[Paper Review] Gluon imaging using azimuthal correlations in diffractive scattering at the Electron-Ion Collider

Heikki Mäntysaari, Kaushik Roy+2 more|arXiv (Cornell University)|Nov 4, 2020

High-Energy Particle Collisions ResearchPhysics and Astronomy164 references69 citations

TL;DR

This paper proposes using azimuthal angle correlations between the scattered electron and produced photon or vector meson in diffractive electron-nucleus scattering at the Electron-Ion Collider (EIC) to probe spatial correlations in the gluon distribution of the target. Using the Color Glass Condensate (CGC) effective field theory with small-x JIMWLK evolution and spatially correlated McLerran-Venugopalan initial conditions, the authors compute differential cross-sections and find significant anisotropies (v₁,₂ ≈ 2–10%) in electron-proton collisions, while nuclear effects suppress these modulations in electron-gold collisions.

ABSTRACT

We study coherent diffractive photon and vector meson production in electron-proton and electron-nucleus collisions within the Color Glass Condensate effective field theory. We show that electron-photon and electron-vector meson azimuthal angle correlations are sensitive to non-trivial spatial correlations in the gluon distribution of the target, and perform explicit calculations using spatially dependent McLerran-Venugopalan initial color charge configurations coupled to the numerical solution of small $x$ JIMWLK evolution equations. We compute the cross-section differentially in $Q^2$ and $|t|$ and find sizeable anisotropies in the electron-photon and electron-$\mathrm{J}/\psi$ azimuthal correlations ($v_{1,2} \approx 2 - 10 \%$) in electron-proton collisions for the kinematics of the future Electron-Ion Collider. In electron-gold collisions these modulations are found to be significantly smaller ($v_{1,2} <0.1 \%$). We also compute incoherent diffractive production where we find that the azimuthal correlations are sensitive to fluctuations of the gluon distribution in the target.

Motivation & Objective

• To develop a method for imaging non-trivial spatial correlations in the gluon distribution of protons and nuclei using exclusive diffractive processes at the EIC.
• To investigate the sensitivity of electron-photon and electron-vector meson azimuthal correlations to the spatial structure of the target's gluon field.
• To quantify the impact of nuclear effects on azimuthal anisotropies in electron-nucleus collisions compared to electron-proton collisions.
• To explore the role of event-by-event fluctuations in the gluon distribution through incoherent diffractive production.
• To provide quantitative predictions for azimuthal correlations at EIC kinematics using realistic CGC-based models.

Proposed method

• Employing the Color Glass Condensate (CGC) effective field theory to describe high-density gluon dynamics at small x in electron-proton and electron-nucleus collisions.
• Using the dipole picture to compute exclusive electroproduction amplitudes, with virtual photon fluctuating into a q¯q pair that scatters off the target's color field.
• Implementing impact parameter-dependent dipole-target scattering amplitudes derived from spatially correlated McLerran-Venugopalan (MV) initial conditions.
• Applying the small-x JIMWLK evolution equations numerically to model the running of the saturation scale and gluon density with increasing energy.
• Computing differential cross-sections in Q² and |t|, and extracting azimuthal anisotropy coefficients v₁ and v₂ from the angular dependence of the electron-vector particle correlation.
• Comparing results from a simple GBW model with those from a more realistic CGC-based dipole amplitude including fluctuating color charge configurations.

Experimental results

Research questions

• RQ1 Can azimuthal correlations in diffractive electron-proton scattering at the EIC reveal non-trivial spatial correlations in the gluon distribution of the proton?
• RQ2 How do the azimuthal anisotropies (v₁, v₂) in electron-photon and electron-J/ψ production depend on the kinematic variables Q² and |t|?
• RQ3 To what extent are the azimuthal modulations suppressed in electron-gold collisions compared to electron-proton collisions due to nuclear effects?
• RQ4 Can incoherent diffractive production be used to probe event-by-event fluctuations in the spatial structure of the gluon distribution?
• RQ5 How do the predictions from the CGC framework with JIMWLK evolution compare to simpler models like GBW in describing azimuthal correlations?

Key findings

• The azimuthal correlations in electron-proton collisions exhibit significant anisotropies with v₁,₂ ≈ 2–10%, indicating sensitivity to non-trivial spatial correlations in the gluon distribution.
• In electron-gold collisions, the azimuthal anisotropies are strongly suppressed, with v₁,₂ < 0.1%, due to averaging over fluctuating nuclear configurations.
• The computed cross-sections show clear differential dependence on Q² and |t|, with the largest anisotropies occurring at moderate |t| values.
• The incoherent diffractive production channel is sensitive to event-by-event fluctuations in the gluon distribution, as demonstrated by the angular dependence of the correlations.
• The CGC-based model with JIMWLK evolution reproduces the qualitative features of the data and provides a quantitative framework for interpreting future EIC measurements.
• The results confirm that azimuthal correlations in exclusive diffractive processes are a viable probe of the elliptic component of the gluon Wigner distribution at small x.