Speaker
Description
The longitudinal local polarization $(P_{z,s2})$ of $\Lambda$ and $\bar{\Lambda}$ hyperons exhibits a second-harmonic azimuthal modulation that is closely related to the anisotropic expansion and velocity-gradient structure of the produced medium. Measurements of local polarization in 200 GeV Au + Au and isobar collisions at RHIC exhibit similar centrality dependence, increasing in more peripheral collisions. However, the results in 8.16 TeV $p$-Pb collisions at LHC indicate that the multiplicity dependence of local polarization follows a trend opposite to that of elliptic flow, raising questions about a simple direct relationship between the two observables. We employ the Event Shape Engineering method, which uses event-by-event variations of the second-order flow vector to select event classes with different elliptic-flow magnitudes within a given centrality interval, to measure $P_{z,s2}$ as a function of $v_{2}$ in Ru + Ru and Zr + Zr at $\sqrt{s_{NN}}$ = 200 GeV by the STAR experiment. In addition, we present a measurement of the covariance between event-by-event $P_{z,s2}$ and $v_{2}$. The results reveal a weak dependence of $P_{z,s2}$ on $v_{2}$. These findings will help improve our understanding of whether the second-harmonic local polarization is simply governed by the magnitude of elliptic flow, or whether a more complex underlying mechanism is responsible.
