Spin polarization and alignment in heavy-ion collisions

TL;DR

This paper reports measurements of hyperon and meson spin polarization in heavy-ion collisions across various energies, revealing complex vortical structures and challenging existing theories of spin alignment.

Contribution

It provides new experimental data on global and local spin polarization, including the first measurements of certain observables at specific energies and harmonic orders, highlighting complex vortical phenomena.

Findings

Hyperon polarization increases at energies below 7.7 GeV.

Net local polarization shows negative values at certain energies.

Unexpected spin alignment patterns challenge current theories.

Abstract

We report the measurements of global spin polarization of hyperons ($P_{\Lambda}$) from FAIR, RHIC and LHC energies. The $P_{\Lambda}$ continue to follow an increasing trend at $\sqrt{s_{\mathrm NN}}$ < 7.7 GeV indicating that enormous vorticity prevail even in hadronic dominant region. New measurements of local spin polarization ($P_{z,2}$) from isobar collisions follow the same pattern observed in previous RHIC and LHC energies. An observable motivated by predicted baryonic Spin Hall Effect, the net local polarization ($P_{z,2}^{\mathrm{net}}$), shows negative value in Au+Au collisions at $\sqrt{s_{\mathrm NN}}$ = 19.6 and 27 GeV. The first $P_{z}$ measurement with respect to third order harmonic in 200 GeV RHIC isobar collisions show a non-zero sine modulation, indicating the presence of complex vortical structure in the medium. Surprising new patterns of spin alignment ($\rho_{00}$) for various vector mesons ($K^{*0}$, $K^{*\pm}$, $\phi$ and $J/\psi$) emerged at both RHIC and LHC energies. The large deviation in $\rho_{00}$ of several species poses challenge to the theory.