Opacity dependence of elliptic flow in kinetic theory

TL;DR

This paper investigates how elliptic flow in small collision systems can develop without full hydrodynamization, showing that a significant portion of flow arises within a mean free path, challenging fluid dynamic interpretations.

Contribution

It demonstrates that a large part of elliptic flow can form in small systems without hydrodynamization, highlighting the importance of kinetic theory in understanding flow development.

Findings

Up to half of elliptic flow forms without hydrodynamization.

System size affects the accuracy of hybrid kinetic-fluid models.

Significant elliptic flow can develop within a single mean free path.

Abstract

The observation of large azimuthal anisotropies $v_n$ in the particle spectra of proton-proton (pp) and proton-nucleus (pA) collisions challenges fluid dynamic interpretations of $v_n$, as it remains unclear how small collision systems can hydrodynamize and to what extent hydrodynamization is needed to build up $v_n$. Here, we study in a simple kinetic theory how the same physics that leads to hydrodynamization in large systems represents itself in small systems. We observe that one third to one half of the elliptic flow signal seen in fully hydrodynamized systems can be built up in collisions that extend over only one mean free path $l_{\rm mfp}$ and that do not hydrodynamize. This is qualitatively in line with observing a sizeable $v_2$ in $pp$ collisions for which other characteristics of soft multi-particle production seem well-described in a free-streaming picture. We further expose a significant system size dependence in the accuracy of hybrid approaches that match kinetic theory to viscous fluid dynamics. The implications of these findings for a reliable extraction of shear viscosity are discussed.