The polarization of the Λ and Λ¯ hyperons along the beam (z) direction, Pz, has been measured in Pb-Pb collisions at ffiffiffiffiffiffiffiffi sNN p ¼ 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 ¼ hPz sinð2φ − 2Ψ2Þi, 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 ffiffiffiffiffiffiffiffi sNN p ¼ 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.
Polarization of Λ and ¯Λ Hyperons along the Beam Direction in Pb-Pb Collisions at √sNN=5.02 TeV
A. De Caro;D. De Gruttola;S. De Pasquale;L. Dello Stritto;N. Funicello;T. Virgili;
2022-01-01
Abstract
The polarization of the Λ and Λ¯ hyperons along the beam (z) direction, Pz, has been measured in Pb-Pb collisions at ffiffiffiffiffiffiffiffi sNN p ¼ 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 ¼ hPz sinð2φ − 2Ψ2Þi, 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 ffiffiffiffiffiffiffiffi sNN p ¼ 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.File | Dimensione | Formato | |
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PhysRevLett.128.172005.pdf
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