Polarization of ? and ? ¯ Hyperons along the Beam Direction in Pb-Pb Collisions at s NN=5.02 TeV

Abstract

The polarization of the ? and ?¯ hyperons along the beam (z) direction, Pz, has been measured in Pb-Pb collisions at sNN=5.02 TeV recorded with ALICE at the Large Hadron Collider (LHC). The main contribution to Pz comes from elliptic flow-induced vorticity and can be characterized by the second Fourier sine coefficient Pz,s2=?Pzsin(2f-2?2) ©, where f is the hyperon azimuthal emission angle and ?2 is the elliptic flow plane angle. We report the measurement of Pz,s2 for different collision centralities and in the 30%-50% centrality interval as a function of the hyperon transverse momentum and rapidity. The Pz,s2 is positive similarly as measured by the STAR Collaboration in Au-Au collisions at sNN=200 GeV, with somewhat smaller amplitude in the semicentral collisions. This is the first experimental evidence of a nonzero hyperon Pz in Pb-Pb collisions at the LHC. The comparison of the measured Pz,s2 with the hydrodynamic model calculations shows sensitivity to the competing contributions from thermal and the recently found shear-induced vorticity, as well as to whether the polarization is acquired at the quark-gluon plasma or the hadronic phase.