Quasi-Particle Degrees of Freedom versus the Perfect Fluid as Descriptors of the Quark-Gluon Plasma

TL;DR

This paper challenges the interpretation of quark number scaling in heavy ion collisions, arguing it does not necessarily indicate the presence of QCD quasi-particles during the quark-gluon plasma phase, due to contradictions with fluid behavior.

Contribution

It introduces a quasi-particle transport stage hypothesis and compares hadronization processes to clarify the interpretation of experimental scaling observations.

Findings

Near-perfect fluid behavior contradicts quasi-particle degrees of freedom.

Quark number scaling can occur without initial quasi-particles.

The duration of the transport stage affects the existence of well-defined quasi-particles.

Abstract

The hot nuclear matter created at the Relativistic Heavy Ion Collider (RHIC) has been characterized by near-perfect fluid behavior. We demonstrate that this stands in contradiction to the identification of QCD quasi-particles with the thermodynamic degrees of freedom in the early (fluid) stage of heavy ion collisions. The empirical observation of constituent quark ``$n_q$'' scaling of elliptic flow is juxtaposed with the lack of such scaling behavior in hydrodynamic fluid calculations followed by Cooper-Frye freeze-out to hadrons. A ``quasi-particle transport'' time stage after viscous effects break down the hydrodynamic fluid stage, but prior to hadronization, is proposed to reconcile these apparent contradictions. However, without a detailed understanding of the transitions between these stages, the ``$n_q$'' scaling is not a necessary consequence of this prescription. Also, if the duration of this stage is too short, it may not support well defined quasi-particles. By comparing and contrasting the coalescence of quarks into hadrons with the similar process of producing light nuclei from nucleons, it is shown that the observation of ``$n_{q}$'' scaling in the final state does not necessarily imply that the constituent degrees of freedom were the relevant ones in the initial state.