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Polarization of Λ and Λˉ Hyperons along the Beam Direction in Pb-Pb Collisions at √sNN = 5.02  TeV

The polarization of the Λ and Λ¯ 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(2φ-2Ψ2)⟩, where φ is the hyperon azimuthal emission angle and Ψ2 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.

Lambda: polarization ; Antilambda: polarization ; heavy ion: scattering ; quark gluon: plasma ; hyperon: transverse momentum ; model: hydrodynamics ; CERN LHC Coll ; elliptic flow ; ALICE ; thermal ; rapidity ; experimental results ; 5020 GeV-cms/nucleon

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