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arxiv: 1906.10049 · v1 · pith:SIGJTGC2new · submitted 2019-06-24 · ✦ hep-ph · hep-ex· nucl-ex· nucl-th

Prompt {J/psi}-pair production at the LHC: impact of loop-induced contributions and of the colour-octet mechanism

Jean-Philippe Lansberg , Hua-Sheng Shao , Nodoka Yamanaka , Yu-Jie Zhang This is my paper

Pith reviewed 2026-05-25 17:29 UTC · model grok-4.3

classification ✦ hep-ph hep-exnucl-exnucl-th
keywords double J/psi productionloop-induced contributionscolor-octet mechanismdouble parton scatteringNRQCDLHCprompt quarkoniumgluon correlations
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The pith

Loop-induced color-singlet contributions become the leading single-parton term at large rapidity separations in double J/psi production but remain too small to explain the data without double-parton scattering.

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

The paper examines two single-parton-scattering mechanisms that could be enhanced in kinematic regions where double-parton scattering is expected to dominate double J/psi production at the LHC. It isolates a gauge-invariant infrared-safe subset of loop-induced color-singlet diagrams and surveys color-octet contributions with multiple sets of long-distance matrix elements. The loop-induced terms are shown to dominate other color-singlet single-parton contributions at large rapidity separation yet fall short of observed yields. Only two choices of matrix elements visibly alter the combined NRQCD prediction, and only in the rapidity-separation and pair-mass distributions; these changes leave the ATLAS control regions for double-parton studies untouched.

Core claim

We have critically examined two mechanisms of single parton scatterings (SPS) that may be kinematically enhanced where DPS are thought to be dominant. First, we have considered a gauge-invariant and infrared-safe subset of the loop-induced contribution via Colour-Singlet transitions. We have found it to become the leading CS SPS contributions at large rapidity separation, yet too small to account for the data without invoking the presence of DPS yields. Second, we have surveyed the possible Colour-Octet contributions using both old and up-to-date non-perturbative long distance matrix elements. We have found that the pure CO yields crucially depend on the LDMEs. Among all the LDMEs we used, 2

What carries the argument

Gauge-invariant infrared-safe subset of loop-induced color-singlet diagrams together with color-octet long-distance matrix elements in NRQCD

If this is right

  • Double-parton scattering remains required to match the observed prompt double-J/psi rates at large rapidity separations.
  • The ATLAS control regions chosen for double-parton-scattering extraction are robust against the examined higher-order single-parton contributions.
  • Only specific long-distance matrix elements produce visible changes, and these appear only in the rapidity-separation and pair-invariant-mass distributions.
  • Studies aiming to extract linearly polarised gluon distributions from double J/psi can treat these single-parton mechanisms as sub-dominant in the relevant phase space.

Where Pith is reading between the lines

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

  • The result supports continued use of double J/psi as a clean probe of gluon-gluon correlations inside the proton.
  • Similar loop-induced subsets may warrant examination in other heavy-quarkonium pair channels at the LHC.
  • Refined global fits of color-octet matrix elements could be tested directly against the two distributions where changes were found.

Load-bearing premise

The assumption that the selected gauge-invariant infrared-safe subset of loop-induced color-singlet diagrams adequately represents the full higher-order SPS contribution, together with the choice of specific non-perturbative long-distance matrix elements taken from external fits.

What would settle it

A measurement of the double-J/psi yield at large rapidity separation that lies significantly above the sum of the computed loop-induced color-singlet plus color-octet single-parton contributions plus the expected double-parton-scattering yield.

read the original abstract

Prompt double-$J/\psi$ production at high-energy hadron colliders can be considered as a golden channel to probe double parton scatterings (DPS) --in particular to study gluon-gluon correlations inside the proton-- and, at the same time, to measure the distribution of linearly-polarised gluons inside the proton. Such studies however require a good control of both single and DPS in the respective regions where they are carried out. In this context, we have critically examined two mechanisms of single parton scatterings (SPS) that may be kinematically enhanced where DPS are thought to be dominant, even though they are either at higher orders in the strong-coupling or velocity expansion. First, we have considered a gauge-invariant and infrared-safe subset of the loop-induced contribution via Colour-Singlet (CS) transitions. We have found it to become the leading CS SPS contributions at large rapidity separation, yet too small to account for the data without invoking the presence of DPS yields. Second, we have surveyed the possible Colour-Octet (CO) contributions using both old and up-to-date non-perturbative long distance matrix elements (LDMEs). We have found that the pure CO yields crucially depend on the LDMEs. Among all the LDMEs we used, only two result into a visible modification of the NRQCD (CS+CO) yield, but only in two kinematical distributions measured by ATLAS, those of the rapidity separation and of the pair invariant mass. These modifications however do not impact the control region used for their DPS study.

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 examines two classes of single-parton-scattering (SPS) contributions to prompt J/ψ-pair production that could be kinematically enhanced at large rapidity separation, where double-parton scattering (DPS) is expected to dominate. It considers a gauge-invariant infrared-safe subset of loop-induced color-singlet (CS) diagrams and concludes that this subset becomes the leading CS SPS term at large Δy yet remains too small to account for the data without DPS. It also surveys color-octet (CO) contributions with multiple published LDME sets and finds that only two sets produce visible modifications to the NRQCD (CS+CO) yield, and only in the Δy and pair-invariant-mass distributions; these modifications do not affect the ATLAS DPS control region.

Significance. If the central conclusions hold, the work provides a useful validation that neither the examined loop-induced CS subset nor the CO contributions contaminate the control region used in existing DPS analyses of double-J/ψ production. The explicit survey across multiple LDME sets and the identification of the loop-induced subset as dominant among CS terms at large Δy are concrete contributions that can inform future DPS and gluon-polarization studies.

major comments (2)
  1. [Abstract] Abstract: The claim that the chosen gauge-invariant infrared-safe subset of loop-induced CS diagrams becomes the leading CS SPS contribution at large rapidity separation (and is still too small to explain the data without DPS) is load-bearing for the overall conclusion. The manuscript does not provide a quantitative estimate of the size of omitted diagrams or a demonstration that the selected subset captures the dominant Δy dependence of the full higher-order CS contribution; if additional terms grow faster with Δy, the 'too small' assessment would change.
  2. [Abstract] Abstract: The statement that only two LDME sets produce visible modifications to the NRQCD yield (and that these do not impact the control region) depends on the specific numerical values and kinematic cuts employed. Without an explicit tabulation of the LDME sets used, the resulting cross sections in the control region, or error bands on the distributions, independent verification of the 'no impact' claim is not possible from the presented information.
minor comments (1)
  1. [Abstract] The abstract would be clearer if it stated the total number of LDME sets surveyed and named the two sets that produce visible effects.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the careful reading and the constructive comments on our manuscript. We address each major comment below.

read point-by-point responses

  1. Referee: [Abstract] Abstract: The claim that the chosen gauge-invariant infrared-safe subset of loop-induced CS diagrams becomes the leading CS SPS contribution at large rapidity separation (and is still too small to explain the data without DPS) is load-bearing for the overall conclusion. The manuscript does not provide a quantitative estimate of the size of omitted diagrams or a demonstration that the selected subset captures the dominant Δy dependence of the full higher-order CS contribution; if additional terms grow faster with Δy, the 'too small' assessment would change.

    Authors: The subset we consider is the minimal gauge-invariant and infrared-safe set containing the t-channel gluon exchanges that produce the potential enhancement at large Δy. Individual diagrams outside this set either violate gauge invariance or lack the same kinematic enhancement, as can be seen from the structure of the amplitudes. A complete higher-order CS calculation lies beyond the scope of the present work, but we will add a dedicated paragraph in the revised manuscript justifying the dominance of this subset at large Δy on the basis of power counting and comparison with analogous processes in the literature. revision: partial

  2. Referee: [Abstract] Abstract: The statement that only two LDME sets produce visible modifications to the NRQCD yield (and that these do not impact the control region) depends on the specific numerical values and kinematic cuts employed. Without an explicit tabulation of the LDME sets used, the resulting cross sections in the control region, or error bands on the distributions, independent verification of the 'no impact' claim is not possible from the presented information.

    Authors: We agree that an explicit tabulation would improve verifiability. In the revised version we will add a table listing all LDME sets employed, the corresponding cross sections evaluated in the ATLAS DPS control region, and uncertainty bands on the Δy and pair-invariant-mass distributions. revision: yes

Circularity Check

0 steps flagged

No significant circularity; derivation relies on external LDMEs and explicit diagram selection

full rationale

The paper computes a gauge-invariant IR-safe subset of loop-induced CS diagrams and surveys CO contributions using published external LDME sets. No step reduces by construction to a fit performed inside this work, nor does any central claim rest on a self-citation chain or imported uniqueness theorem. LDME values are taken from prior independent fits and their spread is reported without re-optimization to the double-J/ψ data; the subset selection follows standard gauge-invariance criteria rather than being defined by the target observable. The reported outcomes (leading CS term at large Δy yet insufficient without DPS) are therefore direct numerical results, not tautological restatements of inputs.

Axiom & Free-Parameter Ledger

1 free parameters · 2 axioms · 0 invented entities

Central claim rests on NRQCD factorization, perturbative QCD loop calculations, and external LDME values; the gauge-invariant subset selection is a modeling choice specific to this work.

free parameters (1)
  • LDMEs
    Non-perturbative long-distance matrix elements for color-octet states, taken from external fits to other data.
axioms (2)
  • domain assumption NRQCD factorization holds for double J/ψ production
    The entire analysis is performed inside the NRQCD framework.
  • ad hoc to paper The chosen subset of loop-induced diagrams is gauge-invariant and infrared-safe
    Paper explicitly selects this subset as the object of study.

pith-pipeline@v0.9.0 · 5843 in / 1382 out tokens · 52851 ms · 2026-05-25T17:29:56.568104+00:00 · methodology

discussion (0)

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Reference graph

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