Polarization of Λ and ¯ Λ Hyperons along the Beam Direction in Pb-Pb Collisions at √ s N N = 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 N N = 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 , s 2 = ⟨ P z sin ( 2 φ − 2 Ψ 2 ) ⟩ , where φ is the hyperon azimuthal emission angle and Ψ 2 is the elliptic flow plane angle. We report the measurement of P z , s 2 for different collision centralities and in the 30%–50% centrality interval as a function of the hyperon transverse momentum and rapidity. The P z , s 2 is positive similarly as measured by the STAR Collaboration in Au-Au collisions at √ s N N = 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 , s 2 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.