Global and local polarization of $\Lambda$ ($\bar{\Lambda}$) hyperons in Pb--Pb collisions in ALICE at the LHC

TL;DR

This paper reports measurements of global and local polarization of hyperons in Pb--Pb collisions at the LHC, finding global polarization consistent with zero and first evidence of non-zero local polarization along the beam direction.

Contribution

It provides the first experimental evidence of non-zero local hyperon polarization along the beam direction in Pb--Pb collisions at the LHC.

Findings

Global polarization consistent with zero at both energies

First evidence of non-zero local polarization along the beam direction

Local polarization exhibits a second harmonic sine modulation

Abstract

The global polarization of the $\Lambda$ and $\bar\Lambda$ hyperons ($P_{\rm H}$) has been measured in Pb--Pb collisions at $\sNN=$ 2.76 TeV and 5.02 TeV in ALICE at the Large Hadron Collider (LHC). The hyperon global polarization is found to be consistent with zero at both collision energies. The local polarization of the $\Lambda$ and $\bar\Lambda$ hyperons along the beam ($z$) direction, $P_{\rm z}$, has also been measured in Pb--Pb collisions at $\sNN=5.02$ TeV. The $P_{\rm z}$, measured as a function of the hyperon emission angle relative to the second harmonic symmetry plane, exhibits a second harmonic sine modulation, as expected due to elliptic flow. The measurements of global and local hyperon polarization are reported for different collision centralities and as a function of transverse momentum in semicentral collisions. These results show the first experimental evidence of a non-zero hyperon $P_{\rm z}$ in Pb--Pb collisions at the LHC.