Hyperon global polarization in isobar Ru+Ru and Zr+Zr collisions at $\sqrt{s_{NN}}$ = 200 GeV

TL;DR

This study measures hyperon polarization in isobar Ru+Ru and Zr+Zr collisions at 200 GeV, revealing centrality, azimuthal angle, and hyperon type dependencies, with results consistent with hydrodynamic models and no significant system size effects.

Contribution

First measurement of hyperon polarization dependence on emission azimuthal angle and comparison between different hyperon species in isobar collisions.

Findings

Polarization increases in more peripheral collisions.

No significant dependence on transverse momentum or pseudorapidity.

Lambda and Anti-Lambda polarizations are consistent, indicating minimal spin-magnetic coupling effects.

Abstract

The polarization of Lambda, Anti-Lambda, Xi, and Anti-Xi hyperons along the angular momentum of the system has been measured in isobar collisions of Ru+Ru and Zr+Zr at $\sqrt{s_{NN}}$ = 200 GeV with the STAR detector at RHIC. The polarization dependence on collision centrality is explored and found to show an increasing trend in more peripheral collisions. Dependencies on transverse momentum and pseudorapidity are investigated for Lambda and Anti-Lambda hyperons, but no significant dependence has been observed. The polarization measurements for Lambda and Anti-Lambda are consistent with each other, indicating little contribution of the spin-magnetic coupling in the observed polarization. The results for Lambda hyperons measurements are qualitatively consistent with hydrodynamic calculations incorporating effects from shear-induced polarization and thermal vorticity, and show no obvious system size dependence in comparison with previous results in Au+Au collisions. For the first time, the dependence of the polarization on the hyperon's emission azimuthal angle with respect to the second harmonic event plane is extracted and shows stronger polarization for the in-plane emitted hyperons at the level of 2.4$\sigma$ significance in 20-50% centrality. The measurements of Xi hyperons polarization via the polarization transfer analysis exhibit a finite positive polarization, 2.9$\sigma$ significance in 20-50% centrality, slightly enhanced compared to the inclusive Lambda polarization.