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

Abstract

We present some analytic results that describe the gluon field, or glasma, that exists at very early times after a collision of relativistic heavy ions at proper time $\ensuremath{\tau}=0$. We use a color glass condensate approach, and perform an expansion in $\ensuremath{\tau}$. The full details of our calculational method are described in our previous paper [Carrington, Czajka, and Mr\'owczy\ifmmode \acute{n}\else \'{n}\fi{}ski, Eur. Phys. J. A 58, 5 (2022)], where we have explained all of the steps that are necessary to obtain the energy-momentum tensor up to sixth order in $\ensuremath{\tau}$. In this paper we present an analysis of various physical quantities that can be obtained from this energy-momentum tensor. We show that the expansion to order ${\ensuremath{\tau}}^{6}$ can be trusted to about $\ensuremath{\tau}=0.05$ fm. We calculate the transverse and longitudinal pressures and show that, for times small enough that the expansion converges, they move towards their equilibrium values of one third of the energy density. We also study the spatial eccentricity of the glasma and the Fourier coefficients of the azimuthal flow. Our results for the Fourier coefficients 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 behavior. Finally, we have calculated the angular momentum of the glasma and obtained results that are many orders of magnitude smaller than the angular momentum of the initial system of colliding ions in a configuration with nonzero impact parameter. This indicates that most of the angular momentum carried by the valence quarks is not transmitted to the glasma. The result is significant because it 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.

Topics & Concepts