arxiv: 2604.26789 · v1 · submitted 2026-04-29 · ✦ hep-ph

Recognition: unknown

Effect of sub-nucleon fluctuations on the DVCS process in proton and nuclear targets at the EIC

J. Cepila , V. P. Goncalves , A. Ridzikova

Authors on Pith no claims yet

Pith reviewed 2026-05-07 13:12 UTC · model grok-4.3

classification ✦ hep-ph

keywords DVCShot-spot modelEICincoherent cross sectionsub-nucleon fluctuationscoherent scatteringnuclear targetsgluon distributions

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The pith

Sub-nucleon fluctuations modeled by hot spots cause a turnover in the energy dependence of the incoherent DVCS cross section at EIC.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

This paper investigates the role of sub-nucleon gluon fluctuations in the Deeply Virtual Compton Scattering process on protons and nuclei at the Electron-Ion Collider. By applying the hot-spot model, the authors calculate how these fluctuations affect the coherent and incoherent cross sections across different energies, photon virtualities, and nuclear sizes. A key result is that the incoherent cross section exhibits a maximum as energy increases, and the location of this maximum shifts depending on the virtuality of the incoming photon. The work also provides predictions for how the ratio of coherent to incoherent cross sections changes with these variables and for the shape of the momentum transfer distributions in nuclear targets.

Core claim

Assuming the hot-spot model for sub-nucleon fluctuations, the DVCS cross sections on proton and nuclear targets at EIC energies show that the incoherent contribution turns over with increasing center-of-mass energy, with the turnover position depending on the photon virtuality Q squared. The ratio of coherent to incoherent cross sections is predicted to increase with energy, atomic number, and lower Q squared. Additionally, the t-distribution for incoherent nuclear scattering displays a maximum whose position depends on the atomic number and Q squared.

What carries the argument

The hot-spot model, in which the proton is treated as a superposition of localized regions of high gluon density whose transverse positions fluctuate from event to event, used to compute the DVCS amplitude.

If this is right

The incoherent DVCS cross section for protons reaches a maximum at an energy that depends on the photon virtuality.
The ratio of coherent to incoherent cross sections increases with rising energy and larger atomic numbers but decreases at higher virtuality.
The t-distribution of the incoherent cross section for nuclei exhibits a peak that shifts with atomic number and photon virtuality at fixed energy.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

Such a turnover could provide a new way to probe the spatial distribution of gluons inside the nucleon using EIC data.
Similar fluctuation effects might influence other exclusive processes like J/psi production at the EIC.
Comparison with EIC measurements could help refine the parameters of the hot-spot model for better accuracy in nuclear physics calculations.

Load-bearing premise

The hot-spot model accurately represents the sub-nucleon gluon density fluctuations that determine the DVCS amplitude in proton and nuclear targets.

What would settle it

An EIC measurement of the energy dependence of the incoherent DVCS cross section at several values of photon virtuality that shows either no maximum or a maximum position independent of virtuality would falsify the central prediction.

Figures

Figures reproduced from arXiv: 2604.26789 by A. Ridzikova, J. Cepila, V. P. Goncalves.

**Figure 1.** Figure 1: FIG. 1. Transverse profile of a proton generated using the energy - dependent hot - spot model at view at source ↗

**Figure 2.** Figure 2: FIG. 2. Transverse profile of Ca (upper plots) and Pb (lower plots) generated using the energy - view at source ↗

**Figure 3.** Figure 3: FIG. 3. Predictions for the energy dependence of the cross - sections associated with coherent (left view at source ↗

**Figure 4.** Figure 4: FIG. 4. Predictions for the differential view at source ↗

**Figure 5.** Figure 5: FIG. 5. Energy dependence of the total DVCS cross - section associated with coherent (left panel) view at source ↗

**Figure 6.** Figure 6: FIG. 6. Predictions for the differential view at source ↗

read the original abstract

The impact of the sub - nucleon fluctuations on the Deeply Virtual Compton Scattering (DVCS) process at the Electron - Ion Collider (EIC) is investigated considering proton and nuclear targets. Assuming the hot - spot model, we estimate the energy dependence of the coherent and incoherent cross - sections for different values of the photon virtuality and atomic number. Predictions for the $t$ - distributions are also presented. We demonstrate that the sub - nucleon fluctuations in the proton, as described by the hot - spot model, implies a turn - over in the energy dependence of the incoherent cross - section, with the position of the maximum being dependent of the photon virtuality. Our results indicate that the ratio between the coherent and incoherent cross - sections increases with energy, atomic numbers and for smaller values of $Q^2$. Moreover, we predict a maximum in the $t$ - distribution of the nuclear incoherent cross - section at a fixed center - of - mass energy, which is dependent on the atomic number and $Q^2$.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Desk Editor's Note private letter to a colleague

The paper applies the hot-spot model to DVCS at EIC and predicts a Q^2-dependent turnover in the incoherent cross-section energy dependence, but the results rest on parameters fitted elsewhere without fresh validation at these kinematics.

read the letter

The main point is that the authors extend the hot-spot model for sub-nucleon gluon fluctuations to DVCS on protons and nuclei at EIC energies. They report a turnover in the W-dependence of the incoherent cross section whose location moves with photon virtuality, along with t-distributions for nuclei that peak at a position set by A and Q^2. The coherent-to-incoherent ratio is also shown to rise with energy, atomic number, and lower Q^2. These are concrete, previously unreported forecasts for EIC measurements. The work does a reasonable job spelling out how configuration averaging produces the turnover and the nuclear t-shape, and it treats both proton and nuclear targets in the same framework. That makes the outputs directly usable for planning experiments that could test whether fluctuations matter in this channel. The soft spot is that the hot-spot number, size, and energy evolution come from earlier fits to other data, and the paper does not demonstrate that the same setup reproduces existing DVCS or vector-meson results at comparable Q^2 before extrapolating to EIC. Because the turnover position depends on how the virtual-photon dipole overlaps with the fluctuating spots, any mismatch in that coupling shifts the predicted maximum. The stress-test concern about missing validation at the new kinematics therefore stands. This paper is for EIC phenomenologists and experimentalists working on small-x structure or nuclear gluon tomography who want specific observables to propose. A reader already using the hot-spot model will get the most out of the plots. I would send it to peer review because the topic is timely and the predictions are falsifiable, even though the model assumptions will need closer scrutiny in revision.

Referee Report

2 major / 2 minor

Summary. The manuscript investigates the impact of sub-nucleon gluon fluctuations, modeled via the hot-spot ansatz, on the DVCS process for both proton and nuclear targets at EIC kinematics. It computes the W-dependence of coherent and incoherent cross sections at varying Q^2 and atomic number A, reports a turnover in the incoherent proton cross section whose location shifts with Q^2, finds that the coherent-to-incoherent ratio grows with W, A, and decreasing Q^2, and predicts a maximum in the t-distribution of the nuclear incoherent cross section.

Significance. If the adopted hot-spot parameters correctly describe the fluctuations that enter the DVCS dipole amplitude, the results supply concrete, Q^2-dependent predictions that could be tested at the EIC to probe sub-nucleon structure in exclusive processes. The explicit linkage of the turnover position to the virtual-photon wave-function overlap and the extension to nuclear targets are useful features.

major comments (2)

[§2] §2 (hot-spot implementation): the model parameters (number, size, and energy evolution of hot spots) are taken directly from earlier fits without demonstrating that they reproduce existing DVCS or exclusive vector-meson data at comparable Q^2 and W; because the reported turnover in the incoherent cross section arises from the configuration average of the dipole scattering matrix element weighted by the photon wave function, this choice is load-bearing for the central claim.
[§4] §4 (incoherent results): the paper does not examine robustness under replacement of the hot-spot ansatz by an alternative fluctuation model (e.g., a fluctuating saturation scale in the IP-Sat framework); the turnover position is stated to depend on Q^2 through the photon-dipole overlap, so lack of such a test leaves open whether the feature is generic to sub-nucleon fluctuations or specific to the chosen parametrization.

minor comments (2)

[Abstract] Abstract: 'dependent of the photon virtuality' should read 'dependent on the photon virtuality'; inconsistent hyphenation in 'sub - nucleon'.
[§2] Notation: the averaging brackets for the incoherent cross section (<|A|^2> − |<A>|^2) are used without an explicit equation defining the configuration average over hot-spot positions; adding this would improve clarity.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the careful reading of our manuscript and the constructive comments. We address the two major comments point by point below. Where appropriate, we have revised the text to improve clarity and acknowledge limitations.

read point-by-point responses

Referee: [§2] §2 (hot-spot implementation): the model parameters (number, size, and energy evolution of hot spots) are taken directly from earlier fits without demonstrating that they reproduce existing DVCS or exclusive vector-meson data at comparable Q^2 and W; because the reported turnover in the incoherent cross section arises from the configuration average of the dipole scattering matrix element weighted by the photon wave function, this choice is load-bearing for the central claim.

Authors: The hot-spot parameters are adopted from earlier global fits to HERA inclusive and diffractive data, as cited in the manuscript. These same parameters have been employed in related calculations of exclusive vector-meson production at comparable kinematics. We agree that an explicit demonstration of consistency with existing DVCS measurements would strengthen the justification for the parameter set, given its central role in the turnover prediction. We will add a short paragraph in §2 summarizing the validation against HERA DVCS and vector-meson data, including a brief comparison of the energy dependence where data exist. This revision directly addresses the load-bearing nature of the choice. revision: yes
Referee: [§4] §4 (incoherent results): the paper does not examine robustness under replacement of the hot-spot ansatz by an alternative fluctuation model (e.g., a fluctuating saturation scale in the IP-Sat framework); the turnover position is stated to depend on Q^2 through the photon-dipole overlap, so lack of such a test leaves open whether the feature is generic to sub-nucleon fluctuations or specific to the chosen parametrization.

Authors: We acknowledge that a cross-check with an alternative fluctuation model would help establish whether the turnover is a generic consequence of sub-nucleon fluctuations or tied to the specific hot-spot implementation. Implementing and calibrating a different framework (such as fluctuating saturation scale within IP-Sat) requires substantial additional development beyond the scope of the present work, which is focused on the hot-spot ansatz. We will expand the discussion in §4 to note this limitation explicitly, reiterate that the turnover arises from configuration averaging weighted by the photon wave function (a feature that should appear in any model with sufficient sub-nucleon variance), and identify testing with other models as a worthwhile direction for future study. This revision clarifies the reach of our results without overclaiming generality. revision: partial

Circularity Check

0 steps flagged

No significant circularity; model application yields independent kinematic predictions

full rationale

The paper takes the hot-spot model as an established input (with parameters drawn from prior literature) and performs explicit calculations of the coherent and incoherent DVCS amplitudes via dipole scattering averaged over hot-spot configurations. The reported turn-over in the W-dependence of the incoherent cross section, and its Q^2 dependence, arises from the structure of the photon wave-function overlap with the fluctuating gluon field and the variance term < |A|^2 > - |<A>|^2; this is a derived dynamical feature, not a re-statement or statistical echo of the input parameters. No self-definitional loops, fitted quantities renamed as predictions, or load-bearing self-citations that close the argument are present. The results are presented as predictions for EIC kinematics and are in principle falsifiable by future data, keeping the derivation chain self-contained.

Axiom & Free-Parameter Ledger

1 free parameters · 1 axioms · 0 invented entities

Central results rest on the phenomenological hot-spot model whose parameters are adjusted to fit existing data, plus standard assumptions of high-energy QCD for DVCS amplitudes.

free parameters (1)

hot-spot model parameters
Number, transverse size, and spatial distribution of hot spots inside the proton and nuclei; these are fitted to previous proton structure data.

axioms (1)

domain assumption The hot-spot model provides a valid description of sub-nucleon fluctuations for calculating DVCS cross sections.
Invoked to compute coherent and incoherent amplitudes for proton and nuclear targets.

pith-pipeline@v0.9.0 · 5492 in / 1262 out tokens · 68081 ms · 2026-05-07T13:12:15.196491+00:00 · methodology

discussion (0)

Reference graph

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