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J/$\psi$ and $\Upsilon$(1S) production in jets at LHC energies

T0 review · 4 major / 5 minor · reviewed 2026-08-15 · deepseek-v4-flash

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Pith's one-line read The new quarkonia parton shower in PYTHIA 8.312 reproduces prompt $J/\psi$ production in jets at LHCb and CMS, and the QCD-based color-reconnection model improves that agreement.

desk verdict First direct test of the PYTHIA 8.312 quarkonia parton shower against LHCb/CMS J/psi-in-jet data, with a testable Upsilon(1S) prediction, but the central 'correctly describes' claim rests on visual agreement pending MC uncertainties. read the letter →

arxiv 2506.15205 v1 pith:MNWHUWBN submitted 2025-06-18 hep-ph

Lizardo Valencia Palomo This is my paper · ORCID
classification hep-ph
keywords hadron-hadroncollisionsquarkoniaproductionpartonshowercolorreconnectionJ/psiinjetsUpsilon(1S)NRQCDPYTHIA8.312
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

The paper addresses a long-standing discrepancy: PYTHIA 8.307 predicts too many prompt $J/\psi$ mesons produced with little or no jet activity, while LHCb and CMS data show them embedded in jets. The paper shows that the new quarkonia parton shower in PYTHIA 8.312, which lets quarkonium states be produced during the shower itself, flattens the jet-momentum-fraction ($z$) distribution and brings the simulation into agreement with both experiments. Replacing the MPI-based color reconnection with the QCD-based model improves the description further and changes the predicted $z$ shape, so the observable can distinguish between the two color-reconnection implementations. For $\Upsilon(1S)$, the paper predicts that distinguishing the old and new showers requires a higher jet transverse-momentum threshold than the one used for $J/\psi$.

What carries the argument

The central object is the quarkonia parton shower newly implemented in PYTHIA 8.312: the splitting kernels $Q \to Q\bar Q[{}^3S_1^{(1)}] Q$, $g \to Q\bar Q[{}^3S_1^{(1)}] g g$, and $g \to Q\bar Q[{}^3S_1^{(8)}]$, with the only implemented color-octet state treated as a massive gluon. The diagnostic observable is $z$, the fraction of the jet's transverse momentum carried by the quarkonium; the new kernels add surrounding radiation and thereby change the predicted $z$ distribution. The paper uses this observable to compare the MPI-based and QCD-based color-reconnection models against LHCb and CMS data.

What would settle it

Measure the $\Upsilon(1S)$ $z$ distribution in jets at LHCb with $p_T^{\rm jet}>50$ GeV/c and at CMS with $p_T^{\rm jet}>70$ GeV/c: the paper predicts that the old and new PYTHIA versions separate near $z\approx1$; if no separation appears, the proposed mechanism and its claimed discriminating power fail.

Watch

Extended reading notes

Core claim

The central claim is that the PYTHIA 8.307 discrepancy was not evidence against NRQCD-based quarkonium production but a symptom of missing shower physics. The new quarkonia parton shower adds three splitting kernels for S-wave quarkonia, $Q \to Q\bar Q[{}^3S_1^{(1)}] Q$, $g \to Q\bar Q[{}^3S_1^{(1)}] g g$, and $g \to Q\bar Q[{}^3S_1^{(8)}]$, creating quarkonia during the parton shower rather than only in the hard scattering. With this shower active, the normalized $z$ distributions for prompt $J/\psi$ at LHCb ($\sqrt{s}=13$ TeV) and CMS ($\sqrt{s}=5.02$ TeV) agree with data, and the QCD-based color reconnection describes the data better than the MPI-based one, especially for CMS. For $\Upsilon(1S)$, the paper finds that the old and new showers give almost identical predictions at the jet $p_T$ thresholds used for $J/\psi$, because the heavier quarkonium needs a more energetic parton to be created during the shower; raising the jet $p_T$ cut to 50 GeV/c for LHCb and 70 GeV/c for CMS should separate the predictions near $z\approx1$.

Load-bearing premise

The central premise is that the simplified quarkonium splitting rules in the new shower---only three kernels, with the lone color-octet state modeled as a massive gluon and the other octet splittings omitted---are accurate enough that the improved agreement with data is real rather than accidental.

Editorial extensions

If this is right

  • If the claim holds, the prompt $J/\psi$ fragmentation function in jets becomes an observable that discriminates between MPI-based and QCD-based color reconnection, and the data favour the QCD-based model.
  • The long-standing excess of isolated $J/\psi$ in PYTHIA 8 disappears once quarkonium production is included in the parton shower, shifting the interpretation from a failing production rate to missing shower radiation.
  • For $\Upsilon(1S)$, measurements at the jet $p_T$ cuts used for $J/\psi$ will not distinguish the old and new showers; LHCb needs $p_T^{\rm jet}>50$ GeV/c and CMS $p_T^{\rm jet}>70$ GeV/c, especially near $z\approx1$.
  • The mass dependence implies that heavier quarkonium states require harder jets for the quarkonia parton shower to leave a visible imprint on the $z$ distribution.

Reading between the lines

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

  • A test not performed in the paper: switch each of the three quarkonia splitting kernels on and off separately in PYTHIA 8.312; if the $z$ distribution still matches data when the octet kernel is removed, that kernel is not the source of the improvement.
  • If the mechanism is genuine, the same shower treatment should also shape $\psi(2S)$ and $\Upsilon(2S,3S)$ fragmentation in jets, with the mass hierarchy dictating the jet $p_T$ values at which the effect appears.
  • The QCD-based color-reconnection preference implied by the data may also show up in other color-flow-sensitive observables, such as baryon production or the underlying event, which this paper does not address.
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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Referee Report

4 major / 5 minor

Summary. This manuscript compares PYTHIA 8.307 and 8.312 predictions for the normalized z distribution of prompt J/ψ mesons inside jets with LHCb (√s=13 TeV) and CMS (√s=5.02 TeV) data, for the MPI and QCD color reconnection schemes. It reports that PYTHIA 8.307 overshoots the data at z≈1 regardless of the CR model, while PYTHIA 8.312 with the new quarkonia parton shower improves the agreement, especially with QCD-CR. It then proposes higher jet pT selections (50 GeV/c for LHCb, 70 GeV/c for CMS) as a way to discriminate CR models for prompt J/ψ, and predicts Υ(1S) production in jets at the same LHCb/CMS kinematics, concluding that high jet pT cuts are needed to distinguish simulations with and without the quarkonia shower. The study uses publicly available generator versions with default tunes and LDMEs and performs no fits.

Significance. If the quantitative concerns below are addressed, this is a useful phenomenological result: it highlights a qualitatively new effect of the NRQCD parton shower on jet fragmentation into quarkonia, and it makes concrete, falsifiable predictions for Υ(1S) at high jet pT that LHC experiments can test. The paper is honest about the approximations in the new shower and does not introduce ad hoc parameters. Its current weakness is that the central claims—'correctly describe the experimental results' and 'the data clearly favours the QCD-CR model'—are supported only by visual comparison of normalized histograms, with no Monte Carlo uncertainty bands and no goodness-of-fit quantification.

major comments (4)
  1. [Abstract; Section 3, Figs. 1-2] The abstract and Section 3 state that PYTHIA 8.312 can 'correctly describe the experimental results' and that 'the data clearly favours the QCD-CR model' (Fig. 2b), but the manuscript provides no Monte Carlo uncertainty bands, no number of generated events, and no goodness-of-fit measure such as a chi-squared or Kolmogorov-Smirnov statistic. The observable is a normalized z distribution written as (1/N) dN/dz, and the discriminating bins at high z contain relatively few entries; without MC statistical uncertainties, the visible differences between the red and blue curves in Fig. 2b, and the apparent flattening in Fig. 2a, could be statistical fluctuations. Please add MC error bars and report a quantitative comparison metric for each data/MC panel.
  2. [Section 3, second paragraph] The new quarkonia parton shower is implemented with only three splitting kernels, with the color-octet state treated as a massive gluon, and with the gluon- and heavy-quark-initiated octet splittings omitted. The text acknowledges these approximations but does not quantify their effect on the z distribution. Because the claim is that the shower provides the 'correct amount of jet activity,' the agreement with data could depend on cancellations among the missing kernels. Please estimate the numerical impact of each omitted kernel (for example, by switching individual kernels on and off, or by comparing with a full NRQCD calculation) before concluding that the shower describes the data.
  3. [Figs. 1-2] The comparison between PYTHIA 8.307 and 8.312 attributes all differences to the new quarkonia parton shower, but PYTHIA 8.312 contains other changes relative to 8.307, and Fig. 1 already uses a 'new beam remnant model' in the QCD-CR simulation. To isolate the effect of the quarkonia shower, the authors should compare, within the same PYTHIA version, runs with the quarkonia shower enabled and disabled, or otherwise demonstrate that no other version change affects the z distribution.
  4. [Figs. 4-5; Section 3] The Υ(1S) predictions and the proposed experimental discrimination inherit their credibility from the J/ψ validation, but they are also presented without uncertainty bands, and the manuscript does not state the number of generated events, the PDF set, the CR parameter values (e.g., Reconnection Range), or the exact PYTHIA run configuration. Please provide these technical details so that the results are reproducible, and include MC uncertainties in the prediction figures.
minor comments (5)
  1. [Throughout] There are numerous typographical errors: 'non of' should be 'none of'; 'chamonium' should be 'charmonium'; 'relavistically' should be 'relativistically'; 'octate' should be 'octet'; 'wether' should be 'whether'; 'taged' should be 'tagged'; 'topologycal' should be 'topological'; 'the later' should be 'the latter'; 'independently if' should be 'regardless of whether'.
  2. [Section 1] The text first lists the NRQCD matrix elements as 3S(1)_1, 1S(8)_0, 3S(8)_1 and 3P(8)_J, but later refers to 3P(8)_0; please make the notation consistent.
  3. [Reference [15]] Reference [15] contains the placeholder text 'Image 1' in the title and should be corrected.
  4. [Section 3] Please clarify how 'prompt J/ψ' is defined in the simulation, in particular whether feed-down from excited charmonia states (e.g., ψ(2S) or χ_c) is included; the experimental prompt distributions include such feed-down.
  5. [Fig. 4 caption] The caption states that the green curve uses 8.307 with MPI-CR while the text says the 8.307 results are independent of the CR choice; this should be stated consistently, and the invisible CR dependence of 8.307 should be shown or explicitly demonstrated.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: the paper benchmarks an external generator implementation against independent LHC data and makes an extrapolated prediction, without fitting parameters to the benchmark data.

full rationale

The paper's central claim is that the new quarkonia parton shower in PYTHIA 8.312, taken from ref. [29] (Cooke et al.), brings PYTHIA into agreement with LHCb and CMS measurements of prompt J/psi production in jets. No parameter is fitted in this paper: the simulations use the generator's default LDMEs and the standard Monash/CP5 tunes, and the comparisons use normalized z distributions, which remove the overall normalization. The new parton shower is not introduced as the author's own prior result and is not justified by a self-citation chain; it is an external implementation cited as independent support. The Upsilon(1S) prediction is a genuine extrapolation of the same generator to a different quarkonium state and to higher jet-pT selections, and it is not equivalent by construction to the J/psi comparison: the different mass changes the shower kinematics, and the predicted discrimination is a testable, non-tautological statement. The paper's acknowledged weaknesses, such as the missing octet splitting kernels and the absence of Monte Carlo uncertainty bands or goodness-of-fit statistics, concern validation and quantitative support for the agreement claim, not circularity. Since no load-bearing step reduces to its own inputs or to a fitted parameter renamed as a prediction, the appropriate circularity score is 0.

Assumptions & free parameters 2 free parameters · 3 assumptions · 0 invented entities

The paper introduces no new fitted parameters or entities. It depends on external tuned parameters in PYTHIA (tunes, LDMEs) and on the physical assumptions of NRQCD and the new parton shower implementation.

free parameters (2)
  • Reconnection Range (RR) for MPI-CR = Not quoted in paper; default from underlying event tune
    The MPI color reconnection probability is governed by this single free parameter, which is fitted in the underlying event tunes used here.
  • Default NRQCD LDMEs for J/psi and Upsilon(1S) = Defaults in PYTHIA 8.312, not quoted
    The simulation uses the default long-distance matrix elements for the color singlet and octet states; these are fitted to other quarkonium production data and could affect the normalization and possibly the z shape.
assumptions (3)
  • standard math NRQCD factorization and the velocity expansion for quarkonium production are valid.
    Used in the introduction and by references [8,9] to justify the LDME approach in PYTHIA.
  • domain assumption The quarkonia parton shower's splitting kernels (Q -> Q g[3S1], g -> QQbar[3S1] g, g -> QQbar[3S8]) are the dominant mechanism for quarkonium production in jets, and the neglected octet kernels enter at higher order.
    Section 3 states the missing kernels are suppressed by alpha_s or heavy-quark suppression; the paper assumes these have negligible numerical impact.
  • domain assumption The event generator's underlying event, beam remnants, and color reconnection models (MPI and QCD) as implemented in PYTHIA 8.307/8.312 faithfully model the data environment.
    The comparisons rely on the tunes (Monash, CP5) and CR models being appropriate, which is not independently verified in this paper.

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Cite this review

Pith. "Pith review of J/$\psi$ and $\Upsilon$(1S) production in jets at LHC energies." pith.science (2026). https://pith.science/paper/MNWHUWBN

@misc{pith2026250615205,
  author       = {Pith},
  title        = {Pith review of: J/$\psi$ and $\Upsilon$(1S) production in jets at LHC energies},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/MNWHUWBN}},
  note         = {Machine review of arXiv:2506.15205}
}
abstract

Quarkonia production in hadronic collisions is far from being understood as none of the existing models can correctly describe the wealth of available data. In particular, LHCb and CMS experiments have reported that PYTHIA 8 cannot reproduce the prompt J/$\psi$ production in jets in proton-proton collisions at two different center of mass energies: the event generator predicts an important amount of the prompt J/$\psi$ to be produced isolated, opposite to the experimental data. This document demonstrates that such effect remains true even if the QCD color reconnection (CR) model is used. Besides that, it is shown that using the new quarkonia parton shower included in PYTHIA 8.312 it is possible to correctly describe the experimental results. This agreement between data and simulation is improved when using the QCD color reconnection approach, opening the possibility to distinguish between the two CR implementations. Finally, a prediction performed for $\Upsilon$(1S) indicates that a higher jet p$_T$ selection should be used by the LHC experiments in order to distinguish between PYTHIA 8 results generated with and without the quarkonia parton shower.

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