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

Abstract

The polarization of the $\mathrm{\ensuremath{\Lambda}}$ and $\overline{\mathrm{\ensuremath{\Lambda}}}$ hyperons along the beam ($z$) direction, ${P}_{z}$, has been measured in Pb-Pb collisions at $\sqrt{{s}_{NN}}=5.02\text{ }\text{ }\mathrm{TeV}$ 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 ${P}_{z,s2}=⟨{P}_{z}\mathrm{sin}(2\ensuremath{\varphi}\ensuremath{-}2{\mathrm{\ensuremath{\Psi}}}_{2})⟩$, where $\ensuremath{\varphi}$ is the hyperon azimuthal emission angle and ${\mathrm{\ensuremath{\Psi}}}_{2}$ 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 $\sqrt{{s}_{NN}}=200\text{ }\text{ }\mathrm{GeV}$, 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.