[Paper Review] Global Polarization Effect in the Extremely Rapidly Rotating QGP in HIC

Zuo-tang Liang, M. A. Lisa+1 more|arXiv (Cornell University)|Dec 17, 2019

High-Energy Particle Collisions Research1 references4 citations

TL;DR

This paper investigates global polarization in the extremely rapidly rotating quark-gluon plasma (QGP) formed in non-central heavy-ion collisions (HIC), demonstrating that the QGP exhibits a vorticity of approximately (9 ± 1) × 10²¹ s⁻¹, making it the most vortical fluid ever observed. The study confirms theoretical predictions linking orbital angular momentum to hyperon polarization, with STAR Collaboration data showing measurable global polarization of Λ and Λ̄ hyperons, providing a new probe of QGP's rotational and electromagnetic properties.

ABSTRACT

This is prepared for a featured article in Nuclear Physics News. Recently, the global polarization of Λand \barΛ hyperons in heavy-ion collisions (HIC) has been observed by the STAR Collaboration at the Relativistic Heavy Ion Collider in Brookhaven National Laboratory. The discovery confirms the theoretical prediction made more than ten years ago and indicates that the quark gluon plasma (QGP) produced in HIC possesses a vorticity as high as 10^21s^-1, much higher than any other fluid observed in nature. This opens a new window to study properties of QGP and a new direction in high energy heavy ion physics. This featured article is aimed to report the basic idea, current status and outlook.

Motivation & Objective

• To investigate the origin and manifestation of global polarization in the quark-gluon plasma (QGP) produced in non-central heavy-ion collisions (HIC).
• To confirm the theoretical prediction that the QGP's large orbital angular momentum couples to spin degrees of freedom, leading to observable hyperon polarization.
• To quantify the vorticity of the QGP using polarization measurements of Λ and Λ̄ hyperons.
• To explore the interplay between vorticity, spin-orbit coupling, and strong magnetic fields in the QGP as a probe of its extreme conditions.

Proposed method

• Theoretical modeling of global orbital angular momentum in non-central HIC using hard-sphere and Wood-Saxon nuclear distributions.
• Estimation of local vorticity via transverse gradients in longitudinal momentum using hydrodynamic simulations.
• Application of the hydrodynamic model to relate observed hyperon polarization to system vorticity.
• Use of STAR Collaboration data from Au-Au collisions at √s = 200 GeV to extract polarization values and infer vorticity.
• Comparison of polarization data with predictions from spin-orbit coupling in a vortical fluid to estimate QGP vorticity.
• Incorporation of magnetic field effects via spin-magnetic coupling to assess their influence on Λ and Λ̄ polarization patterns.

Experimental results

Research questions

• RQ1 What is the magnitude of vorticity in the quark-gluon plasma formed in non-central heavy-ion collisions?
• RQ2 How does the global orbital angular momentum of the QGP manifest as measurable hyperon polarization?
• RQ3 To what extent do spin-orbit coupling and magnetic fields contribute to the observed polarization of Λ and Λ̄ hyperons?
• RQ4 Can the observed polarization be consistently explained by a vortical ideal fluid model?
• RQ5 What is the role of magnetic fields in modifying the polarization pattern of hyperons in the QGP?

Key findings

• The STAR Collaboration measured global polarization of Λ hyperons at 1.08 ± 0.15 (stat) ± 0.11 (sys) per cent, confirming theoretical predictions.
• For Λ̄ hyperons, the global polarization was measured at 1.38 ± 0.30 (stat) ± 0.13 (sys) per cent, indicating a larger polarization than for Λ.
• The inferred QGP vorticity is (9 ± 1) × 10²¹ s⁻¹, the highest ever observed in nature, surpassing all other known fluids.
• The polarization pattern suggests a possible fine-structure effect from strong magnetic fields, with B ~ 10¹⁴ T, though higher-precision data is needed for confirmation.
• Hydrodynamic simulations show that local vorticity forms a ring-like structure in the transverse plane, with non-zero net vorticity perpendicular to the reaction plane.
• The results establish global hyperon polarization as a robust probe of QGP's rotational and electromagnetic properties in extreme conditions.