Hydrodynamic effects on spin polarization along the beam direction in Au+Au and p+Pb collisions

Abstract

We investigate hydrodynamic effects on the spin polarization of $\Lambda$ hyperons in Au+Au collisions at $\sqrt{s_{NN}}=200$ GeV and p+Pb collisions at $\sqrt{s_{NN}}=8.16$ TeV using the CLVisc hydrodynamic framework. We present numerical results for the second Fourier sine coefficient of the longitudinal spin polarization, $⟨P_z\sin 2({\varphi }_p-{\Psi }_2)⟩$, as a function of multiplicity (centrality) under three equilibrium scenarios: $\Lambda$ equilibrium, $s$-quark equilibrium, and isothermal equilibrium. We highlight the respective roles of thermal vorticity and the thermal-shear tensor in generating $⟨P_z\sin 2({\varphi }_p-{\Psi }_2)⟩$ across collision systems and scenarios.

Comments: 4 pages, 2 figures. Contribution to the proceedings of Quark Matter 2025