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arxiv: 1907.06471 · v1 · pith:72ZOUN6Hnew · submitted 2019-07-15 · ✦ hep-ph

Towards a determination of the low x gluon via exclusive J/psi production

Chris A. Flett , Stephen P. Jones , Alan D. Martin , Misha G. Ryskin , Thomas Teubner This is my paper

Pith reviewed 2026-05-24 21:20 UTC · model grok-4.3

classification ✦ hep-ph
keywords exclusive J/psi productionlow x gluonphotoproductioncoefficient functionsglobal PDF fitsQ0 cutMS-bar scheme
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The pith

A Q0 cut tames the MS-bar coefficient functions to stabilize exclusive J/ψ photoproduction predictions.

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

The paper demonstrates that inserting a Q0 cut into the known MS-bar coefficient functions accounts for a formally power-suppressed but numerically large correction in exclusive J/ψ photoproduction. This taming improves the stability of the cross-section prediction. With greater stability the exclusive data become suitable for inclusion in a global PDF fit. The ultimate aim is to extract tighter constraints on the gluon at small x from these measurements. A sympathetic reader cares because small-x gluon knowledge directly affects high-energy collider predictions for many processes.

Core claim

By systematically incorporating a Q0 cut to account for the power-suppressed correction in the MS-bar coefficient functions, the stability of the theoretical prediction for exclusive J/ψ photoproduction is improved, opening the way to include such data in global fits to determine the low-x gluon.

What carries the argument

The Q0 cut applied to the MS-bar coefficient functions, which encodes the dominant power-suppressed correction and tames their numerical behavior.

If this is right

  • Exclusive J/ψ photoproduction data can be added to global PDF fits while preserving fit stability.
  • The low-x gluon distribution receives additional constraints from the photoproduction measurements.
  • Theoretical uncertainties in the predicted cross sections are reduced by the tamed coefficient functions.

Where Pith is reading between the lines

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

  • The same Q0-cut procedure could be applied to other exclusive vector-meson channels to test consistency of the small-x constraints.
  • If the method works, future fits might combine photoproduction data with inclusive deep-inelastic scattering to reduce extrapolation uncertainties below x of 10 to the minus 4.

Load-bearing premise

The chosen Q0 cut correctly captures the dominant power-suppressed correction in a way that remains valid when the improved coefficient functions are inserted into a global PDF fit without introducing uncontrolled biases or double-counting.

What would settle it

Performing a global PDF fit that includes the exclusive J/ψ data with the Q0-improved coefficient functions and finding that the resulting gluon distribution produces inconsistent predictions for other measured processes would falsify the claim that the cut provides a reliable improvement.

Figures

Figures reproduced from arXiv: 1907.06471 by Alan D. Martin, Chris A. Flett, Misha G. Ryskin, Stephen P. Jones, Thomas Teubner.

Figure 1
Figure 1. Figure 1: The two independent subprocesses contributing to the ultraperipheral cross section. The exchange at energy W+ allows for a probe of smaller x, while the W− exchange is typically at larger x, where there is already significant data support from HERA. The qT of the photon is small. γ C LO g V (X + ξ)P + (X − ξ)P + p p Fg 0 γ C NLO q V (X + ξ)P + (X − ξ)P + p p Fq 0 l [PITH_FULL_IMAGE:figures/full_fig_p003_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: Five leg pomeron-like exchange diagrams at LO (left panel) and NLO (right panel). The pertur￾batively calculable coefficients are denoted by Cg,q and the GPDs by Fg,q, with parton momentum fractions x = X +ξ and x 0 = X −ξ . that the input distribution has no singularities in the right-half of the Mellin-N plane, one may re￾late the two outside the timelike region X ∈ [−ξ ,ξ ] with O(x) accuracy via the Sh… view at source ↗
Figure 3
Figure 3. Figure 3: MS scale variations of ImA/W2 at LO and NLO generated using CTEQ6.6 global partons at µ 2 F = µ 2 R = 1.2,2.4,4.8GeV2 . ImA is the imaginary part of the amplitude. lies in the high energy asymptotics of the NLO contribution, which contains a double logarithm αs ln(1/ξ )ln(µF/mc). In the small x (small ξ ) domain, this term leads to a large enhancement of the amplitude. However, we find that by choosing the… view at source ↗
Figure 4
Figure 4. Figure 4: Results of ImA/W2 vs. W for the scale fixing procedure (left panel) and the implementation of a Q0 cut (right panel), using CTEQ6.6 partons with µ 2 F = m 2 c = 2.4GeV2 fixed. at Q0. It ends at µF = mc, our ‘optimal’ factorisation scale. Therefore, to maintain the typical hiearchy of scales, one would like Q0 ≤ µF. Clearly, however, Q0 should be of the order of the starting scale of the PDF fit. We thus ha… view at source ↗
Figure 5
Figure 5. Figure 5: The quark contribution is seen to be almost vanishing and the scale dependence is small throughout the HERA and LHCb regions (left panel). Plot showing the large uncertainty between the global parton predictions in the LHCb regime. The typical x ' 2ξ values probed are shown on the upper axis. (right panel) Within a given global fit, we obtain stability of the cross section prediction with Q0 = µF = mc and … view at source ↗
read the original abstract

We discuss how the stability of the theoretical prediction for exclusive $J/\psi$ photoproduction has been improved through a systematic taming of the known $\overline{\text{MS}}$ coefficient functions by accounting for a formally power suppressed, but numerically significant, correction encoded within a $Q_0$ cut. The phenomenological implications of this will be emphasised meaning, ultimately, the possibility to include the exclusive data into a global fitter framework to provide constraints on the small $x$ gluon.

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.

Referee Report

2 major / 1 minor

Summary. The manuscript proposes improving the stability of theoretical predictions for exclusive J/ψ photoproduction by taming the known MS-bar coefficient functions via a Q0 cut that encodes a formally power-suppressed but numerically significant correction. The goal is to enable inclusion of exclusive data in global PDF fits to better constrain the small-x gluon.

Significance. If the Q0 cut can be shown to capture the dominant correction without introducing biases or double-counting when inserted into a global fit, the approach would offer a valuable independent constraint on the low-x gluon, which remains poorly determined by inclusive DIS data alone. The paper's emphasis on phenomenological implications highlights this potential, though the absence of explicit cross-checks against other datasets limits the assessed impact.

major comments (2)
  1. [Abstract] Abstract: the central claim that the Q0 cut systematically tames the coefficient functions and improves stability is stated without an explicit derivation, numerical validation, or comparison to data or prior calculations, making it impossible to assess whether the improvement is robust or merely an adjustment.
  2. The introduction of the Q0 cut (free parameter) to account for power-suppressed terms risks circularity or bias in global fits; no cross-check is provided demonstrating that the chosen Q0 value preserves separation from inclusive DIS or other small-x data and does not shift the extracted gluon outside prior uncertainties.
minor comments (1)
  1. Notation for the Q0 cut and its relation to the coefficient functions should be defined more explicitly at first use to aid readability.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the careful reading and constructive comments. We respond point-by-point to the major comments below.

read point-by-point responses

  1. Referee: [Abstract] Abstract: the central claim that the Q0 cut systematically tames the coefficient functions and improves stability is stated without an explicit derivation, numerical validation, or comparison to data or prior calculations, making it impossible to assess whether the improvement is robust or merely an adjustment.

    Authors: The abstract is a concise summary. The explicit derivation of the Q0 cut to account for power-suppressed corrections, together with numerical validation showing improved stability of the coefficient functions and comparisons to prior calculations without the cut, are provided in the body of the manuscript (Sections 2 and 3). We can revise the abstract to include a brief reference to these validations for improved clarity. revision: partial

  2. Referee: The introduction of the Q0 cut (free parameter) to account for power-suppressed terms risks circularity or bias in global fits; no cross-check is provided demonstrating that the chosen Q0 value preserves separation from inclusive DIS or other small-x data and does not shift the extracted gluon outside prior uncertainties.

    Authors: The Q0 value is fixed by the theoretical structure of the power-suppressed terms in the coefficient functions rather than being adjusted to the exclusive data. The manuscript demonstrates the resulting stability improvement but does not perform a global PDF fit, so no shift of the gluon distribution is claimed or shown. The kinematics of exclusive J/psi photoproduction are distinct from inclusive DIS, supporting separation. We agree that explicit cross-checks within a global fit would be valuable but lie beyond the scope of the present work, which focuses on the coefficient-function improvement to enable such fits. revision: no

Circularity Check

0 steps flagged

No significant circularity in the derivation chain.

full rationale

The paper proposes a methodological improvement to the stability of exclusive J/ψ photoproduction predictions by incorporating a Q0 cut to account for power-suppressed corrections in the known MS-bar coefficient functions, with the goal of enabling future inclusion of such data in global PDF fits. No load-bearing step reduces by construction to a fitted parameter, self-definition, or self-citation chain; the Q0 correction is presented as an external adjustment to existing coefficient functions rather than a quantity derived from or tuned to the target exclusive data within this work. The central claim remains a forward-looking phenomenological discussion without equations or results that are forced by the paper's own inputs.

Axiom & Free-Parameter Ledger

1 free parameters · 0 axioms · 0 invented entities

The approach rests on the assumption that a single cutoff scale Q0 can systematically encode the dominant power-suppressed terms without further free parameters or ad-hoc adjustments.

free parameters (1)
  • Q0
    Cutoff scale introduced to tame the coefficient functions; its specific value is not stated in the abstract but is central to the claimed improvement.

pith-pipeline@v0.9.0 · 5623 in / 1111 out tokens · 26147 ms · 2026-05-24T21:20:11.335110+00:00 · methodology

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Reference graph

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