Opportunities for Imaging Light Nuclei with a Second Interaction Region at the Electron-Ion Collider

TL;DR

This paper investigates the potential of a second Electron-Ion Collider interaction region (IR-8) to image light nuclei via coherent diffractive vector-meson production in $e$A collisions. Using the eSTARlight generator and a pre-conceptual IR-8 layout with a secondary focus and far-forward detectors, the study assesses kinematic reach and tagging efficiencies across multiple light nuclei and collision energies, highlighting substantial gains in low-$p_T$ acceptance for light ions. The results quantify detection efficiencies as functions of $W$, $Q^{2}$, and $x$, and show how different vector mesons extend the accessible phase space; crucially, the $|t|$-distributions can be Fourier-transformed to image the transverse gluon distributions $F(b)$, illustrating the imaging potential of the IR-8 configuration. Overall, the work demonstrates that incorporating a secondary focus at IR-8 can substantially enhance exclusive, tagging, and diffractive measurements for light nuclei at the EIC, enabling complementary physics to the existing IR-6 program and guiding future detector-and-optics optimization for spatial-parton imaging.

Abstract

The upcoming Electron-Ion Collider (EIC) will address several outstanding puzzles in modern nuclear physics. Key questions, such as the partonic structure of nucleons and nuclei and the origin of their mass and spin, can be explored through high-energy electron-proton and electron-nucleus collisions. To maximize its scientific reach, the EIC community has advocated for the addition of a second interaction region equipped with a detector complementary to the EIC general purpose collider detector, ePIC. The pre-conceptual design of this interaction region aims to provide a different configuration from the first interaction region, which enhances forward acceptance at very small scattering angles ($θ\sim 0$ mrad). This machine configuration would significantly benefit exclusive, tagging, and diffractive physics programs, complementing those of the ePIC experiment. In particular, accessing coherent diffractive processes on light nuclei by tagging of the full, intact nucleus is essential for mapping their spatial parton distributions. In this work, we present a systematic study of the detection capabilities for light nuclei at a second EIC interaction region, with a detailed discussion of the accessible kinematic phase space and its implications for imaging.

Figures (7)

• Figure 1: The layout of the far-forward IR design for the outgoing hadron beam direction at the second interaction region (IR-8) of the EIC. It includes four dipole (green), five quadrupole (blue) magnets in the ion beamline, and four far-forward detector subsystems. The figure is taken from PhysRevD.111.072013.
• Figure 2: The detection efficiency as a function of $W$ for coherent $J/\psi$ production in various light nuclei. The results for different coherent nuclei are shown with different colors.
• Figure 3: The correlation of $Q^{2}$ and $x$ for detection efficiency for coherent $J/\psi$ production in $e$$^{2}$D, $e$$^{7}$Li, and $e$$^{16}$O collisions with the top collision energies.
• Figure 4: The correlation of $Q^{2}$ and $x$ for coherent $J/\psi$ production in $e$$^{3}$He collisions with different energies: $18\times183~\rm{GeV}^{2}$, $10\times100~\rm{GeV}^{2}$, and $5\times41~\rm{GeV}^{2}$. Top row: the distributions for MC-generated events by eSTARlight. Bottom row: the distributions for detection efficiency of the far-forward detectors of IR-8 at EIC.
• Figure 5: The detection efficiency as a function of $W$ for different coherent VM productions of $e^{3}\rm{He}$ with collision energy of $18\times183~\rm{GeV}^{2}$. The results with different VM productions are represented by different colors.