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Momentum transfer squared dependence of exclusive quarkonia photoproduction in UPCs

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Cheryl Henkels, Emmanuel G. de Oliveira, Roman Pasechnik, Haimon Trebien

High Energy Physics - Phenomenology · Submitted 29 Sep 2020 · 90 references

Pith's verdict: A standard but useful dipole-model calculation: the BK comparison with H1 J/psi data is genuine, the excited-state potential-model wave functions are a real addition, and the main caveats are untested angular correlations and post-hoc model selection.

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arXiv ID2009.14158 (version 2)
Pith IDSYGNPLQZ, Pith's permanent record of this paper and its review
CitationCheryl Henkels, Emmanuel G. de Oliveira, Haimon Trebien, Roman Pasechnik. "Momentum transfer squared dependence of exclusive quarkonia photoproduction in UPCs." arXiv:2009.14158 (2020). https://pith.science/paper/SYGNPLQZ
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The paper

The paper claims

A dipole model that combines potential-model quarkonium wave functions with an impact-parameter-dependent BK solution reproduces the measured t-dependence of exclusive J/psi photoproduction and predicts the same observable for heavier…

The reading

Exclusive quarkonium photoproduction, where a quasi-real photon turns into a J/psi, psi(2S), Upsilon(1S), or Upsilon(2S) while the target stays intact, is one of the cleanest ways to image the transverse distribution of gluons inside a proton or nucleus. This paper argues that the full momentum-transfer-squared (t) dependence of such processes can be computed in the dipole picture, using quarkonium wave functions from a potential model (with Melosh spin rotation) and an impact-parameter-dependent dipole amplitude taken from the Balitsky-Kovchegov equation. With the Buchmuller-Tye potential and the b-dependent BK solution, the computed differential cross sections reproduce the H1 data for J/psi photoproduction on protons at W=55 and 100 GeV. The same setup yields t-dependent predictions for the excited psi and Upsilon states, and, through a Glauber-Gribov treatment with gluon shadowing, for coherent photoproduction off lead nuclei in gamma-A and A-A ultraperipheral collisions at sqrt(s)=5.02 TeV.

What carries the argument

The central object is the b-dependent dipole amplitude N(x,r,b), the imaginary part of the quark-antiquark dipole scattering amplitude at impact parameter b. The differential cross section is obtained from its Fourier transform over b, specifically from the amplitude integral $d^{2}$ b e^{-i[b-(1-z)r] . $\Delta$} N(x,r,b) that appears in Eq. (2.9), so the b-profile of the dipole amplitude directly sets the |t|-slope. Two inputs for N are used: the bSat saturation model, which employs a Gaussian proton shape function T(b), and the numerical solution of the BK equation with the collinearly improved kernel from Ref. [19], which avoids unphysical Coulomb tails. The quarkonium wave functions come from solving the Schrodinger equation for five interquark potentials (power-like, harmonic oscillator, Cornell, Buchmuller-Tye, logarithmic) and boosting them to the infinite-momentum frame with the Melosh spin rotation, which substantially enhances excited-state production.

Extended reading notes

Core claim

The central claim is that the differential cross section dsigma/dt of exclusive quarkonia photoproduction, whose t-dependence is the Fourier conjugate of the impact-parameter profile of the target's gluon density, is reliably computable in the dipole picture provided the dipole amplitude depends on the impact parameter and the vector-meson wave functions are realistic. The paper shows that combining the Buchmuller-Tye interquark potential, used for the light-front wave functions including the Melosh spin transformation, with the numerical b-dependent solution of the Balitsky-Kovchegov equation with a collinearly improved kernel reproduces the H1 differential J/psi photoproduction data on the proton. The same setup provides t-dependent predictions for psi(2S), Upsilon(1S) and Upsilon(2S) on the proton and, via a Glauber-Gribov average over the nuclear thickness function together with an EPPS16-based gluon-shadowing factor, for coherent gamma-Pb to V-Pb and Pb-Pb to V-Pb-Pb at sqrt(s)=5.02 TeV. The authors emphasize that this is the first calculation combining a realistic potential-model treatment of excited-state wave functions with the latest b-dependent BK solution, and they use the alternative bSat model as a benchmark to expose the spread of model predictions.

Load-bearing premise

The calculation's t-dependence is controlled entirely by the assumed impact-parameter profile of the dipole amplitude, N(x,r,b); if that profile is inaccurate, the Fourier transform yields the wrong slope of dsigma/dt even when the forward cross section is correct, and all proton and nuclear predictions inherit the error.

Editorial extensions

If this is right

  • The H1 differential J/psi photoproduction data are reproduced by the BK-plus-Buchmuller-Tye setup at W=55 and 100 GeV, establishing the b-dependent BK dipole amplitude as a viable input for t-dependent observables on the proton.
  • The same setup yields concrete t-dependent predictions for psi(2S), Upsilon(1S), and Upsilon(2S) photoproduction on protons at W=55-120 GeV, with the spread from five interquark potentials giving a theory band.
  • For ultraperipheral lead-lead and photon-lead collisions at sqrt(s)=5.02 TeV and central rapidity, the framework predicts coherent differential cross sections for all four quarkonia states, with destructive-interference dips appearing at nearly the same |t| for the psi and Upsilon families.
  • The comparison with ALICE and CMS data shows that the bSat model gives a closer description of the nuclear and p-Pb data, whereas the BK model is the one that matches the HERA proton data, so the preferred b-profile may depend on the target.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • If the BK-plus-Buchmuller-Tye description survives confrontation with future differential data, the slope of dsigma/dt becomes a direct, quantitative measurement of the gluon's impact-parameter profile at low x, effectively a transverse image of the nucleon at scales currently probed only indirectly.
  • A target-dependent preference (bSat for nuclei, BK for protons) suggests the b- and angle-dependence of the dipole amplitude is not universal, and implementing an r-b angular correlation is the natural next test of the framework.
  • The predicted Upsilon(2S)-to-Upsilon(1S) ratio could serve as a wave-function-sensitive discriminant among interquark potentials, turning exclusive photoproduction into a probe of quarkonium spectroscopy at the LHC or a future electron-ion collider.
  • Because the nuclear calculation embeds a proton dipole amplitude in a Glauber-Gribov average, a mismatch at the predicted dip positions in Pb-Pb data would point specifically to new nuclear effects beyond EPPS16 shadowing rather than to the proton b-profile.
Original abstract

In this paper, we study fully differential quarkonia photoproduction observables in ultraperipheral collisions (UPCs) as functions of momentum transfer squared. We employ the dipole picture of the QCD part of the scattering with proton and nucleus targets, with the projectile being a quasi-real photon flux emitted by an incoming hadron. We analyse such observables for ground $J/\psi$, $\Upsilon(1S)$ and excited $\psi'$, $\Upsilon(2S)$ states whose light-front wave functions are obtained in the framework of interquark potential model incorporating the Melosh spin transformation. Two different low-$x$ saturation models, one obtained by solving the Balitsky--Kovchegov equation with the collinearly improved kernel and the other with a Gaussian impact-parameter dependent profile, are used to estimate the underlined theoretical uncertainties of our calculations. The results for the proton target and with charmonium in the final state are in agreement with the available HERA data, while in the case of nucleus target we make predictions for $\gamma A$ and $AA$ differential cross sections at different $W$ and at $\sqrt{s}=5.02$ TeV, respectively.

The compiled record

The paper's own sentences, arranged, kept beside the review and separate from it. Each label is drawn from a closed vocabulary; the number beside it is the probability the classifier gave that label. Numbers in the tables are the authors', checked by code. No sentence in this record was written by a model.

Compiled by classifier-1.13.0 via TypeSafe API, question bank abstract-p1-20260923, 2026-09-23: 1 request, 4,570 input tokens.

What the abstract itself states, as probabilities: claims first 0.03 · states limitation 0.64 · human participants 0.01 · controlled comparison 0.10 · randomized assignment 0.01 · states quantitative result 0.17 · compares with prior results 0.94 · mentions code or data availability 0.02.

The abstract, sentence by sentence

arXiv listing abstract v2. Results and the main claim are underlined; .

In this paper, we study fully differential quarkonia photoproduction observables in ultraperipheral collisions (UPCs) as functions of momentum transfer squared. We employ the dipole picture of the QCD part of the scattering with proton and nucleus targets, with the projectile being a quasi-real photon flux emitted by an incoming hadron. We analyse such observables for ground $J/\psi$, $\Upsilon(1S)$ and excited $\psi'$, $\Upsilon(2S)$ states whose light-front wave functions are obtained in the framework of interquark potential model incorporating the Melosh spin transformation. Two different low-$x$ saturation models, one obtained by solving the Balitsky--Kovchegov equation with the collinearly improved kernel and the other with a Gaussian impact-parameter dependent profile, are used to estimate the underlined theoretical uncertainties of our calculations. The results for the proton target and with charmonium in the final state are in agreement with the available HERA data, while in the case of nucleus target we make predictions for $\gamma A$ and $AA$ differential cross sections at different $W$ and at $\sqrt{s}=5.02$ TeV, respectively.

background contribution method result interpretation main claim

What the classifier reports for each sentence. The number is the probability it gave the label.
#sentencerolethe authors' own workstates the main conclusionstrength
s0 In this paper, we study fully differential quarkonia photoproduction observables in ultraperipheral collisions (UPCs) as functions of momentum transfer squared. contribution 1.00 0.90 0.05 asserted 0.82
s1 We employ the dipole picture of the QCD part of the scattering with proton and nucleus targets, with the projectile being a quasi-real photon flux emitted by an incoming hadron. method 1.00 0.73 0.02 asserted 0.98
s2 We analyse such observables for ground $J/\psi$, $\Upsilon(1S)$ and excited $\psi'$, $\Upsilon(2S)$ states whose light-front wave functions are obtained in the framework of interquark potential model incorporating the Melosh spin transformation. method 0.73 0.87 0.03 asserted 0.97
s3 Two different low-$x$ saturation models, one obtained by solving the Balitsky--Kovchegov equation with the collinearly improved kernel and the other with a Gaussian impact-parameter dependent profile, are used to estimate the underlined theoretical uncertainties of our calculations. method 0.99 0.79 0.04 asserted 0.98
s4 The results for the proton target and with charmonium in the final state are in agreement with the available HERA data, while in the case of nucleus target we make predictions for $\gamma A$ and $AA$ differential cross sections at different $W$ and at $\sqrt{s}=5.02$ TeV, respectively. result 1.00 0.90 0.47 asserted 0.96

What kind of paper the classifier reports this is: simulation study 0.52 · evidence from simulation or computation 1.00.

What was examined, and what was not

sources read
  • arXiv listing abstract v2
not examined
  • full text
  • figures and tables
  • references and citing sentences
  • relations to other papers
levels run
  • P abstract
not run
  • S sections
  • T body sentences
  • C citations
  • R cross-paper relations

Every quoted sentence on this record is the authors'. Every label is from Pith's closed vocabulary, with the probability shown beside it. Every number in the measurements table is the authors', checked by code. Record as JSON

Figures
Figure 1. shows the differential cross section for J/ψ ≡ ψ(1S) (upper curves) and ψ(2S) (lower curves) production as a function of the momentum transfer squared |t| for W = 100 GeV (left) and W = 55 GeV (right). Here, the results are obtained using a numerical solution of the BK equation of the b-dependent partial dipole amplitude discussed above. The ground-state charmonium results were compared to the experimental data available from the H1 Collaboration [53, 54] yielding a very good description. The corresponding observables have been evaluated with the LF quarkonia wave functions obtained…
Figure 2. FIG. 2: Predictions for the differential cross section for Υ(1
Figure 3. FIG. 3: Differential cross section for
Figure 4. FIG. 4: Predictions for the differential cross section for Υ(1
Figure 5. FIG. 5: Differential cross section for
Figure 6. FIG. 6: Differential cross section for the
Figure 7. FIG. 7: Differential cross sections for the
Figure 8. FIG. 8: Predictions for the differential cross sections for the
Figure 9. FIG. 9: Predictions for the differential cross sections for

Figures reproduced from arXiv:2009.14158 by the authors.

References
How the literature uses it

3 arXiv papers cite this in 8 sentences

Each citing sentence is taken from the citing paper's LaTeX source and typed by Pith's classifier. A label appears at probability 0.8 or more (0.95 for support), a contest at 0.7 or more; the quoted words are the citing authors'. How each label was checked · JSON

Papers citing it · 6 sentences without a confident label

Cites it as support 1 paper
Cites it as background 1 paper

What it cites · 40 sentences without a confident label

Uses the method or tool of 9 papers
Uses data or values from 1 paper
  • Besides, we considered a conventional Gaussian form for the proton shape function $T(b)$ [equation] where the slope parameter $B_G=4.25 GeV^-2$ is found at Ref. [ref] .

    An Impact Parameter Dipole Saturation Model · Partial dipole amplitude · 0.86

Compares with 7 papers
Cites as support 1 paper
Cites as background 5 papers
  • In an attempt to improve the color dipole models available in the literature, the authors of Ref. [ref] included a correlation between the impact parameter and the dipole separation in the elastic dipole-proton amplitude, by adding a dependence on the angle between $r⃗$ and $b⃗$ to the color dipole cross section.

    Electroproduction of heavy quarkonia: significance of dipole orientation · Results for $γA$ and $AA$ collisions · 0.92

  • A very recent one [ref] executes the calculations at the NLO level in $γp$ collisions for longitudinally polarized photons making use of the CGC framework and proposing a wave function based upon NRQCD matrix elements [ref] .

    Relativistic corrections to the vector meson light front wave function · Elastic photoproduction off a proton · 0.84

  • A very recent one [ref] executes the calculations at the NLO level in $γp$ collisions for longitudinally polarized photons making use of the CGC framework and proposing a wave function based upon NRQCD matrix elements [ref] .

    Exclusive heavy vector meson production at next-to-leading order in the dipole picture · Elastic photoproduction off a proton · 0.84

Discussion

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