Global polarization of $Λ$, $Ξ$, and $Ω$ hyperons in Au+Au collisions from the RHIC Beam Energy Scan-II

STAR Collaboration

AI summary · arXiv: 2611.03044v1 · nucl-ex

Global polarization of $Λ$, $Ξ$, and $Ω$ hyperons in Au+Au collisions from the RHIC Beam Energy Scan-II

STAR Collaboration

Original paper Submitted to arXiv on 02 Nov. 2026 6 pages, 4 figures License: CC BY-NC-ND 4.0 Also in: fr

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The essentials

STAR measures hyperon global polarization across seven collision energies, finding a significant positive rapidity dependence for Λ and first significant Ξ polarization in this energy range.

Question

How do global polarization and its energy, rapidity, and particle-species dependence behave in Au+Au collisions, and do Λ and anti-Λ polarizations differ in a way that constrains magnetic-field effects?

Method

The STAR Collaboration analyzed BES-II Au+Au data at √sNN = 7.7, 9.2, 11.5, 14.6, 17.3, 19.6, and 27 GeV, with 110–668 million minimum-bias events per dataset. They reconstructed Λ, anti-Λ, Ξ, and Ω hyperons and extracted polarization from decay-product angular distributions relative to the first-order event plane; Ω polarization was inferred through spin transfer from its daughter Λ. Λ rapidity dependence was measured at 20–50% centrality, and systematic uncertainties were evaluated for reconstruction, backgrounds, event-plane determination, acceptance, and decay parameters.

Findings

Result Λ and anti-Λ polarizations are consistent with each other across the measured collision-energy range.
See the passage in the paper
With significantly reduced uncertainties, the difference $\Delta P$ is further confirmed to be consistent with zero across a wider energy range.
Result Λ polarization increases with absolute rapidity, with the combined linear-fit slope differing from zero by 4.8 standard deviations.
See the passage in the paper
The fit yields a slope of $b=0.498\pm 0.103$, including both statistical and systematic uncertainties, which differs from zero by $4.8$ standard deviations.
Result Ξ− plus anti-Ξ+ polarization is observed with significance above five standard deviations and decreases as collision energy rises.
See the passage in the paper
A clear polarization signal of $\Xi^{-}+\bar{\Xi}^{+}$ is observed and exhibits a collision energy dependence, rising toward lower energies.
Authors’ interpretation The Ω measurements favor the positive decay-parameter solution under the assumed spin-polarization comparison, but the species comparison is not highly precise.
See the passage in the paper
Under this assumption, the measurements favor the $\gamma_{\Omega}\approx+1$ solution.
Authors’ interpretation The similar energy trends measured for Λ, Ξ, and Ω are interpreted as supporting a common vorticity-driven origin of global polarization.
See the passage in the paper
Such a unified energy dependence trend for $\Lambda$, $\Xi$, and $\Omega$ supports a common vorticity-driven origin of global polarization.

Limitations

The measurements are not corrected for feed-down from particle decays.
See the passage in the paper
We note that the polarization measurements are not corrected for feed-down contributions from particle decays.
The Ω polarization results have relatively large uncertainties, limiting the precision of comparisons with Λ polarization.
See the passage in the paper
a hint of larger $\Omega^{-}+\bar{\Omega}^{+}$ polarization than that of $\Lambda+\bar{\Lambda}$ at lower collision energies.
Anti-Λ rapidity dependence could not be presented with comparable coverage because of limited statistical precision at larger rapidity.
See the passage in the paper
Only the $\Lambda$ results are presented here because the $\bar{\Lambda}$ measurements have limited statistical precision at larger $|y|$.

Why it matters

The measurements add constraints on the energy, rapidity, and species dependence of hyperon polarization, including a first significant Ξ signal in this energy range and evidence for increasing Λ polarization with rapidity. The authors argue that the shared energy trend supports a common vorticity-driven mechanism, while further theoretical work and higher-precision measurements are needed.

Written by AI from the paper. Passages are checked word for word against the text we analysed; the rest may contain errors.

AI-generated keywords: Global polarization Hyperons Relativistic heavy-ion collisions Vorticity STAR experiment

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Comprehensive Summary
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Original abstract

The paper reports measurements of global polarization in Lambda, anti-Lambda, Xi, and Omega hyperons produced in gold–gold collisions at seven energies between 7.7 and 27 GeV. Global polarization describes the alignment of particle spins with the direction of the collision system’s orbital angular momentum. The study examines how polarization varies with collision energy, rapidity, and hyperon species, and whether differences between Lambda and anti-Lambda polarization indicate a contribution from the magnetic field. It also tests whether polarization patterns across hyperon species are consistent with a common underlying mechanism. The analysis uses high-statistics data collected by the STAR experiment during the second phase of the RHIC Beam Energy Scan. The datasets contain approximately 110 million events at 7.7 GeV, 204 million at 9.2 GeV, 269 million at 11.5 GeV, 413 million at 14.6 GeV, 275 million at 17.3 GeV, 668 million at 19.6 GeV, and 521 million at 27 GeV. Hyperons were reconstructed from their characteristic decay products. Their polarization was inferred from the daughter-baryon angular distributions relative to an event plane estimated using forward particles and spectator signals. For Xi and Omega hyperons, the researchers used direct angular measurements where possible, as well as polarization transferred to the daughter Lambda. The Omega measurement assumes a decay spin-transfer coefficient, with particular attention to the solution corresponding to gamma Omega near +1. The results are not corrected for feed-down from heavier-particle decays. Lambda and anti-Lambda polarization decrease as collision energy increases, with no statistically significant difference between the two across the measured range. Under a thermal freeze-out interpretation, this result yields magnetic-field estimates consistent with zero. The reported 95% confidence upper limits range from 3.8 × 10^13 tesla at 7.7 GeV to 0.9 × 10^13 tesla at 19.6 GeV. These limits depend on the assumed emission temperature and the thermal relationship between polarization splitting and magnetic field; they do not show that the collision’s magnetic field is absent. The study finds that Lambda polarization increases with the magnitude of rapidity. A straight-line fit to measurements combined across 7.7–17.3 GeV gives a slope of 0.498 ± 0.103, including statistical and systematic uncertainties. This nonzero slope has a significance of 4.8 standard deviations, and the trend is more pronounced at lower energies. The result provides new evidence about the longitudinal structure of polarization and differs from some model predictions, including one that predicts decreasing polarization with rapidity at 7.7 GeV. The researchers also report the first significant Xi plus anti-Xi polarization signal in this energy range. Its energy dependence resembles that of Lambda, with larger polarization at lower energies. Averaged over 7.7–27 GeV, the direct measurement is 1.389 ± 0.163% (statistical) ± 0.212% (systematic), while the indirect measurement using the daughter Lambda is 1.565 ± 0.094% (statistical) ± 0.200% (systematic). Their significances are 5.2 and 7.1 standard deviations, respectively, and the results agree within one standard deviation. At the current level of precision, Xi polarization is not significantly different from Lambda polarization. Omega polarization also trends downward with increasing energy, although the uncertainties are large and the result at the lowest energy is negative. Assuming the positive spin-transfer solution, the energy-averaged value is 3.731 ± 1.062% (statistical) ± 1.289% (systematic).

What hyperon spins reveal about nuclear collisions

In collisions between gold nuclei, the particles produced can carry a subtle imprint of the collision’s motion. A new STAR Collaboration study, “Global polarization of Λ, Ξ, and Ω hyperons in Au+Au collisions from the RHIC Beam Energy Scan-II,” measures how the spins of several short-lived particles align with the orbital angular momentum of the colliding system. This alignment, known as global polarization, offers a way to investigate the extreme conditions created in these collisions.

The researchers analyzed data from seven collision energies, spanning 7.7 to 27 billion electron volts. Their measurements track how polarization changes with energy, particle type, and rapidity—a quantity related to a particle’s motion along the collision axis. Together, the results provide new evidence about the structure and possible origins of polarization in the hot matter produced at RHIC.

How the measurements were made

The analysis uses high-statistics data collected by the STAR experiment during the second phase of the RHIC Beam Energy Scan. The datasets include roughly 110 million collision events at 7.7 GeV and 668 million at 19.6 GeV, with hundreds of millions more at each of the other energies. Hyperons were identified through the characteristic decay products they leave behind.

Because hyperon decays are sensitive to the parent particle’s spin, researchers can infer polarization from the angular distribution of a decay product. They compared these directions with an estimate of the collision’s orbital-angular-momentum direction, reconstructed using forward-moving particles and spectator signals. For Ξ and Ω hyperons, the team also used polarization transferred to a daughter Λ. The Ω result depends on an assumed spin-transfer solution, with special attention to the case where the transfer coefficient γΩ is near +1.

The reported measurements are not corrected for hyperons produced indirectly in decays of heavier particles. That detail matters when interpreting the numbers, but it does not prevent comparisons among the patterns observed in this analysis.

Λ and anti-Λ: energy and magnetic-field clues

Λ and anti-Λ polarization both decrease as collision energy rises. Across the measured range, the study finds no statistically significant difference between the two. This comparison is of interest because a magnetic field could affect particles and antiparticles differently, potentially creating a polarization splitting.

Interpreted using a thermal freeze-out framework, the lack of a significant splitting gives magnetic-field estimates consistent with zero. The study reports 95% confidence upper limits ranging from 3.8 × 1013 tesla at 7.7 GeV to 0.9 × 1013 tesla at 19.6 GeV. These are limits, not evidence that the collision’s magnetic field is absent. They depend on the assumed emission temperature and on the thermal relationship used to connect polarization differences to magnetic-field strength.

Polarization grows farther from midrapidity

The Λ results also reveal a pronounced dependence on rapidity. Polarization increases with the magnitude of rapidity, meaning it is larger farther from the central region of the collision. Combining measurements from 7.7 to 17.3 GeV, the researchers find a straight-line slope of 0.498 ± 0.103, with statistical and systematic uncertainties included. The slope differs from zero by 4.8 standard deviations.

The rise is more pronounced at lower collision energies. It offers new evidence about how polarization is distributed along the collision axis and poses a challenge to some model expectations. In particular, one model predicts that polarization should decrease with rapidity at 7.7 GeV, unlike the observed trend.

Comparing Λ, Ξ, and Ω

The study reports the first significant combined Ξ and anti-Ξ polarization signal in this energy range. Its energy dependence resembles that of Λ: polarization is greater at lower energies. Averaged over 7.7–27 GeV, the direct Ξ measurement is 1.389 ± 0.163% (statistical) ± 0.212% (systematic). The indirect measurement, using polarization transferred to a daughter Λ, is 1.565 ± 0.094% (statistical) ± 0.200% (systematic). Their significances are 5.2 and 7.1 standard deviations, respectively, and the two results agree within one standard deviation.

At the current precision, Ξ polarization is not significantly different from Λ polarization. This similarity is consistent with a common underlying mechanism, though more precise measurements will be needed to test that interpretation more sharply.

Ω polarization also trends downward as energy increases, but its uncertainties are large, and the result at the lowest energy is negative. Under the assumed positive spin-transfer solution, its energy-averaged value is 3.731 ± 1.062% (statistical) ± 1.289% (systematic).

A clearer picture, with open questions

Across several hyperon species, the measurements establish a consistent broad pattern: polarization is stronger at lower collision energies, while Λ polarization grows with the magnitude of rapidity. The Λ–anti-Λ comparison places limits on a possible magnetic-field contribution, but does not rule out a field. Meanwhile, the agreement between direct and indirect Ξ measurements and the similarity of Ξ and Λ polarization provide useful tests of shared explanations.

The results deepen the experimental picture of spin alignment in nuclear collisions, while leaving important questions open. Better precision—especially for Ω hyperons and for differences between particle species—will help determine which mechanisms best explain how the collision’s motion becomes encoded in the spins of the particles it produces.

Created on 09 Nov. 2026

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Weekly letter Presented in "Particles, nuclei and gravitation this week" of 9 November 2026. Particles, nuclei and gravitation this week: rare decays and the shape of the unseen

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Global polarization of $Λ$, $Ξ$, and $Ω$ hyperons in Au+Au collisions from the RHIC Beam Energy Scan-II

The essentials

Question

Method

Findings

Limitations

Why it matters

Read further

Definitions

What hyperon spins reveal about nuclear collisions

How the measurements were made

Λ and anti-Λ: energy and magnetic-field clues

Polarization grows farther from midrapidity

Comparing Λ, Ξ, and Ω

A clearer picture, with open questions

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