Bulk Viscosity Effects in Event-by-Event Relativistic Hydrodynamics

TL;DR

This paper investigates how bulk viscosity influences collective flow harmonics in heavy ion collisions using a new 2+1 Lagrangian hydrodynamic code with SPH, revealing that bulk viscosity enhances flow coefficients even at small viscosity ratios.

Contribution

It introduces a novel 2+1 Lagrangian hydrodynamic code with SPH and derives a new formula for bulk viscous corrections at freeze-out, advancing the modeling of viscous effects in heavy ion collisions.

Findings

Bulk viscosity enhances flow coefficients from v2 to v5 at pT ~ 1-3 GeV.

The new formula for bulk viscous corrections is derived from the Boltzmann equation.

Bulk viscosity effects are significant even at low ζ/s ratios.

Abstract

Bulk viscosity effects on the collective flow harmonics in heavy ion collisions are investigated, on an event by event basis, using a newly developed 2+1 Lagrangian hydrodynamic code named v-USPhydro which implements the Smoothed Particle Hydrodynamics (SPH) algorithm for viscous hydrodynamics. A new formula for the bulk viscous corrections present in the distribution function at freeze-out is derived starting from the Boltzmann equation for multi-hadron species. Bulk viscosity is shown to enhance the collective flow Fourier coefficients from $v_2(p_T)$ to $v_5(p_T)$ when $% p_{T}\sim 1-3$ GeV even when the bulk viscosity to entropy density ratio, $% \zeta/s$, is significantly smaller than $1/(4\pi)$.