# The ideal relativistic fluid limit for a medium with polarization

## TL;DR

This paper develops an effective field theory for an ideal relativistic fluid with microscopic polarization, aiming to describe polarization phenomena in heavy ion collisions, and discusses stability and viscosity implications.

## Contribution

It introduces a novel EFT framework for polarized relativistic fluids, relevant for polarization observables in heavy ion collision experiments.

## Key findings

- First EFT for polarized relativistic fluids
- Higher-order non-dissipative dynamics may cause stability issues
- Implications for lower bounds on viscosity

## Abstract

We use Lagrangian effective field theory techniques to construct the equations of motion for an ideal relativistic fluid whose constituent degrees of freedom have microscopic polarization. We discuss the meaning of such a system, and argue that it is the first term in the EFT appropriate for describing polarization observables in heavy ion collisions, such as final state particle polarization and chiral magnetic and vortaic effects. We show that this system will generally require non-dissipative dynamics at higher order in gradient than second order, leading to potential stability issues known with such systems. We comment on the significance of this in the light of conjectured lower limits on viscosity.