A Barnett-Like Effect in Relativistic Heavy-Ion Collisions

The Barnett effect refers to the spontaneous magnetization or polarization of rotating matter via spin-orbit coupling, a fundamental phenomenon observed across a multitude of physical systems down to atomic scales. Here, we demonstrate that this effect extends to subatomic regimes within relativistic heavy-ion collisions, where the most vortical fluid ever created in terrestrial laboratories is produced and the orbital angular momentum is transferred to emitted hadrons via spin-vorticity coupling. By performing transport simulations spanning a broad energy range, we discover a nearly-linear scaling relation between the global spin polarization of $Λ$ hyperons and the source tilt angle extracted via two-pion femtoscopic correlations. This scaling reveals that global $Λ$ polarization induced by the local vorticity field is modulated by the inclined geometry of the produced strongly-interacting nuclear matter in these collisions. Supported by existing experimental measurements, our results unravel a geometry-modulated Barnett-like effect at the femtometre ($10^{-15}$ m) scale.