On differences between even and odd anisotropic-flow harmonics in non-equilibrated systems

TL;DR

This study compares kinetic models and analytical calculations to understand how anisotropic flow harmonics develop in out-of-equilibrium systems, revealing distinct behaviors between even and odd harmonics.

Contribution

It demonstrates that even and odd flow harmonics originate from different mechanisms, with odd harmonics being less influenced by escape probabilities.

Findings

Even flow harmonics are consistent across models and calculations.

Odd flow harmonics vary significantly between approaches.

Escape probability mainly influences even harmonics, not odd ones.

Abstract

To assess how anisotropic transverse flow is created in a system out of equilibrium, we compare several kinetic-theoretical models in the few-rescatterings regime. We compare the flow harmonics $v_n$ from three types of transport simulations, with either $2\to 2$ or $2\to 0$ collision kernels and in the former case allowing the particles to rescatter several times or not, and from analytical calculations neglecting the gain term of the Boltzmann equation. We find that the even flow harmonics are similar in all approaches, while the odd ones differ significantly. This suggests that while even $v_n$ harmonics may to a large extent be due to the anisotropic escape probability of particles, this is not the predominant mechanism underlying the odd $v_n$ coefficients.