Measurements of local polarization of $\Lambda$ hyperons with event shape engineering in Au+Au collisions at RHIC-STAR

Abstract

Anisotropic flow is expected to generate local vorticities along the beam direction, which in turn lead to local polarization of produced particles. Although the local polarization of hyperons has already been observed at RHIC and the LHC, its mechanism is not yet understood: model calculations indicate that the shear-induced and vorticity-induced contributions can have opposite signs and may substantially cancel, so that the resulting polarization depends sensitively on the theoretical implementation. Since both are driven by the anisotropic flow, we employ the Event Shape Engineering (ESE) method, which selects events with different flow magnitudes within a narrow centrality range, to measure the local polarization as a function of the elliptic flow at a fixed multiplicity. We report the measurements of the elliptic flow ($v_2$) and the local polarization ($P_{z,2}$) of $\Lambda$ and $\bar{\Lambda }$ as a function of the reduced flow vector magnitude $q_2$ used for the ESE event selection, in Au+Au collisions at $\sqrt{s_{\mathrm{NN}}}=19.6\mathrm{GeV}$ from the STAR experiment. The $v_2$ shows a clear positive dependence on $q_2$, while $P_{z,2}$ indicates a weak positive dependence on $q_2$ and $v_2$. This is the first measurement to directly investigate the relation between anisotropic flow and local polarization, providing new insight into the origin of local polarization.

arXiv comment5 pages, 5 figures, Proceedings of the 22nd International Conference on Strangeness in Quark Matter (SQM2026)

References in corpus (4)

• Methods for analyzing anisotropic flow in relativistic nuclear collisions
• Ultra-relativistic nuclear collisions: event shape engineering
• Polarization phenomenon in heavy-ion collisions
• Hyperon polarization along the beam direction relative to the second and third harmonic event planes in isobar collisions at $\sqrt{s_{NN}}$ = 200 GeV