channel $^4$He$(\gamma,J/psi p)X$ across a wide $E_\gamma$ range and reconstructing the initial proton momentum, the program aims to constrain possible large-$x$ gluon modifications in bound nucleons and to explore exotic gluonic mechanisms such as hidden-color configurations. Building on prior threshold measurements and the first observation of threshold J/psi production from nuclei, this effort seeks to deliver high-precision cross sections and differential observables ($\alpha_{miss}$, $|t|$) that will dramatically improve our understanding of nuclear gluon structure and its connection to the EMC effect. The expected outcome includes tighter constraints on gluon PDFs in nuclei at large $x$, enhanced insights into SRC-driven dynamics, and potential discovery of new gluonic phenomena in dense nuclear environments, with broad impact on nuclear QCD modeling.

Abstract

The nuclear EMC effect is the observation that quark distributions in bound nucleons experience significant modification at large $x$ relative to free nucleons. Despite decades of measurements verifying the presence of this effect in quarks across a wide range of nuclei, behavior of large-$x$ gluons in nuclei remains almost completely unknown. As the nuclear physics community seeks out new observables to try to elucidate the mechanisms behind the EMC effect, it becomes striking that we remain ignorant regarding the impact of nuclear effects on gluonic behavior. Recent photonuclear data using the Hall D photon beam have enabled the first measurement of $J/ψ$ photoproduction from nuclei near and below the energy threshold, with the results highlighted in Physical Review Letters as an Editors' Suggestion. These data have placed the first, and currently only, constraints on the behavior of large-$x$ gluons within bound nucleons. However, compared to the quantity of data which currently informs our knowledge of the quark-sector EMC effect, these data are extremely limited, and remain unable to conclusively observe or exclude large modification of gluon distributions. A high-luminosity photonuclear experiment will enable a precision measurement of incoherent $J/ψ$ photoproduction at and below the threshold region. This data will provide the first stringent constraints on nuclear modification of gluon structure or other exotic effects which could impact the production of $J/ψ$ from nuclei. We request 85 PAC days at Hall D using the GlueX detector with a 12 GeV electron beam energy and a coherent photon peak energy of $8$ GeV, split into 80 days using a $^4$He target and 5 calibration days using a $^2$H target.

Figures (12)

• Figure 1: Ratios of nPDFs for $^{208}$Pb to the free-proton PDFs for different parton flavors, from the analysis of Ref. 2660149. The shaded bands on the full nPDF models show the constraints given the current experimental data. We note in particular the figure on the top-right, which shows the gluon nPDF ratio. It can be seen that gluon distributions in nuclei at large $x$ are poorly-constrained at present.
• Figure 2: Measurements of $J/\psi$ photoproduction a Jefferson Lab. Left: The total cross section $\sigma(\gamma p\rightarrow J/\psi p)$ as a function of $E_\gamma$ from Ref. adhikari2023measurement. Right: Kinematic coverage of the measurement $\frac{d\sigma}{dt}(\gamma p\rightarrow J/\psi p)$ in bins of $E_\gamma$ and $t$ from Ref. HallCJPsi.
• Figure 3: Left: Averaged A$(\gamma,J/\psi p)X$ cross section for $^4$He and $^{12}$C as a function of beam photon energy, compared with plane-wave calculations for mean-field (dotted red) and SRC (dashed blue) contributions as well as the total (dot-dashed grey). Data also include a common 23% normalization uncertainty (not shown). Right: Differential A$(\gamma,J/\psi p)X$ cross section as a function of light-cone momentum fraction $\alpha_\text{miss}$, for $^4$He and $^{12}$C, separated into the above-threshold (top) and below-threshold (bottom) energy regions. Insets shows the differential cross section as a function of momentum transfer $|t|$. Measured data (black points) are compared with plane-wave calculations (blue solid line), as well as calculations assuming a modified proton density (orange dashed) and a modified form factor (green dot-dashed). Data also include a common 23% normalization uncertainty (not shown). Figures from Ref. pybus2024measurementnearsubthresholdjpsi.
• Figure 4: Left: Diagram of possible hidden-color state with coupling to $J/\psi$ photoproduction Brodsky_2001. In this process gluons can be exchanged with both "color octet" states within the color-sharing system, resulting in color-neutral nucleons in the final-state. Right: Calculated cross section for incoherent $\gamma D\rightarrow J/\psi pn$. The solid line shows the calculation for quasi-free photoproduction from the proton, and the dashed (dotted) lines show the contribution from hidden-color components as 0.1 (1)% of the nucleus.
• Figure 5: Simulated kinematic distributions for the final-state particles for $(\gamma, J/\psi p)$ production from mean-field protons. The electron (left) and positron (center) have a wide distribution of kinematics but a strong correlation between the momentum and the angle of the lepton. The proton (right) consistently is produced at moderate angles and low momentum.