Quark-to-gluon composition of the quark-gluon plasma in relativistic heavy-ion collisions

TL;DR

This paper investigates the evolution of quark and gluon composition in the quark-gluon plasma during heavy-ion collisions, revealing a rapid transition from gluon to quark dominance near the phase transition temperature, with implications for QGP properties.

Contribution

It introduces a partonic transport model incorporating mean fields and inelastic collisions to study the dynamic quark-gluon ratio in QGP, highlighting its significant variation near T_c.

Findings

Quark-to-gluon ratio increases by a factor of ~20 near T_c.

The plasma becomes quark-dominated (>80%) close to T_c.

The quark composition impacts key QGP phenomena like viscosity and jet quenching.

Abstract

We study the evolution of the quark-gluon composition of the plasma created in ultra-Relativistic Heavy-Ion Collisions (uRHIC's) employing a partonic transport theory that includes both elastic and inelastic collisions plus a mean fields dynamics associated to the widely used quasi-particle model. The latter, able to describe lattice QCD thermodynamics, implies a "chemical" equilibrium ratio between quarks and gluons strongly increasing as $T\rightarrow T_c$, the phase transition temperature. Accordingly we see in realistic simulations of uRHIC's a rapid evolution from a gluon dominated initial state to a quark dominated plasma close to $T_c$. The quark to gluon ratio can be modified by about a factor of $\sim 20$ in the bulk of the system and appears to be large also in the high $p_T$ region. We discuss how this aspect, often overflown, can be important for a quantitative study of several key issues in the QGP physics: shear viscosity, jet quenching, quarkonia suppression. Furthermore a bulk plasma made by more than $80\%$ of quarks plus antiquarks provides a theoretical basis for hadronization via quark coalescence.