Spin polarization and alignment in heavy-ion collisions

Institute of Modern Physics Chinese Academy of Sciences, Lanzhou, China

^* e-mail: subhash@impcas.ac.cn

Published online: 1 March 2023

Abstract

We report the measurements of global spin polarization of hyperons (P_Λ) from FAIR, RHIC and LHC energies. The P_Λ continues to follow an increasing trend at √S_NN < 7.7 GeV indicating that enormous vorticity prevails 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^net), show a negative value in Au+Au collisions at √S_NN = 19.6 and 27 GeV. The first P_z measurement with respect to third order harmonic in 200 GeV RHIC isobar collisions shows a non-zero sine modulation, indicating the presence of complex vortical structure in the medium. Surprising new patterns of spin alignment (ρ₀₀) for various vector mesons (K^*0, K^*±, ϕ and J/ψ emerged at both RHIC and LHC energies. The large deviation in ρ₀₀ of several species poses challenge to the theory.