[Paper Review] Global polarization and parity violation study in Au+Au collisions

I. Selyuzhenkov|arXiv (Cornell University)|Oct 25, 2005

High-Energy Particle Collisions Research19 citations

TL;DR

This study investigates global polarization and parity violation in Au+Au collisions at √sNN = 62 GeV using the STAR detector. It employs three-particle azimuthal correlations and hyperon decay angular distributions relative to the reaction plane, finding no significant polarization or parity violation, with Λ hyperon polarization constrained to PΛ = (−1.44 ± 9.66) × 10⁻³, far below theoretical predictions.

ABSTRACT

We present results on the parity violation effects and global system polarization measurements in Au+Au collisions at sqrt(s_NN) = 62 GeV obtained with the STAR detector at RHIC. The parity violation effects are studied by three particle azimuthal correlations of charged particles. The global polarization of the system is examined by measuring the polarization of strange hyperons with respect to the collision reaction plane.

Motivation & Objective

• To measure global polarization of Λ hyperons in non-central Au+Au collisions at √sNN = 62 GeV using decay product angular distributions relative to the reaction plane.
• To probe strong parity violation via three-particle azimuthal correlations of charged particles, sensitive to topological charge fluctuations.
• To test theoretical predictions of global spin polarization and CP-violating effects in hot QCD matter formed in heavy-ion collisions.
• To set an upper limit on the magnitude of parity violation and global polarization effects in the quark-gluon plasma regime.

Proposed method

• Measures Λ hyperon polarization using the angular distribution of decay protons relative to the reaction plane, parameterized as dN/dcosθ* ∝ 1 + αPHcosθ*.
• Extracts polarization via two-particle azimuthal correlation: PH = (8/πα)⟨sin(φ − ΨRP)⟩, where φ is the decay product azimuthal angle and ΨRP is the reaction plane.
• Applies the mixed harmonic method to extract charged particle asymmetry parameters a± from ⟨cos(φi + φj − 2ΨRP)⟩, sensitive to parity-violating effects.
• Uses reaction plane reconstruction from directed flow in TPC (|η| < 1.3) and forward detectors (FTPC and ZDC-SMD) for high-precision alignment.
• Analyzes data in centrality bins defined by charged particle multiplicity in |η| < 0.5, with systematic uncertainties assessed via control studies.
• Compares experimental asymmetry parameters a±² and a+a− to theoretical predictions assuming minimal topological charge |Q| = 1.

Experimental results

Research questions

• RQ1 Is there measurable global polarization of Λ hyperons in non-central Au+Au collisions at √sNN = 62 GeV?
• RQ2 Can parity-violating effects in charged particle emission be observed via three-particle azimuthal correlations?
• RQ3 Do the observed asymmetries in particle emission align with theoretical predictions of strong CP violation in hot QCD matter?
• RQ4 What is the upper limit on the magnitude of global polarization and parity violation effects in this energy regime?

Key findings

• The measured Λ hyperon polarization is PΛ = (−1.44 ± 9.66) × 10⁻³, consistent with zero within large uncertainties.
• The upper limit on Λ polarization is significantly below the theoretical prediction of PΛ = −0.3, indicating no evidence for strong global spin polarization.
• Charged particle asymmetry parameters a±² and a+a− show no significant deviation from zero across all centrality bins.
• The experimental data for a±² and a+a− are consistent with theoretical predictions assuming |Q| = 1, but with large uncertainties preventing definitive confirmation.
• The analysis sets a stringent upper bound on strong C and CP violation effects in Au+Au collisions at √sNN = 62 GeV.
• Systematic uncertainties remain significant, but the current data are sufficient to rule out the largest predicted polarization effects.