Initial Temperature and Extent of Chemical Equilibration of Partons in Relativistic Collision of Heavy Nuclei

TL;DR

This paper investigates the initial temperature and chemical equilibration of partons in quark-gluon plasma formed during relativistic heavy ion collisions, using experimental data to estimate densities and analyze equilibration processes.

Contribution

It provides a method to determine formation temperature and fugacities from energy and entropy densities, and explores the limits of chemical equilibration during the plasma's evolution.

Findings

Energy and entropy densities fix formation temperature and fugacities.

Chemical equilibration extent is sensitive to initial energy density estimates.

Different scenarios affect dilepton mass distributions, offering experimental signatures.

Abstract

We emphasize that a knowledge of energy and entropy densities of quark gluon plasma - a thermalized de-confined matter, formed in relativistic heavy ion collisions fixes the formation temperature and the product of gluon fugacity and formation time uniquely, {\em provided} we know the relative fugacities of quarks and gluons. This also provides that a smaller formation time would imply larger fugacities for partons. Next we explore the limits of chemical equilibration of partons during the initial stages in relativistic collision of heavy nuclei. The experimentally measured rapidity densities of transverse energy and charged particle multiplicity at RHIC and LHC energies are used to estimate the energy and number densities with the assumption of formation of a thermally equilibrated quark gluon plasma which may be chemically equilibrated to the same or differing extents for quarks and gluons. The estimates are found to be very sensitive to the correction factor used for the Bj\"{o}rken energy density for identifying it with the initial energy density. The extent of chemical equilibration near the end of the QGP phase is inferred by solving master equations by including the processes $gg \leftrightarrow ggg$ and $gg \leftrightarrow q\overline{q}$ along with expansion and cooling of the plasma. The possible consequences for invariant mass distribution of intermediate mass dileptons radiated from the plasma are discussed which could distinguish between different scenarios.