Measurements of global and local spin polarization of $\Lambda$ and $\bar{\Lambda}$ in Au+Au collisions from the RHIC Beam Energy Scan

Abstract

We report the measurements of $\Lambda$ hyperons' global and local spin polarization from second phase of the RHIC Beam Energy Scan (BES-II) in Au+Au collisions at $\sqrt{s_{NN}}$= 7.7--27 GeV. Global polarization measurements of $\bar{\Lambda}$ and $\Lambda$ show no significant differences, offering insights into the late-stage evolution of the magnetic field. The new measurements of the local polarization of $\Lambda$ perpendicular to the reaction plane ($\langle P_{2,y} \rangle$) shows a monotonic increase with decreasing collision energy, while the component along the beam direction ($\langle P_{2,z} \rangle$) for both $\Lambda$ and $\bar{\Lambda}$ is small in magnitude with no strong energy dependence. The net local polarization observable, $\langle P_{2,y}^{net} \rangle =\langle P_{2,y}(\Lambda) \rangle - \langle P_{2,y}(\bar{\Lambda})\rangle$ and $\langle P_{2,z}^{net} \rangle = \langle P_{2,z}(\Lambda) \rangle - \langle P_{2,z}(\bar{\Lambda} \rangle)$ designed to probe baryonic spin Hall effect, is consistent with zero with large uncertainty.