# Landau and Eckart frames for relativistic fluids in nuclear collisions

## TL;DR

This paper examines how the choice of Landau or Eckart frames affects relativistic hydrodynamic simulations of nuclear collisions, finding that flow observables vary with frame but particle distributions remain largely unaffected.

## Contribution

It provides a detailed comparison of Landau and Eckart frames in second-order relativistic hydrodynamics and assesses their impact on flow observables in nuclear collision simulations.

## Key findings

- Flow differs between frames in simulations.
- Particle distributions are frame-independent at freeze-out.
- Frame choice influences flow observable predictions.

## Abstract

The quark matter created in relativistic nuclear collisions is interpreted as a nearly-perfect fluid. The recent efforts to explore its finite-density properties in the beam energy scan programs motivate one to revisit the issue of the local rest frame fixing in off-equilibrium hydrodynamics. I first investigate full second-order relativistic hydrodynamics in the Landau and the Eckart frames. Then numerical hydrodynamic simulations are performed to elucidate the effect of frame choice on flow observables in relativistic nuclear collisions. The results indicate that the flow can differ in the Landau and the Eckart frames but charged particle and net baryon rapidity distributions are mostly frame independent when off-equilibrium kinetic freeze-out is considered.