Physical characteristics of glasma from the earliest stage of relativistic heavy ion collisions

TL;DR

This paper analytically investigates the early-time properties of the glasma in relativistic heavy ion collisions using a Colour Glass Condensate framework, revealing insights into pressure evolution, flow anisotropy, and angular momentum transfer.

Contribution

It provides a detailed analytic expansion of the gluon field at early times, challenging assumptions about anisotropy development and angular momentum transfer in the initial glasma state.

Findings

Transverse and longitudinal pressures approach equilibrium values quickly.

Azimuthal flow coefficients are larger than expected, indicating early anisotropy.

Angular momentum in the glasma is negligible compared to the initial ions.

Abstract

We present analytic results that describe the gluon field, or glasma, at very early times after a collision of relativistic heavy ions at proper time $\tau=0$. We use a Colour Glass Condensate approach, and perform an expansion in $\tau$. The full details of our method are described in our previous paper [1]. In this paper we present an analysis of various physical quantities that can be obtained from the energy-momentum tensor. We show that the expansion to order $\tau^6$ can be trusted to about $\tau=0.05$ fm. For times small enough that the expansion converges, the transverse and longitudinal pressures move towards their equilibrium values of one third of the energy density. The Fourier coefficients of the azimuthal flow are larger than expected, which contradicts the usual assumption that anisotropy is mostly generated during the hydrodynamic evolution of the plasma. We find a significant correlation between the elliptic flow coefficient and the eccentricity, which indicates that the spatial asymmetry introduced by the initial geometry is effectively transmitted to the azimuthal distribution of the gluon momentum field, even at very early times. This result is interesting because correlations of this kind are characteristic of the onset of hydrodynamic behaviour. We show that the angular momentum of the glasma is orders of magnitude smaller than the angular momentum of the initial system of ions colliding with non-zero impact parameter. This indicates that most of the angular momentum carried by the valence quarks is not transmitted to the glasma, and contradicts the picture of a rapidly rotating initial glasma state that has been proposed by several authors, but agrees with the current lack of experimental evidence for a significant polarization effect of the hyperons and vector mesons produced in heavy ion collisions at the highest accessible energies.