Resolve the sign and mechanism of beam-direction polarization

Resolve the incomplete understanding of the polarization mechanism governing the sine modulation of hyperon polarization along the beam direction, including the relative contributions of thermal vorticity and shear-induced polarization and the conditions determining the sign of the modulation.

Background

Local polarization along the beam direction receives contributions from thermal vorticity and shear-induced polarization in theoretical descriptions of anisotropically expanding heavy-ion matter. The paper explains that these contributions can largely cancel, and that different implementations of the models can predict opposite signs for the measured sine modulation. This model dependence indicates that the underlying polarization mechanism has not been fully resolved.

References

However, it has been pointed out that these two contributions are largely cancelled out and that the sign of $P_z$ sine modulation, defined as $P_{z,s2}=\langle P_z\sin[2(\phi_\Lambda-\Psi_2)]\rangle$, could be flipped depending on the specific model implementation, indicating an incomplete understanding of the polarization mechanism (see reviews ).

— Experimental review on global polarization and spin alignment in heavy-ion collisions  (2610.21947 - Niida, 15 Oct 2026) in Section 3, Hyperon polarization along the beam direction