arxiv: 2604.07005 · v1 · submitted 2026-04-08 · ⚛️ nucl-ex · nucl-th

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Measurement of inclusive J/psi polarization in Ru+Ru and Zr+Zr collisions at sqrt{s_{rm NN}}=200 GeV at STAR

STAR Collaboration

Pith reviewed 2026-05-10 17:59 UTC · model grok-4.3

classification ⚛️ nucl-ex nucl-th

keywords J/ψ polarizationheavy-ion collisionsSTAR experimentRu+Ru collisionsZr+Zr collisionsRHICquarkonium productiondi-electron decay

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The pith

J/ψ mesons show no net polarization in Ru+Ru and Zr+Zr collisions at 200 GeV.

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

This paper presents the first measurement of inclusive J/ψ polarization at mid-rapidity in 200 GeV Ru+Ru and Zr+Zr collisions with the STAR detector. J/ψ are reconstructed via their electron-positron decay, and the polarization parameters are extracted as functions of transverse momentum and centrality in both the helicity and Collins-Soper frames. The parameters are found to be consistent with zero over 0.2 to 10 GeV/c in p_T and 0-80 percent centrality. A sympathetic reader would care because the null result tests whether heavy-quark spin alignment arises in the nuclear medium and directly compares to both proton-proton data and transport-model expectations at the same energy.

Core claim

The central claim is that the inclusive J/ψ polarization parameters λ_θ and λ_φ remain consistent with zero across the measured transverse-momentum range 0.2 < p_T < 10 GeV/c and across collision centralities 0-80 percent in both the helicity and Collins-Soper frames. This is the first such measurement in Ru+Ru and Zr+Zr isobar collisions at RHIC, and the null result agrees with corresponding measurements in p+p collisions at the same collision energy as well as with transport-model calculations.

What carries the argument

The polarization parameters λ_θ and λ_φ, obtained from the angular distribution of J/ψ decay electrons in the helicity and Collins-Soper frames.

If this is right

The null polarization indicates that the dominant J/ψ production mechanisms generate no measurable spin alignment in these heavy-ion systems.
Agreement with p+p results implies that any medium-induced polarization effects are smaller than experimental precision.
Consistency with transport-model calculations supports the use of those models for describing J/ψ spin dynamics at RHIC energies.
The result holds across a wide centrality range, showing that varying medium density does not induce detectable polarization.

Where Pith is reading between the lines

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

The absence of polarization may mean that any initial-state spin correlations from gluon fusion are randomized by interactions in the medium.
Extending the measurement to higher p_T or to other quarkonia such as the Υ could test whether the null result is universal.
Higher-luminosity runs at RHIC or the LHC could reveal small non-zero values that current statistics cannot resolve.
The result constrains the parameter space of models that link quarkonium polarization to the presence of a quark-gluon plasma.

Load-bearing premise

Detector acceptance, efficiency corrections, and background subtraction introduce no significant bias in the extracted polarization parameters.

What would settle it

A high-statistics measurement in the same kinematic range that finds λ_θ or λ_φ significantly different from zero would falsify the claim of consistency with zero.

Figures

Figures reproduced from arXiv: 2604.07005 by STAR Collaboration.

**Figure 2.** Figure 2: Upper: raw J/ψ yield and its acceptance and efficiency (A × ϵ) from the final iteration as a function of daughter positron’s cos θ (left) and ϕ (right) in the helicity frame for 0.2 < p J/ψ T < 10 GeV/c, where N raw total denotes the total raw J/ψ yield. Lower: J/ψ yields corrected for acceptance and efficiency, along with the simultaneous fit, where N corr total denotes the total corrected J/ψ yield. −1 −… view at source ↗

**Figure 3.** Figure 3: Upper: raw J/ψ yield and its A×ϵ from the final iteration as a function of daughter positron’s cos θ (left) and ϕ (right) in the Collins-Soper frame for 0.2 < p J/ψ T < 10 GeV/c, where N raw total is total J/ψ yield of raw data. Lower: J/ψ yields corrected for acceptance and efficiency, along with the simultaneous fit, where N corr total denotes the total corrected J/ψ yield. of the invariant mass spectrum… view at source ↗

**Figure 5.** Figure 5: illustrates the dependence of the inclusive J/ψ polarization parameters within 0.2 < p J/ψ T < 10 GeV/c on collision centrality in the HX and CS frames for Ru+Ru and Zr+Zr collisions at √ sNN = 200 GeV. The numerical values can be found in [PITH_FULL_IMAGE:figures/full_fig_p006_5.png] view at source ↗

**Figure 4.** Figure 4: Inclusive J/ψ polarization parameters (from top to bottom: λθ, λϕ, λinv) as a function of J/ψ pT for Ru+Ru and Zr+Zr collisions at √ sNN = 200 GeV. The bars indicate statistical uncertainties, while the boxes denote systematic uncertainties. Polarization parameters in the helicity frame are presented on the left and those from Collins-Soper frame are shown on the right [PITH_FULL_IMAGE:figures/full_fig_… view at source ↗

**Figure 6.** Figure 6: Extracted polarization parameter for regenerated [PITH_FULL_IMAGE:figures/full_fig_p007_6.png] view at source ↗

read the original abstract

The first measurement of inclusive J/psi polarization at mid-rapidity (|y^{J/psi}| < 0.8) in 200 GeV Ru+Ru and Zr+Zr collisions at sqrt(s_NN) = 200 GeV with the STAR experiment at RHIC is presented. J/psi mesons are reconstructed through their di-electron (e+e-) decay channel. The polarization parameters (lambda_theta, lambda_phi) are measured as a function of the J/psi transverse momentum (p_T) and collision centrality in both the helicity and the Collins-Soper frames. These polarization parameters are found to be consistent with zero across the measured J/psi p_T range of 0.2 < p_T < 10 GeV/c and across collision centralities within 0-80 percent in both frames. These results are consistent with corresponding measurements p+p collisions at the same collision energy and with transport-model calculations.

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

This is a clean first measurement of J/ψ polarization in Ru+Ru and Zr+Zr collisions that simply confirms the zero result already seen in p+p.

read the letter

The paper reports the first inclusive J/ψ polarization measurement at mid-rapidity in 200 GeV Ru+Ru and Zr+Zr collisions. The extracted λ_θ and λ_φ parameters are consistent with zero across 0.2 < p_T < 10 GeV/c and 0-80% centrality in both the helicity and Collins-Soper frames. This matches the STAR p+p results at the same energy and the expectations from transport models. The analysis uses the standard di-electron reconstruction channel, which is appropriate for this observable at STAR. Extending the measurement to the isobar systems is the clear novelty; those runs were taken to test nuclear structure effects, so the polarization data adds a modest constraint on heavy-quark production pictures even if it does not shift any models. The result is presented directly against external benchmarks with no internal fitting or circular definitions. The central claim rests on acceptance and efficiency corrections plus background subtraction, which are standard steps but would need the usual control plots and uncertainty tables to be fully evaluated. From the abstract alone those details are not visible, so the support for the zero result cannot be checked at the level of a full review. This is incremental work rather than a resolution of an open question. It is useful for the subset of heavy-ion physicists who follow quarkonium polarization and want the complete set of RHIC data points. A serious referee should see it to verify the analysis cuts and systematic treatment, but the paper does not require major revision on the basis of what is shown. I would send it to peer review.

Referee Report

0 major / 3 minor

Summary. The manuscript presents the first measurement of inclusive J/ψ polarization in Ru+Ru and Zr+Zr collisions at √s_NN = 200 GeV with the STAR experiment. J/ψ mesons are reconstructed via the di-electron decay channel at mid-rapidity (|y| < 0.8). Polarization parameters λ_θ and λ_φ are extracted as functions of J/ψ p_T (0.2–10 GeV/c) and collision centrality (0–80%) in both the helicity and Collins-Soper frames. The central result is that both parameters are consistent with zero across the measured kinematic range in both collision systems, and the measurements agree with prior p+p data at the same energy as well as with transport-model calculations.

Significance. This is the first polarization measurement in isobar collisions at RHIC. If the result holds, it provides direct experimental input constraining the relative contributions of different J/ψ production mechanisms (color-singlet vs. color-octet) and the role of the medium in heavy-ion collisions. The consistency with zero polarization and with p+p data is a clear, falsifiable statement that can be used to test model predictions for quarkonium dynamics.

minor comments (3)

The manuscript should explicitly state the functional form used for the angular distribution fit (e.g., the exact expression for the decay angular distribution in terms of λ_θ and λ_φ) and confirm that the fit is performed in the full 3D acceptance-corrected space rather than projected distributions.
Section describing systematic uncertainties: the contribution from background subtraction and from the choice of signal and sideband regions should be quantified separately for each p_T and centrality bin, with the total systematic uncertainty shown as a separate band on the figures.
The comparison to transport models would benefit from overlaying the model predictions with their theoretical uncertainty bands on the same plots as the data points.

Simulated Author's Rebuttal

0 responses · 0 unresolved

We thank the referee for the positive summary of our manuscript and the recommendation for minor revision. The referee correctly identifies this as the first inclusive J/ψ polarization measurement in isobar collisions at RHIC, with results showing λ_θ and λ_φ consistent with zero across the measured p_T and centrality ranges in both frames, in agreement with p+p data and transport models. No specific major comments were raised in the report.

Circularity Check

0 steps flagged

No circularity: direct experimental measurement with external comparisons

full rationale

The paper reports a direct experimental extraction of inclusive J/ψ polarization parameters (λ_θ, λ_φ) from di-electron decays in Ru+Ru and Zr+Zr collisions. The analysis chain consists of reconstruction, acceptance/efficiency corrections, background subtraction, and fits to the angular distribution in helicity and Collins-Soper frames. The central claim—that the parameters are consistent with zero over the measured p_T and centrality range—is a data-driven result, not a derivation that reduces to its own inputs by construction. Consistency with prior p+p measurements and transport models is an external comparison, not a self-referential step. No self-definitional relations, fitted inputs renamed as predictions, or load-bearing self-citations appear in the derivation. The result is self-contained against external benchmarks.

Axiom & Free-Parameter Ledger

0 free parameters · 2 axioms · 0 invented entities

The analysis relies on established experimental techniques and theoretical frameworks from heavy-ion physics with no new free parameters or invented entities.

axioms (2)

domain assumption J/ψ mesons are reconstructed via their decay to electron-positron pairs.
This is the standard channel used for J/ψ identification in the STAR experiment.
standard math The angular distribution of decay products is parameterized by λ_θ and λ_φ in the helicity and Collins-Soper frames.
This is the conventional way to quantify polarization in vector meson studies.

pith-pipeline@v0.9.0 · 5473 in / 1314 out tokens · 77049 ms · 2026-05-10T17:59:36.083586+00:00 · methodology

discussion (0)

Lean theorems connected to this paper

Citations machine-checked in the Pith Canon. Every link opens the source theorem in the public Lean library.

IndisputableMonolith/Cost/FunctionalEquation.lean washburn_uniqueness_aczel unclear
These polarization parameters are found to be consistent with zero across the measured J/ψ p_T range of 0.2 < p_T < 10 GeV/c and across collision centralities within 0-80 percent in both frames.

Reference graph

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