Exclusive diffractive processes in electron-ion collisions

TL;DR

The paper tackles how to probe non-linear QCD and gluon saturation in nuclei via exclusive diffractive processes in electron-ion collisions. It develops a dipole-model framework (bSat) extended to eA and implements the Sartre Monte Carlo event generator to compute coherent and incoherent diffractive cross-sections, predicting differential distributions in Q^2 and t and enabling extraction of the nuclear gluon spatial distribution. The authors show that the coherent t-spectrum can be Fourier-transformed to obtain F(b), the transverse gluon density, while the incoherent component reveals nuclear fluctuations; phi production offers heightened sensitivity to saturation, with J/ψ serving as a complementary probe. The framework is also applicable to Ultra Peripheral Collisions, linking eA physics to UPC measurements at RHIC/LHC, and exhibits reasonable agreement with existing data within the validity range of the model.

Abstract

We present a new technique to calculate the cross-section for diffractive vector meson production and DVCS in electron-ion collisions based on the dipole model. The measurement of these processes can provide valuable information on non-linear QCD phenomena, such as gluon saturation, and is the the only known way to gain insight into the spatial distribution of gluons in nuclei. We present predictions of differential cross-section distribution $dσ/dQ^2$ and $dσ/dt$ for $J/ψ$ and $φ$ meson production for diffractive processes of heavy nuclei and demonstrate the feasibility of extracting the gluon source distribution of heavy nuclei, F(b), from coherent diffraction. We briefly introduce a new event generator based on our method that can be used for studying exclusive diffractive processes at a future electron-ion collider.

Figures (8)

• Figure 1: (Color online) A schematic picture of the dipole model and its variables. See text for details.
• Figure 2: (Color online) (a) The resulting coherent and (b) total cross-section for $\gamma^*A\rightarrow\gamma^*J/\psi A$, averaged over 10, 100, 500 and 800 configurations. As reference, the coherent analytical average described by eq. \ref{['eq:analytical']} is also shown.
• Figure 3: (Color online) In (a) and (b) the wave-overlap between the virtual photon and produced vector mesons are shown for transverse and longitudinal polarizations respectively, as functions of dipole radius $r$. In the third panel the dipole cross-section is shown as a function of $r$, with bSat (solid) and bNonSat (dashed) for protons (black) and gold ions (red/grey).
• Figure 4: (Color online) (a) Cross-sections for $J/\psi$ production differential in $Q^2$ for $ep$ and $e$Au collisions for both bSat and bNonSat dipole models. The cross-sections are scaled by 1/A$^{4/3}$. (b) Ratio of $e$A to $ep$ cross-sections for both models.
• Figure 5: (Color online) (a) Cross-sections for $\phi$ production differential in $Q^2$ for $ep$ and $e$Au collisions for both bSat and bNonSat dipole models. The cross-sections are scaled by 1/A$^{4/3}$. (b) Ratio of $e$A to $ep$ cross-sections for both models.