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

The polarization of the Λ and ̄Λ hyperons along the beam (z) direction, P z, has been measured in Pb-Pb collisions at √s_NN = 5.02 TeV recorded with ALICE at the Large Hadron Collider (LHC). The main contribution to P z comes from elliptic flow-induced vorticity and can be characterized by the second Fourier sine coefficient P z;s2 ¼ hP z sinð2φ − 2Ψ2Þi, where φ is the hyperon azimuthal emission angle and Ψ2 is the elliptic flow plane angle. We report the measurement of P z;s2 for different collision centralities and in the 30%–50% centrality interval as a function of the hyperon transverse momentum and rapidity. The P z;s2 is positive similarly as measured by the STAR Collaboration in Au-Au collisions at √s_NN = 200 GeV, with somewhat smaller amplitude in the semicentral collisions. This is the first experimental evidence of a nonzero hyperon P z in Pb-Pb collisions at the LHC. The comparison of the measured P z;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.