Global and local polarization of $\Lambda$ hyperons in Au+Au collisions at 200 GeV from STAR

TL;DR

This paper reports new measurements of global and local polarization of $\Lambda$ hyperons in Au+Au collisions at 200 GeV, revealing a decrease in polarization with energy and a quadrupole modulation consistent with elliptic flow.

Contribution

It provides the first measurement of local polarization along the beam direction and explores the charge asymmetry dependence of global polarization.

Findings

Global polarization decreases at higher collision energies.

Larger polarization in peripheral and in-plane collisions.

First observation of quadrupole modulation in local polarization.

Abstract

New results on global polarization of $\Lambda$ hyperons in Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV reveal non-zero signal on the order of a few tenths of a percent. Compared with lower energy results, the current measurement shows that the polarization decreases at higher collision energy, the trend being well reproduced by theoretical calculations. The polarization is found to be larger in more peripheral collisions, as well as in the in-plane than in the out-of-plane direction. The signal seems to depend on the asymmetry between positive and negative charge in each event, which may indicate an influence of the axial current induced by the magnetic field. In addition, the first measurement of a local polarization along the beam direction was performed. The results show a quadrupole modulation relative to the second-order event plane, as expected from the elliptic flow.