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

¹ Institute of Particle Physics and Key Laboratory of Quark and Lepton Physics (MOE), Central China Normal University, Wuhan 430079, China
² Department of Physics, McGill University, Montreal, QC, Canada H3A 2T8
³ Fakultät für Physik, Universität Bielefeld, D-33615 Bielefeld, Germany
⁴ Department of Modern Physics, University of Science and Technology of China, Hefei, Anhui 230026, China

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Published online: 17 April 2026

Abstract

We investigate hydrodynamic effects on the spin polarization of Λ hyperons in Au+Au collisions at √s_NN = 200 GeV and p+Pb collisions at √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(ϕ_p – Ψ₂)〉, as a function of multiplicity (centrality) under three equilibrium scenarios: A 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(ϕ_p – Ψ₂)〉 across collision systems and scenarios.