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Photoproduction of heavy vector mesons in peripheral $PbPb$ collisions at the Large Hadron Collider

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Pith's one-line read Comprehensive model comparison predicts Upsilon production and cross-section ratios in peripheral PbPb collisions that future LHC data can use to discriminate among photon-flux and dipole models.

arxiv 2505.14019 v2 pith:A6TPR7EZ submitted 2025-05-20 nucl-ex hep-exhep-phnucl-th

Pedro E. A. da Costa , André V. Giannini , Victor P. Goncalves , Bruno D. Moreira This is my paper · ORCID
classification nucl-exhep-exhep-phnucl-th
keywords collisionsmesonsperipheralphotonanalysiscolliderhadronlarge
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

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

The reading

At the LHC, lead nuclei move so fast that they are surrounded by a cloud of virtual photons. When two lead nuclei pass very close to each other without colliding head-on, these photons can interact with the other nucleus and produce a heavy quark-antiquark particle called a vector meson, such as J/Psi or Upsilon. In peripheral collisions, the nuclei actually touch, and the photons involved are also accompanied by hadronic debris, making the calculation harder.

The authors take the standard equivalent photon approximation and change each of its ingredients: how the photon flux is modified when nuclei overlap, whether the photon-nucleus scattering amplitude uses only spectator nucleons or also the overlap region, which dipole-proton scattering model is used (bCGC, IP-SAT, or a linearized version), and which wave function describes the meson. They compute rapidity distributions for centralities 50-70% and 70-90% at 2.76 and 5.02 TeV.

The spread of predictions is large, sometimes a factor of 2 to 3, and the current ALICE data are not precise enough to single out one model. The paper therefore proposes ratios: Upsilon over J/Psi, and peripheral over ultraperipheral yields. These ratios vary significantly across models, so future measurements, especially of Upsilon, could tell which assumptions about the photon flux in peripheral collisions are correct.

Extended reading notes

Core claim

The simultaneous analysis of J/Psi and Upsilon production, and the corresponding ratios, in peripheral PbPb collisions will allow future LHC data to discriminate between different models of the effective nuclear photon flux, dipole-proton scattering amplitude, and overlap function. This is stated in the abstract and in Section III: a future experimental analysis of these ratios could be very useful to discriminate between the different approaches.

Load-bearing premise

The mapping between centrality and impact parameter, c = b^2/(4 R_A^2), together with the assumption in Eq. (22) that only spectator nucleons outside the overlap region act as the target. If a Glauber Monte Carlo mapping is used instead, the predictions shift significantly, as shown in Fig. 8, and the comparison with ALICE data could change. The paper explicitly acknowledges this sensitivity in Appendix B.

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Editorial analysis

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Assumptions & free parameters 5 free parameters · 8 assumptions · 0 invented entities

All parameters used in the calculations are either standard nuclear constants (Wood-Saxon parameters), model parameters fitted to HERA data in prior literature, or wave-function parameters fixed by normalization and decay width. The paper introduces no new fitted parameters and no new physical entities. The centrality-impact parameter relation and the spectator-only assumption are explicit model choices that are tested only partially.

free parameters (5)
  • bCGC saturation model parameters (N0, gamma_s, x0, lambda, BCGC) = From HERA fit in Ref. [31], values not reproduced in the text
    Used in Eqs. (11)-(12) for the dipole-proton amplitude; fixed by HERA data, not by peripheral PbPb data.
  • IP-SAT parameters (Bp, Ag, lambda_g, C, mu0^2) = Bp = 4 GeV^-2, other values from HERA fit in Ref. [35]
    Used in Eqs. (15)-(17) for the gluon distribution and proton profile; fitted to HERA exclusive J/Psi data.
  • IPnon-SAT gluon parameters = Same as IP-SAT, Ref. [35]
    The linearized amplitude in Eq. (18) uses the same fitted gluon distribution as IP-SAT.
  • GLC wave function parameters (NT, R_T) = Table I: J/Psi NT=1.4532, R_T^2=5.5175 (one of three mass choices); Upsilon NT=0.7654, R_T^2=1.9211
    Fixed by wave function normalization and decay width, used in Eq. (21).
  • BG wave function parameters = From Refs. [35,40]
    Boosted Gaussian parameters for Eq. (20), fixed by normalization and decay width.
assumptions (8)
  • domain assumption Equivalent photon approximation (EPA) is valid for peripheral heavy-ion collisions.
    Used in Eq. (1) to factor the cross section into a photon flux times a photon-nucleus cross section.
  • standard math Wood-Saxon charge distribution describes the nuclear form factor.
    Used in Eq. (3) for F(q^2), with parameters RA=6.62 fm, a=0.549 fm, rho0=0.1603 fm^-3 from electron scattering.
  • domain assumption Glauber-Gribov formalism gives the nuclear dipole amplitude.
    Eq. (9) relates the nuclear scattering amplitude N_B to the dipole-proton cross section and nuclear profile T_B.
  • ad hoc to paper Centrality c maps to impact parameter b via c = b^2/(4R_A^2).
    Assumed in Section III; Appendix B shows that a Glauber Monte Carlo mapping gives different b ranges and predictions.
  • ad hoc to paper Only spectator nucleons outside the overlap region act as target in Eq. (22).
    The theta function theta(b1 - R_A) removes the overlap region from the photon-nucleus cross section, a model choice from Ref. [16].
  • domain assumption QGP effects can be ignored for centralities above 50%.
    Explicitly stated in Section III; for smaller centralities the predictions are flagged as upper bounds because QGP formation would modify the yields.
  • domain assumption The dipole-proton amplitudes (bCGC, IP-SAT, IPnon-SAT) are valid models for high-energy QCD.
    These models are imported from prior HERA fits and used as inputs, not derived in this paper.
  • ad hoc to paper Effective photon flux models N(1)-N(3) with theta functions capture the geometry of peripheral collisions.
    Eqs. (4)-(6) from Refs. [13,16]; the paper uses them as alternative assumptions without derivation.

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Pith. "Pith review of Photoproduction of heavy vector mesons in peripheral $PbPb$ collisions at the Large Hadron Collider." pith.science (2026). https://pith.science/paper/A6TPR7EZ

@misc{pith2026250514019,
  author       = {Pith},
  title        = {Pith review of: Photoproduction of heavy vector mesons in peripheral $PbPb$ collisions at the Large Hadron Collider},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/A6TPR7EZ}},
  note         = {Machine review of arXiv:2505.14019}
}
abstract

A comprehensive analysis of the photoproduction of $J/\Psi$ and $\Upsilon$ mesons in peripheral $PbPb$ collisions at the center - of - mass energies of the Large Hadron Collider (LHC) is performed, considering distinct assumptions for the modeling of the nuclear photon flux, photon - nucleus cross - section, overlap function and dipole - proton scattering amplitude. The comparison of these predictions with the ALICE data is also performed. Our results indicate that a detailed analysis of the production of both mesons will be very useful to improve the description of photon - induced processes in peripheral collisions.

Figures

Figures reproduced from arXiv: 2505.14019 by the authors.

Figure 1
Figure 1. FIG. 1: Photoproduction of heavy vector mesons in peripheral [PITH_FULL_IMAGE:figures/full_fig_p003_1.png] view at source ↗
Figure 2
Figure 2. FIG. 2: Transverse - plane view of the vector meson photoproduc [PITH_FULL_IMAGE:figures/full_fig_p004_2.png] view at source ↗
Figure 3
Figure 3. FIG. 3: Predictions for the rapidity distributions associated with th [PITH_FULL_IMAGE:figures/full_fig_p009_3.png] view at source ↗
Figures from the paper (5 more)
Figure 4
Figure 4. Figure 4: FIG. 4: Predictions for the rapidity distributions associated with th [PITH_FULL_IMAGE:figures/full_fig_p022_4.png]
Figure 5
Figure 5. Figure 5: FIG. 5: Predictions for the rapidity distributions associated with th [PITH_FULL_IMAGE:figures/full_fig_p023_5.png]
Figure 6
Figure 6. Figure 6: FIG. 6: Predictions for the rapidity distributions associated with th [PITH_FULL_IMAGE:figures/full_fig_p024_6.png]
Figure 7
Figure 7. Figure 7: FIG. 7: Predictions for the rapidity distributions associated with th [PITH_FULL_IMAGE:figures/full_fig_p025_7.png]
Figure 8
Figure 8. Figure 8: FIG. 8: Predictions for the rapidity distributions associated with th [PITH_FULL_IMAGE:figures/full_fig_p026_8.png]

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