Pith. sign in

REVIEW 3 major objections 4 minor 13 references

Recent results from STAR for parton distribution functions at low and high $x$ in proton-proton collisions

T0 review · 3 major / 4 minor · reviewed 2026-08-12 · deepseek-v4-flash

Challenge this review Re-run · record.json Download PDF Read on arXiv ↗ Review JSON

Pith's one-line read STAR's RHIC measurements place new constraints on the proton's gluon distribution at high x and its sea-quark asymmetry, while finding no saturation broadening at low x.

desk verdict A clean, honest proceedings summary of three STAR results; the only new physics hint is a 20% jet cross-section deficit, which is not yet supported without the missing systematic budget. read the letter →

arxiv 2412.00614 v2 pith:J5BIR4RK submitted 2024-11-30 hep-ex nucl-ex

Zilong Chang (for the STAR collaboration) This is my paper · ORCID
classification hep-exnucl-ex
keywords partondistributionfunctionsgluonPDFinclusivejetcrosssectionWbosoncross-sectionratiodbar/ubarasymmetrydi-pi0correlationscolorglasscondensateforwardupgrade
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

This proceeding argues that three STAR measurements in $pp$ collisions at RHIC sharpen the empirical picture of the proton's parton distributions at kinematic extremes. At high $x$ ($0.1 < x < 0.5$), inclusive jet cross sections at $\sqrt{s}=200$ and $510$ GeV are presented as direct probes of the gluon PDF; the preliminary data sit about 20% below CT14 NLO perturbative QCD predictions after hadronization corrections, while matching Pythia6 in shape. At intermediate $x$, the $W^\pm$ cross-section ratio $R_W=\sigma(W^+)/\sigma(W^-)$ at $\sqrt{s}=510$ GeV agrees with modern NLO PDF sets and constrains $\bar{d}/\bar{u}$ at $Q^2=M_W^2$. At low $x$ (down to $\sim 10^{-4}$), forward di-$\pi^0$ correlations in $pp$, $p\mathrm{Al}$, and $p\mathrm{Au}$ collisions show a linear $A^{1/3}$ trend in relative area but no azimuthal broadening, which complicates simple saturation expectations. Together the three measurements give new handles on the high-$x$ gluon, the sea-quark asymmetry, and low-$x$ gluon dynamics ahead of the STAR forward upgrade.

What carries the argument

The chain that carries the argument is the detector-level reconstruction folded with theory comparisons. Jets are clustered with the anti-$k_T$ algorithm ($R=0.6$ at 200 GeV, $R=0.5$ at 510 GeV) from TPC tracks and electromagnetic-calorimeter towers; an off-axis cone method subtracts the underlying event, and a Pythia6 Perugia 2012 tune provides the unfolding and hadronization corrections used before comparing with CT14 NLO. $W^\pm$ bosons are identified through their high-energy electron decay, a large missing transverse momentum from the neutrino, and electromagnetic-calorimeter clusters, and $R_W$ is binned in reconstructed electron $\eta$. Forward $\pi^0$ pairs are reconstructed from decay photons in the Forward Meson Spectrometer, and the correlation function $C(\Delta\phi)=N_{\rm pair}(\Delta\phi)/(N_{\rm trig}\Delta\phi_{\rm bin})$ is used to define the relative area between $\Delta\phi=\pi/2$ and $3\pi/2$, plotted against $A^{1/3}$ for $pp$, $p\mathrm{Al}$, and $p\mathrm{Au}$. These objects let the paper translate raw detector counts into PDF-sensitive cross sections and ratios.

What would settle it

Re-derive the jet energy scale for the same 2012 $pp$ data using an independent in-situ calibration such as photon-plus-jet transverse momentum balance; if the inclusive jet cross sections shift upward by about 20% and come into agreement with CT14 NLO, the claimed high-$x$ gluon constraint would collapse.

Watch

Extended reading notes

Core claim

The paper's central results are three preliminary STAR measurements. Inclusive jet cross sections $d^2\sigma/(dp_T\,d\eta)$ at $\sqrt{s}=200$ GeV with $|\eta|<0.8$ and anti-$k_T$ radius $R=0.6$, and at $\sqrt{s}=510$ GeV with $|\eta|<0.9$ and $R=0.5$, are corrected for the underlying event with an off-axis cone method; they sit roughly 20% below CT14 NLO perturbative QCD calculations after hadronization corrections while matching the shape of Pythia6 Perugia 2012, with the jet energy scale as the dominant systematic. The $W^\pm$ cross-section ratio at $\sqrt{s}=510$ GeV, built from $700~\mathrm{pb}^{-1}$ of 2011--2017 data, agrees with CT18, MSHT20, and NNPDF4.0 NLO predictions, supporting the sea-quark asymmetry $\bar{d}/\bar{u}$ at $Q^2=M_W^2$ over $x\approx0.06$--$0.4$. Forward di-$\pi^0$ azimuthal correlations at $\sqrt{s_{NN}}=200$ GeV in $pp$, $p\mathrm{Al}$, and $p\mathrm{Au}$, using trigger $p_T=1.5$--$2$ GeV/$c$ and associate $p_T=1$--$1.5$ GeV/$c$, show the relative area under $C(\Delta\phi)$ from $\Delta\phi=\pi/2$ to $3\pi/2$ growing linearly with $A^{1/3}$ but no angular broadening, a result the paper connects to color-glass-condensate expectations.

Load-bearing premise

The load-bearing premise is that the jet energy scale calibration and the off-axis cone underlying-event subtraction are accurate enough that the 20% deficit against CT14 NLO is a real physics signal rather than an experimental artifact.

Editorial extensions

If this is right

  • If the 20% jet deficit is real, global PDF fits will need a lower or reshaped gluon at $x>0.1$ at RHIC scales, or the NLO calculation must absorb the difference.
  • The $W$ ratio agreement with three independent NLO PDF sets strengthens $\bar{d}/\bar{u}$ constraints at $Q^2=M_W^2$ and $x\approx0.06$--$0.4$, complementing fixed-target Drell-Yan measurements.
  • The absence of angular broadening in forward di-$\pi^0$ correlations implies low-$x$ gluon saturation at $\sqrt{s_{NN}}=200$ GeV does not appear as a simple widening of the back-to-back peak; models must instead reproduce the $A^{1/3}$ scaling of the relative area.
  • The STAR forward upgrade extends these probes to asymmetric $x_1\gg x_2$ collisions and can reach valence-quark distributions at $x>0.5$, a region no current measurement covers.

Reading between the lines

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

  • A direct next step the paper leaves implicit is to compare the same jet datasets with NNLO predictions or to take the 200-to-510 GeV cross-section ratio; the ratio cancels luminosity normalization and much of the jet energy scale uncertainty and would show whether the 20% deficit is a scale shift or a $p_T$-dependent shape effect.
  • The missing azimuthal broadening could be turned into a quantitative CGC test by predicting the relative area as a function of $p_T$ and collision-system size in a wider set of $pA$ species and comparing with the published correlation functions.
  • If the jet deficit is interpreted as a gluon-PDF effect, it predicts that future deep-inelastic or $ep$ measurements at comparable $Q^2$ will also prefer a lower high-$x$ gluon; that is a testable consequence for the Electron-Ion Collider program.
  • The $W$ ratio at 510 GeV could be extended with additional integrated luminosity and forward charge-separation to map $\bar{d}/\bar{u}$ over a wider $x$ range, especially where SeaQuest and NuSea disagree at high $x$.
Share X Bluesky LinkedIn Reddit HN

Signed reviews

No signed human review yet.

Editorial analysis

A structured set of objections, weighed in public.

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

Referee Report

3 major / 4 minor

Summary. This proceedings contribution from the STAR Collaboration reports three preliminary measurements relevant to proton PDFs: (i) mid-pseudorapidity inclusive jet cross sections in pp collisions at sqrt(s)=200 and 510 GeV, which are compared with CT14 NLO predictions and found to lie roughly 20% below them after hadronization corrections; (ii) the W^+/W^- cross-section ratio at sqrt(s)=510 GeV, which agrees with NLO PDFs (CT18, MSHT20, NNPDF4.0); and (iii) forward di-pi0 azimuthal correlations in pp, pAl, and pAu collisions at sqrt(s_NN)=200 GeV, where no angular broadening is observed as a function of A^{1/3}. The paper concludes that jet production constrains the gluon PDF at x>0.1, the W ratio constrains dbar/ubar, and forward di-pi0 correlations probe low-x gluon dynamics.

Significance. If fully supported, the inclusive jet cross-section deficit relative to CT14 NLO would be an interesting constraint on the high-x gluon PDF at RHIC energies, where global fits have large uncertainties. The W-boson ratio provides a complementary, clean probe of sea-quark PDFs and is consistent with modern global fits. The forward di-pi0 result is a potentially important null test for CGC-driven suppression and broadening at low x. The paper merits credit for presenting these comparisons with up-to-date PDF sets and for reporting preliminary results from large STAR datasets. However, the central quantitative claim—the 20% jet deficit—is presented without the systematic uncertainty budget needed to assess whether it is a physics effect or an experimental artifact, and the di-pi0 null result is given without quantitative uncertainties. The significance of the contribution is therefore conditional on the missing supporting information.

major comments (3)
  1. [Sec. 2, Fig. 1] The central claim that the inclusive jet cross sections sit about 20% below CT14 NLO is not supported by the evidence presented. The paper states that "the dominant systematic uncertainties come from the jet energy scale for both results," but it gives no magnitude for this uncertainty, no error bars on Fig. 1, and no systematic uncertainty table. Because the cross section falls steeply with pT, a jet-energy-scale bias of only a few percent can shift the cross section by roughly 20%, so without a quantified JES uncertainty the observed deficit cannot be attributed to the gluon PDF. Please provide the full systematic budget, including the UE correction uncertainty and the JES uncertainty, and show the data points with their total uncertainties.
  2. [Sec. 2, CT14 NLO comparison] The comparison to CT14 NLO is presented without any renormalization or factorization scale variation. The magnitude of the 20% deficit depends on the choice of scale in the NLO calculation, and a scale band could easily cover a significant part of the reported difference. Please show the theoretical prediction with a scale-uncertainty band, and clarify whether the hadronization corrections applied are those from the Pythia6 Perugia 2012 tune and how their uncertainty is estimated. Without this, the claim that the deficit is a genuine constraint on the gluon PDF is premature.
  3. [Sec. 4, Fig. 3] The statement that "surprisingly no angular broadening is observed" is not accompanied by quantitative uncertainties on the correlation functions or on the extracted relative areas. The right panel of Fig. 3 appears to show a linear trend in A^{1/3} with a slope P = -0.09 ± 0.01, but the uncertainties on the individual points, the chi-squared of the linear fit, and the significance of the null result versus CGC expectations are not given. Please report the statistical and systematic uncertainties for C(delta phi) and state explicitly what magnitude of broadening would be excluded at the current precision.
minor comments (4)
  1. [Title] The title reads "at low and highx"; there should be a space before "x".
  2. [Sec. 2, Fig. 1 caption] The caption does not specify the luminosity values or the overall normalization uncertainties (10% at 200 GeV and 5.2% at 510 GeV are given in the text but should be visible in the figure or caption for completeness).
  3. [Sec. 3, Fig. 2] The W-ratio figure shows error bars but the numerical values and the uncertainties are not listed in the text. A short table or explicit statement of the quoted precision would improve the reader's ability to judge the agreement with the PDF predictions.
  4. [Sec. 4, Eq. for C(delta phi)] The correlation function is defined only in terms of the ratio of counts; it would be helpful to define how the underlying-event or combinatorial background is subtracted, since this affects the magnitude and shape of C(delta phi) and the extraction of the relative areas.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: the paper compares independent STAR measurements to external theoretical predictions and does not derive its results from fitted inputs or self-citation chains.

full rationale

The paper reports three preliminary measurements: inclusive jet cross sections at 200 and 510 GeV, the W boson cross-section ratio at 510 GeV, and forward di-pi0 correlations in pp, pAl, and pAu collisions. Each measurement is compared against external predictions that do not include the STAR data being presented: jets are compared to CT14 NLO perturbative QCD and Pythia6, the W ratio is compared to CT18, MSHT20, and NNPDF4.0 NLO PDF sets, and di-pi0 correlations are compared to the CGC expectation of suppression and broadening. No parameter is fitted to a subset of the presented data and then relabeled as a prediction, and no quantity is defined in terms of the result it is claimed to test. The references to prior STAR publications supply detector methods, previous measurements, and theoretical context, but they do not carry the load of making the current measurements equivalent to their inputs. The statement that the dominant systematic uncertainties come from the jet energy scale is an uncertainty caveat, not a circular construction; its completeness is a correctness issue, not circularity. Because the paper does not attempt a first-principles derivation and instead presents data comparisons, no circular reduction exists.

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

The paper introduces no new theoretical entities and fits no new parameters. It relies on standard QCD factorization, PDF universality, and the CGC picture as background assumptions.

assumptions (3)
  • domain assumption QCD factorization of inclusive jet cross sections into PDFs and partonic cross sections holds at RHIC energies.
    Section 2 states the cross sections can be factorized into PDFs and partonic scattering cross sections, relying on the standard QCD factorization theorem.
  • domain assumption PDFs are universal and can be extracted from global fits to other experiments.
    Section 2 says 'Given their universality, the parameters are determined by fits to global experimental data.'
  • domain assumption The CGC framework predicts suppression and azimuthal broadening of back-to-back di-hadrons in pA collisions.
    Section 4 cites ref. [12] for this prediction, used to interpret the di-pi0 correlation result.

how reviews work

0 comments
Cite this review

Pith. "Pith review of Recent results from STAR for parton distribution functions at low and high $x$ in proton-proton collisions." pith.science (2026). https://pith.science/paper/J5BIR4RK

@misc{pith2026241200614,
  author       = {Pith},
  title        = {Pith review of: Recent results from STAR for parton distribution functions at low and high $x$ in proton-proton collisions},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/J5BIR4RK}},
  note         = {Machine review of arXiv:2412.00614}
}
abstract

According to perturbative quantum chromodynamic calculations, in $pp$ collisions at $\sqrt{s} = $~200 and 510 GeV studied at RHIC, jet production in mid-pseudorapidity, $|\eta| <$ 1, is dominated by quark-gluon and gluon-gluon scattering processes. Therefore jets at RHIC are direct probes of the gluon parton distribution functions (PDFs) for momentum fractions 0.01 $<x<$ 0.5. Moreover, $W$ boson cross section ratio, $\sigma(W^+)/\sigma(W^-)$, in $pp$ collisions at $\sqrt{s} = 510$~GeV, is an effective tool to explore anti-quark PDFs, $\bar{d}/\bar{u}$. Last but not least, di-$\pi^{0}$ correlation in forward pseudorapidity, $2.6 < \eta <4.0$, is an important indication of the non-linear gluon dynamics at low $x$ where the gluon density is high in protons and nuclei. In this proceeding, we present recent STAR results of mid-pseudorapidity inclusive jet cross-sections at $\sqrt{s} =$~200 and 510 GeV in $pp$ collisions, $W$ boson cross-section ratios at $\sqrt{s} = 510$~GeV in $pp$ collisions, and forward di-$\pi^0$ correlations in $pp$, $p\textrm{Al}$ and $p\textrm{Au}$ collisions at $\sqrt{s_{\textrm{\tiny NN}}} = 200$~GeV.

Figures

Figures reproduced from arXiv: 2412.00614 by the authors.

Figure 1
Figure 1. STAR preliminary inclusive jet cross section [PITH_FULL_IMAGE:figures/full_fig_p003_1.png] view at source ↗
Figure 2
Figure 2. STAR preliminary W± cross-section ratio, RW , vs. reconstructed ηe from pp collisions at √ s = 510 GeV [9]. 4. Di-π 0 correlations in the forward region High energy physics experiments have shown that gluons dominate at low x inside the proton, and the number of gluons grows as x decreases [2, 10, 11]. An interesting theoretical expectation is that the gluon density must begin to saturate at a certain x where the ra… view at source ↗
Figure 3
Figure 3. STAR di-π 0 correlation function C(∆ϕ) in selected pT ranges of trigger and associate π 0 s from pp, pAu and pAl collisions at √ sNN = 200 GeV on the left panel, and the corresponding relative areas on the right panel [12]. As plotted in [PITH_FULL_IMAGE:figures/full_fig_p005_3.png] view at source ↗

Discussion (0). Continue with ORCID to comment.

Reference graph

Works this paper leans on

13 extracted references · 12 canonical work pages

  1. [1]

    K. H. Ackermann et al., Nucl. Instr. and Meth. A499 624 (2003)

    work page 2003

  2. [2]

    R. D. Ball et al., (NNPDF) EPJC 82 (2022), 428

    work page 2022

  3. [3]

    Adam et al., (STAR) Phys

    J. Adam et al., (STAR) Phys. Rev. D100 052005. (2019)

    work page 2019

  4. [4]

    Chang for the STAR Collaboration PoS (PANIC2021)377

    Z. Chang for the STAR Collaboration PoS (PANIC2021)377

    work page

  5. [5]

    Van Der Meer, ISR-PO/68-31 (1968)

    S. Van Der Meer, ISR-PO/68-31 (1968)

    work page 1968

  6. [6]

    Allasia et al

    D. Allasia et al. (NMC), Phys. Lett. B249, 366-372 (1990)

    work page 1990

  7. [7]

    Dove et al

    J. Dove et al. (SeaQuest), Nature 590, 561-565 (2021) [erratum: Nature 604, E26 (2022)]

    work page 2021

  8. [8]

    Adam et al

    J. Adam et al. (STAR), Phys. Rev. D103, 012001 (2021)

    work page 2021

Show all 13 references
  1. [9]

    J. D. Nam for the STAR Collaboration, SciPost Phys. Proc.8, 140 (2022)

    2022

  2. [10]

    T. J. Hou, et al. Phys. Rev. D103, 014013 (2021)

    2021

  3. [11]

    Bailey, et al

    S. Bailey, et al. Eur. Phys. J. C81, 341 (2021)

    2021

  4. [12]

    M. S. Abdallah et al. (STAR), Phys. Rev. Lett.129, 092501 (2022)

    2022

  5. [13]

    E. C. Aschenauer et al. (RHIC SPIN), arXiv:2302.00605 [nucl-ex]

    arXiv

Pith tools

Reviewed August 12, 2026 · model on record in the stance chip above.