LHC Seminar

Global and local polarization of $\Lambda$ and anti-$\Lambda$ hyperons in Pb-Pb collisions in ALICE

by Debojit Sarkar (Wayne State University (US))





The extreme temperatures and energy densities generated in the ultrarelativistic nuclear collisions produce a state of partonic matter, the quark-gluon plasma (QGP), which behaves almost like an ideal fluid. The created system may possess large orbital angular momentum leading to the global polarization of particles perpendicular to the reaction plane. Also, local asymmetries in the velocity fields due to anisotropic flow can generate vorticity and particle polarization along the beam direction. In parity-violating weak decays of hyperons, the momentum direction of the decay baryon is correlated with the hyperon spin. This feature can be used to measure the hyperon polarization and estimate the local and global vorticity of the system created in relativistic heavy-ion collisions.

This talk will present the recent experimental measurements of the $\Lambda$ and $\overline \Lambda$ hyperons' local and global spin polarization in Pb-Pb collisions in ALICE [1, 2]. The first experimental evidence of a non-zero hyperon spin polarization along the beam direction (local polarization) in Pb-Pb collisions at the LHC will be presented. Also, the global hyperon spin polarization measurement in ALICE, consistent with zero within experimental uncertainties, will be discussed. The comparison of the measured local polarization with the hydrodynamic model calculations, including competing contributions from thermal and shear-induced vorticity, will be shown. A comparison of the ALICE results with the previous STAR measurements and the collision energy dependence of the local and global hyperon spin polarization will be discussed.

[1] ALICE Collaboration, arXiv:2107.11183v1 [nucl-ex] [2] ALICE Collaboration, Phys. Rev. C 101, 044611 (2020)


Organized by

Michelangelo Mangano, Monica Pepe-Altarelli and Pedro Silva.

LHC Seminar - ALICE
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Claudia Dupraz
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