Polarization of Λ and Λ ¯ Hyperons along the Beam Direction in Pb-Pb Collisions at √{s}N N=5.02 TeV
Abstract
The polarization of the Λ and Λ ¯ hyperons along the beam (z ) direction, Pz, has been measured in Pb-Pb collisions at √{sN N}=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 ,s 2=⟨Pzsin (2 φ -2 Ψ2) ⟩, where φ is the hyperon azimuthal emission angle and Ψ2 is the elliptic flow plane angle. We report the measurement of Pz ,s 2 for different collision centralities and in the 30%-50% centrality interval as a function of the hyperon transverse momentum and rapidity. The Pz ,s 2 is positive similarly as measured by the STAR Collaboration in Au-Au collisions at √{sN N}=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 ,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.
- Publication:
-
Physical Review Letters
- Pub Date:
- April 2022
- DOI:
- 10.1103/PhysRevLett.128.172005
- arXiv:
- arXiv:2107.11183
- Bibcode:
- 2022PhRvL.128q2005A
- Keywords:
-
- Nuclear Experiment;
- High Energy Physics - Experiment
- E-Print:
- 18 pages, 4 captioned figures, authors from page 12, published version, figures at http://alice-publications.web.cern.ch/node/7262