# Global $\Lambda$ hyperon polarization in nuclear collisions: evidence for the most vortical fluid

## TL;DR

This paper reports the first experimental evidence of extreme fluid vorticity in heavy ion collisions, demonstrated by hyperon polarization measurements that reveal the most vortical fluid ever observed, aligning with hydrodynamic predictions.

## Contribution

It provides the first measurement linking collision angular momentum to hyperon spin polarization, confirming the existence of highly vortical quark-gluon plasma.

## Key findings

- Hyperons show a positive polarization of a few percent.
- Results are consistent with hydrodynamic models predicting vorticity.
- Previous null results at higher energies are explained by statistical uncertainties.

## Abstract

The extreme temperatures and energy densities generated by ultra-relativistic collisions between heavy nuclei produce a state of matter with surprising fluid properties. Non-central collisions have angular momentum on the order of 1000$\hbar$, and the resulting fluid may have a strong vortical structure that must be understood to properly describe the fluid. It is also of particular interest because the restoration of fundamental symmetries of quantum chromodynamics is expected to produce novel physical effects in the presence of strong vorticity. However, no experimental indications of fluid vorticity in heavy ion collisions have so far been found. Here we present the first measurement of an alignment between the angular momentum of a non-central collision and the spin of emitted particles, revealing that the fluid produced in heavy ion collisions is by far the most vortical system ever observed. We find that $\Lambda$ and $\overline{\Lambda}$ hyperons show a positive polarization of the order of a few percent, consistent with some hydrodynamic predictions. A previous measurement that reported a null result at higher collision energies is seen to be consistent with the trend of our new observations, though with larger statistical uncertainties. These data provide the first experimental access to the vortical structure of the "perfect fluid" created in a heavy ion collision. They should prove valuable in the development of hydrodynamic models that quantitatively connect observations to the theory of the Strong Force. Our results extend the recent discovery of hydrodynamic spin alignment to the subatomic realm.