Suppression of elliptic flow without viscosity

TL;DR

This paper demonstrates that even ideal fluids without viscosity can exhibit elliptic flow suppression at high transverse momenta due to local equilibrium distribution choices, challenging the attribution of all v2 suppression to viscosity.

Contribution

It introduces a non-viscous mechanism for elliptic flow suppression by using generalized local equilibrium distributions, questioning the traditional viscous interpretation.

Findings

Elliptic flow suppression occurs in ideal fluids without viscosity.

Suppression increases with transverse momentum and is lighter for heavier particles.

Results suggest viscous effects may not fully explain observed v2 suppression.

Abstract

We investigate fluid-to-particle conversion using the usual Cooper-Frye approach but with more general local equilibrium distributions than the Boltzmann or Bose/Fermi distributions typically used. Even though we study ideal fluids (i.e., shear stress and bulk pressure are zero everywhere), we find a suppression of elliptic flow (v2) at high transverse momenta (pT>1.5 GeV/c), relative to results obtained with the traditional Boltzmann distributions. The non-viscous suppression shows qualitatively similar features to the well-known shear viscous suppression of v2; for example, it increases with pT, and it is smaller for heavier species as seen in self-consistent kinetic theory calculations. Our results question whether all of the v2 suppression seen in the data can be attributed to viscous effects, and indicate that shear viscosities extracted from RHIC and LHC elliptic flow data might be overestimated.