EMMI Workshop: Light-ion collisions 2026

The polarization of Λ hyperons provides a sensitive probe of the space-time structure of the quark-gluon plasma, and in particular of the gradients of the hydrodynamic fields. Longitudinal polarization is especially sensitive to the interplay between vorticity, shear, and transport properties such as the bulk viscosity. In this talk, we review a model based on (3+1)D average event viscous hydrodynamics, coupled to a hadronic afterburner, with spin polarization evaluated through an integration over the freeze-out hypersurface. The framework provides a good description of global and local polarization observables in heavy-ion collisions. We then apply the same setup to light-ion collisions, focusing on Λ polarization in O–O collisions. The calculation reproduces the observed centrality dependence, but systematically underestimates the measured polarization magnitude. We discuss possible origins of this discrepancy by critically assessing some of the limitations of the model. This analysis suggests that spin polarization can be used to improve our understanding of the dynamical ingredients needed to properly describe light-ion collisions.