Effect of bulk viscosity on Elliptic Flow near QCD phase transition

TL;DR

This paper investigates how bulk viscosity influences elliptic flow in heavy-ion collisions near the QCD phase transition, revealing complex effects depending on transverse momentum and highlighting limitations of certain theoretical methods.

Contribution

It introduces a realistic modeling of bulk viscosity effects on elliptic flow using lattice QCD data and critiques the applicability of Grad's 14-moment method.

Findings

Bulk viscosity reduces elliptic flow at low pT and enhances it at high pT.

Critical behaviors of the equation of state and transport coefficients drive the non-trivial effects.

Grad's 14-moment approximation is inadequate for freeze-out distribution estimates.

Abstract

Effects of the bulk viscosity on the elliptic flow are studied. To introduce a realistic equation of state and transport coefficients, we apply the results of the lattice QCD and hadron resonance gas calculations for these quantities. We found that the bulk viscosity acts in a non trivial manner on the elliptic flow $v_{2}$. The reduction of $v_{2}$ is more effective at low $p_{T}$ compared to the case of shear viscosity, whereas at high $p_{T}$, the situation is reversed, leading to $v_{2}$ enhancement. We argue that this is caused by the competition of the critical behaviors of the equation of state and the transport coefficients. We further found that Grad's method with the 14 moments approximation is not applicable to estimate the viscous effects for the one-particle distribution function at the freeze out.