Early-time dynamics of gluon fields in high energy nuclear collisions

Nuclei colliding at very high energy create a strong, quasiclassical gluon field during the initial phase of their interaction. We present an analytic calculation of the initial space-time evolution of this field in the limit of very high energies using a formal recursive solution of the Yang-Mills equations. We provide analytic expressions for the initial chromoelectric and chromomagnetic fields and for their energy-momentum tensor. In particular, we discuss event-averaged results for energy density and energy flow as well as for longitudinal and transverse pressure of this system. For example, we find that the ratio of longitudinal to transverse pressure very early in the system behaves as pL/pT=-[1-32a(Qτ)2]/[1-1a(Qτ)2]+O(Qτ)4, where τ is the longitudinal proper time, Q is related to the saturation scales Qs of the two nuclei, and a=ln(Q2/m2) with mascale to be defined later. Our results are generally applicable if τ 1/Q. As already discussed in a previous paper, the transverse energy flow Si of the gluon field exhibits hydrodynamiclike contributions that follow transverse gradients of the energy density . In addition, a rapidity-odd energy flow also emerges from the non-Abelian analog of Gauss' law and generates nonvanishing angular momentum of the field. We discuss the space-time picture that emerges from our analysis and its implications for observables in heavy-ion collisions.