arxiv: 2604.22184 · v1 · submitted 2026-04-24 · ⚛️ nucl-ex

Recognition: unknown

Cold Nuclear Matter Effects on Inclusive J/psi Production in p+Au Collisions at sqrt{s_NN} = 200 GeV with the STAR Experiment

STAR Collaboration

Authors on Pith no claims yet

Pith reviewed 2026-05-08 08:53 UTC · model grok-4.3

classification ⚛️ nucl-ex

keywords J/psi productioncold nuclear matternuclear modification factorSTAR experimentp+Au collisions200 GeVquarkonium

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The pith

The nuclear modification factor R_pAu for inclusive J/ψ production in p+Au collisions is consistent with unity.

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

This paper reports measurements of inclusive J/ψ production in both p+p and p+Au collisions at a center-of-mass energy of 200 GeV per nucleon pair using the STAR detector. The nuclear modification factor R_pAu is calculated as the ratio of the J/ψ yield in p+Au to the yield in p+p, scaled by the number of binary nucleon-nucleon collisions. In the transverse momentum range of 4 to 12 GeV/c and within |y| < 1, this factor is found to be consistent with 1. This result indicates that cold nuclear matter effects cause negligible modification to the J/ψ yield in this kinematic region, while also providing improved precision on the cross sections and invariant yields.

Core claim

The R_pAu is consistent with unity, suggesting negligible modification of the yield by CNM effects in this kinematic region. Various model calculations agree with the R_pAu measurement, although they are less satisfactory in describing the individual invariant yields from p+p and p+Au collisions. The analysis improves the precision of the cross-section and invariant-yield measurements.

What carries the argument

The nuclear modification factor R_pAu, which is the ratio of the inclusive J/ψ yield in p+Au collisions to that in p+p collisions scaled by the average number of binary nucleon-nucleon collisions.

If this is right

The measured R_pAu being consistent with unity implies that cold nuclear matter effects do not significantly alter J/ψ production in the covered kinematic range.
Model calculations incorporating CNM effects match the observed R_pAu value.
These models, however, provide a less accurate description of the absolute invariant yields in p+p and p+Au collisions.
Improved precision is achieved in both the p+p cross section and the p+Au invariant yield measurements.

Where Pith is reading between the lines

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

Any observed suppression of J/ψ in heavy-ion collisions at similar energies is likely dominated by hot medium effects rather than cold nuclear matter.
The mismatch between model fits to R_pAu and to the raw yields suggests possible shortcomings in the models' handling of absolute production rates.
Extending these measurements to lower pT or different rapidities could reveal where CNM effects become more prominent.

Load-bearing premise

The long-established presumption that the energy density and temperature in proton-nucleon collisions are too low to form quark-gluon plasma droplets, allowing R_pAu to be attributed solely to cold nuclear matter effects.

What would settle it

A precise measurement of R_pAu significantly deviating from unity in the same pT and y range, or independent evidence that quark-gluon plasma forms in p+Au collisions at this energy.

Figures

Figures reproduced from arXiv: 2604.22184 by STAR Collaboration.

**Figure 1.** Figure 1: FIG. 1. The invariant mass distributions of view at source ↗

**Figure 3.** Figure 3: FIG. 3. Inclusive view at source ↗

**Figure 2.** Figure 2: FIG. 2. Inclusive view at source ↗

**Figure 4.** Figure 4: FIG. 4. The view at source ↗

read the original abstract

In this paper, a study of cold nuclear matter (CNM) effects is reported based on the new STAR measurement of inclusive $J/\psi$ production in $p+p$ and $p+\text{Au}$ collisions at $\sqrt{s_\text{NN}}$ = 200 GeV, and a combined $J/\psi\rightarrow e^{+}e^{-}$ cross section in $p+p$ collisions at $\sqrt{s}$ = 200 GeV is provided. Given the long-established presumption that the energy density and the temperature produced in proton-nucleon collisions are insufficient to form QGP droplets, CNM effects in $p+\text{Au}$ collisions are quantified by the nuclear modification factor ($R_{p\text{Au}}$), defined as the ratio of the yield of the inclusive $J/\psi$ in $p+\text{Au}$ collisions to that in $p+p$ collisions, scaled by the average number of binary nucleon-nucleon collisions. The $R_{p\text{Au}}$ is derived as a function of transverse momentum ($p_\text{T}$) in the range 4--12 GeV/$c$ and is averaged within the rapidity ($y$) and azimuthal angle ($\varphi$) coverage of $|y|<1$, $0 \leq \varphi < 2\pi$. The result is consistent with unity, suggesting negligible modification of the yield by CNM effects in this kinematic region. Various model calculations are in agreement with the $R_{p\text{Au}}$ measurement, yet the same calculations are less satisfactory in describing the individual invariant yields from $p+p$ and $p+\text{Au}$ collisions. In the covered kinematic region, this analysis has improved the precision of cross-section and invariant-yield measurements in $p+p$ and $p+\text{Au}$ collisions, respectively, and consequently the $R_{p\text{Au}}$.

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.

Desk Editor's Note private letter to a colleague

STAR's updated R_pAu for J/ψ in p+Au at 200 GeV lands consistent with unity in the 4-12 GeV/c window, with tighter errors on the p+p reference and p+Au yields.

read the letter

The key point is that this STAR measurement finds the nuclear modification factor R_pAu for inclusive J/ψ consistent with no cold nuclear matter modification in p+Au collisions at 200 GeV, for pT from 4 to 12 GeV/c and |y|<1. They also report a combined p+p cross section that tightens the reference baseline compared to earlier data sets. The result follows the standard ratio of invariant yields scaled by average binary collisions, and the data support the claim directly within uncertainties. Models line up with the ratio even while they underperform on the separate p+p and p+Au yields, which is a useful data point for the field. The improved precision on both the cross section and the p+Au invariant yield is the main advance here, giving a cleaner reference for later heavy-ion suppression studies. The paper sticks to the usual assumption that proton-nucleus collisions at this energy do not produce QGP conditions, so the ratio can be read as a pure CNM probe. That assumption is long-standing and not tested in the work itself, but it is the standard framing. The noted tension where models fit the ratio better than the absolute yields is flagged in the abstract and deserves checking in the full systematics section. Nothing in the reported approach looks circular or internally inconsistent. This paper is for experimentalists and theorists working on quarkonium production in nuclear collisions who need the latest reference numbers. A reader focused on relativistic heavy-ion physics will find the updated yields and model comparisons worth having. It is a direct experimental result with no obvious load-bearing flaws in the central claim. I would send it for peer review.

Referee Report

1 major / 2 minor

Summary. The manuscript reports a new STAR measurement of inclusive J/ψ production in p+p and p+Au collisions at √s_NN = 200 GeV. The nuclear modification factor R_pAu is extracted as a function of p_T (4–12 GeV/c) within |y| < 1 and found to be consistent with unity, implying negligible cold nuclear matter modification of the yield in this kinematic region. An updated combined p+p cross section is provided, and the data are compared to several model calculations that describe the ratio better than the absolute invariant yields.

Significance. If the central result holds, the work supplies a high-precision, data-driven constraint on CNM effects for charmonium at mid-rapidity and moderate p_T. The direct ratio measurement (scaled only by <N_coll>) and the reported improvement in p+p and p+Au precision constitute clear strengths that will be useful for future global analyses of nuclear PDFs and CNM mechanisms.

major comments (1)

Abstract and Results section: the observation that the same models describe R_pAu well but fit the separate p+p and p+Au invariant yields less satisfactorily is noted but not quantified; a table or figure showing the χ² per degree of freedom for each observable would clarify whether this tension is statistically significant and whether it affects the interpretation of the R_pAu result.

minor comments (2)

Figure captions and text should explicitly state the integrated luminosity and trigger conditions used for the p+Au data set to allow direct comparison with other experiments.
The definition of the rapidity and azimuthal acceptance should be repeated in the Results section for readers who skip the experimental setup.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for the positive assessment of the work and for the constructive comment on model comparisons. We address the point below.

read point-by-point responses

Referee: [—] Abstract and Results section: the observation that the same models describe R_pAu well but fit the separate p+p and p+Au invariant yields less satisfactorily is noted but not quantified; a table or figure showing the χ² per degree of freedom for each observable would clarify whether this tension is statistically significant and whether it affects the interpretation of the R_pAu result.

Authors: We agree that a quantitative assessment of the model agreement strengthens the presentation. We have added a new table in the Results section that lists the χ² per degree of freedom for each model relative to the p+p invariant cross section, the p+Au invariant yield, and the R_pAu. The values confirm better agreement with the ratio than with the absolute yields and show no statistically significant tension that would change the conclusion of negligible CNM effects in the reported kinematic region. revision: yes

Circularity Check

0 steps flagged

Direct data ratio; no derivation reduces to inputs by construction

full rationale

The paper computes R_pAu directly from measured invariant yields in p+p and p+Au collisions, scaled by the independently determined <N_coll>. No parameter is fitted to a subset of the data and then re-used to 'predict' the same ratio. The interpretation as a pure CNM probe rests on the external, long-standing presumption that p+N collisions at 200 GeV do not produce QGP; this presumption is not derived inside the paper nor justified by self-citation. Model comparisons are presented as external checks, not as load-bearing steps. The central result is therefore an empirical ratio whose value is not forced by the paper's own equations or prior self-references.

Axiom & Free-Parameter Ledger

0 free parameters · 2 axioms · 0 invented entities

The analysis rests on the standard heavy-ion physics assumption that binary nucleon-nucleon collision scaling provides the correct normalization for cold nuclear matter studies and that p+A collisions do not produce QGP.

axioms (2)

domain assumption Binary collision scaling defines the nuclear modification factor R_pAu
Invoked in the definition of R_pAu as the ratio scaled by average number of binary collisions.
domain assumption Proton-nucleon collisions do not produce QGP droplets
Stated as the long-established presumption allowing CNM interpretation of R_pAu.

pith-pipeline@v0.9.0 · 5657 in / 1359 out tokens · 58056 ms · 2026-05-08T08:53:35.483353+00:00 · methodology

discussion (0)

Reference graph

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