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arxiv: 2106.11465 · v2 · submitted 2021-06-22 · ✦ hep-ph

DPS mechanism for associated cbar{c} l^+l^- production in AA UPCs as a probe for photon density inside the nucleus

Edgar Huayra , Emmanuel G. de Oliveira , Roman Pasechnik , Bruna O. Stahlh\"ofer This is my paper

Pith reviewed 2026-05-24 13:39 UTC · model grok-4.3

classification ✦ hep-ph
keywords double-parton scatteringultraperipheral collisionsphoton densitycharm productiondilepton productionheavy-ion collisionssmall-x physics
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The pith

An analog of the pp pocket formula for double-parton scattering in ultraperipheral AA collisions lets associated charm-dilepton production probe the photon density inside nuclei.

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

The paper examines associated production of charm-anticharm pairs and lepton pairs in ultraperipheral heavy-ion collisions at high energies. It derives a pocket formula analogous to the standard one for proton-proton collisions, incorporating nuclear photon fluxes and both elastic and non-elastic contributions. The work studies how the effective cross section depends on kinematics and presents differential predictions in charm rapidity and dilepton mass and rapidity. A sympathetic reader would care because this channel supplies a direct experimental handle on the photon content of nuclei at small momentum fractions.

Core claim

The central claim is that double-parton scattering of photons emitted from nuclei produces associated c cbar l+l- final states in ultraperipheral AA collisions, thereby providing a novel probe of the photon density inside the nucleus; this is established by deriving the analog pocket formula, determining the kinematical dependence of the effective cross section, and computing explicit differential cross sections for LHC and future-collider kinematics.

What carries the argument

The analog of the standard central pp pocket formula for the DPS cross section, adapted by inserting the nuclear photon flux.

If this is right

  • The effective cross section exhibits a specific kinematical dependence that can be tested in data.
  • Both elastic and non-elastic photon contributions must be included to obtain reliable predictions.
  • Differential cross sections in charm-quark rapidity and in dilepton invariant mass and rapidity are calculable for current and future colliders.

Where Pith is reading between the lines

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

  • Data on this final state could be used to extract or constrain the photon parton distribution inside the nucleus.
  • The same framework might be applied to other DPS final states to test whether the effective cross section remains universal in the nuclear environment.
  • Higher-energy colliders would extend the reach to smaller x values for the nuclear photon density.

Load-bearing premise

The double-parton scattering framework and the effective cross section calibrated in pp collisions transfer directly to ultraperipheral AA collisions once the nuclear photon flux is inserted.

What would settle it

A measurement of the associated c cbar l+l- cross section in AA ultraperipheral collisions that deviates substantially from the predicted value obtained by combining the pp-calibrated effective cross section with the nuclear photon flux.

Figures

Figures reproduced from arXiv: 2106.11465 by Bruna O. Stahlh\"ofer, Edgar Huayra, Emmanuel G. de Oliveira, Roman Pasechnik.

Figure 1
Figure 1. Figure 1: FIG. 1: An illustration of the cross section in the DPS mechanism of the dilepton [PITH_FULL_IMAGE:figures/full_fig_p003_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: FIG. 2: The photon number distribution as a function of impact parameter [PITH_FULL_IMAGE:figures/full_fig_p004_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: FIG. 3: The elastic and inelastic photon distributions integrated over impact parameter [PITH_FULL_IMAGE:figures/full_fig_p006_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: FIG. 4: The SPS dimuon production cross section in PbPb UPCs at typical LHC ( [PITH_FULL_IMAGE:figures/full_fig_p007_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: FIG. 5: The SPS [PITH_FULL_IMAGE:figures/full_fig_p008_5.png] view at source ↗
Figure 6
Figure 6. Figure 6: FIG. 6: The impact parameter picture of DPS in [PITH_FULL_IMAGE:figures/full_fig_p009_6.png] view at source ↗
Figure 7
Figure 7. Figure 7: FIG. 7: The DPS effective cross section [PITH_FULL_IMAGE:figures/full_fig_p010_7.png] view at source ↗
Figure 8
Figure 8. Figure 8: FIG. 8: The DPS [PITH_FULL_IMAGE:figures/full_fig_p012_8.png] view at source ↗
Figure 9
Figure 9. Figure 9: FIG. 9: The DPS [PITH_FULL_IMAGE:figures/full_fig_p013_9.png] view at source ↗
read the original abstract

We discuss the associated $c\bar{c}$ and $l^+l^-$ pairs production in ultraperipheral heavy-ion collisions at high energies. Such a channel provides a novel probe for double-parton scattering (DPS) at small $x$ enabling one to probe the photon density inside the nucleus. We have derived an analog of the standard central $pp$ pocket formula and studied the kinematical dependence of the effective cross section. Taking into account both elastic and non-elastic contributions, we have shown predictions for the DPS $c\bar c l^+l^-$ production cross section differential in charm quark rapidity and dilepton invariant mass and rapidity for LHC and a future collider.

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 / 2 minor

Summary. The paper claims that associated production of c cbar and l+l- pairs in ultraperipheral AA collisions can probe the photon density in the nucleus via the DPS mechanism. It derives an analog of the standard pp pocket formula for the DPS cross section, studies the kinematical dependence of the effective cross section, accounts for both elastic and non-elastic photon contributions, and presents predictions for differential cross sections in charm quark rapidity, dilepton invariant mass, and rapidity at the LHC and a future collider.

Significance. If the analog formula is valid, this offers a novel probe for nuclear photon densities at small x via DPS in UPCs. The derivation of the pocket-formula analog and explicit inclusion of elastic plus non-elastic contributions are strengths; the differential predictions in rapidity and invariant mass also provide concrete, falsifiable observables.

major comments (2)
  1. [Formalism] Formalism section: the analog pocket formula is constructed by substituting the nuclear photon flux into the pp expression σ_DPS = (σ_1 σ_2)/σ_eff with σ_eff taken from pp data. No explicit argument or calculation is supplied showing that impact-parameter correlations between the two photon emissions (or between photon-parton scatterings) leave the numerical value and kinematic dependence of σ_eff unchanged.
  2. [Results] Results section: all quoted differential cross sections (charm rapidity, dilepton mass and rapidity) rest on the assumption that σ_eff is unmodified by nuclear geometry. Because the manuscript states that elastic and non-elastic contributions are included but does not demonstrate invariance of σ_eff under the equivalent-photon approximation in AA, the quantitative predictions cannot yet be regarded as robust.
minor comments (2)
  1. The abstract refers to predictions at 'a future collider' without quoting the center-of-mass energy or collider name (e.g., FCC-hh parameters).
  2. An explicit equation defining the nuclear analog of σ_eff and its kinematic variables should appear in the main text rather than only in the abstract.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the careful review and the constructive comments on our manuscript. We respond to the major comments point by point below.

read point-by-point responses

  1. Referee: [Formalism] Formalism section: the analog pocket formula is constructed by substituting the nuclear photon flux into the pp expression σ_DPS = (σ_1 σ_2)/σ_eff with σ_eff taken from pp data. No explicit argument or calculation is supplied showing that impact-parameter correlations between the two photon emissions (or between photon-parton scatterings) leave the numerical value and kinematic dependence of σ_eff unchanged.

    Authors: The derivation of the analog pocket formula in our work follows the standard approach used in the literature for DPS in hadronic collisions by replacing the parton fluxes with photon fluxes under the equivalent photon approximation. While we have studied the kinematical dependence of the effective cross section in the nuclear environment, we acknowledge that an explicit discussion of why impact-parameter correlations do not alter σ_eff significantly was not provided. In the revised manuscript, we will include a new paragraph or subsection justifying this assumption, noting that the photon emissions are independent and the hard scatterings occur at much smaller scales, similar to justifications in other nuclear DPS studies. This will be supported by a brief estimate of the correlation effects. revision: yes

  2. Referee: [Results] Results section: all quoted differential cross sections (charm rapidity, dilepton mass and rapidity) rest on the assumption that σ_eff is unmodified by nuclear geometry. Because the manuscript states that elastic and non-elastic contributions are included but does not demonstrate invariance of σ_eff under the equivalent-photon approximation in AA, the quantitative predictions cannot yet be regarded as robust.

    Authors: We agree that the robustness of the predictions depends on this assumption. Our study of the kinematical dependence was intended to address variations, but we recognize the need for a more explicit demonstration of the invariance. We will revise the results section to include additional discussion or a supporting calculation showing that the effective cross section's value and dependence remain consistent when incorporating the nuclear photon fluxes in both elastic and inelastic cases. This will make the predictions more robust. revision: yes

Circularity Check

0 steps flagged

No significant circularity; derivation of analog pocket formula is self-contained

full rationale

The paper derives an analog of the standard pp DPS pocket formula by incorporating the nuclear photon flux into the double-parton scattering framework for AA UPCs, then computes the kinematic dependence of the effective cross section from the photon densities and presents differential cross-section predictions. No quoted step reduces a prediction to a fitted parameter by construction, nor does any load-bearing premise rely on a self-citation chain or ansatz smuggled from prior work by the same authors. The central construction treats σ_eff as an input calibrated externally in pp collisions and transferred under the stated assumption; the resulting predictions are therefore independent of the derivation itself. This is the most common honest finding for a paper whose core result is an explicit formula applied to external fluxes.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The central construction rests on transferring the pp DPS pocket formula to the nuclear UPC setting; no explicit free parameters, new entities, or additional axioms are stated in the abstract.

axioms (1)
  • domain assumption The standard DPS pocket formula from pp collisions applies analogously once the nuclear photon flux is substituted.
    The paper states it has derived an analog, implying this transferability is taken as given.

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Forward citations

Cited by 2 Pith papers

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    hep-ph 2025-10 unverdicted novelty 5.0

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  2. Momentum fraction and hard scale dependence of double parton scattering

    hep-ph 2025-06 unverdicted novelty 5.0

    Global fit of an x- and μ-dependent Gaussian model for transverse double parton distributions to LHC and Tevatron data extracts parameters for calculating effective cross sections in other observables.

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