Aspects of the dilute Glasma

Abstract: The Glasma is a semiclassical nonequilibrium state describing the earliest stage in relativistic heavy-ion collisions predicted by the Color Glass Condensate effective theory. It is characterized by strong color fields, which are sourced by color currents pertaining to hard partons in the colliding nuclei. We introduce the (3+1)D dilute Glasma framework, which incorporates the longitudinal and transverse structure of colliding particles and describes the rapidity-dependence of observables like the energy-momentum tensor. This is in stark contrast to the canonical picture of boost-invariance, where nuclei are infinitesimally thin in longitudinal direction, and the rapidity-dependence of observables is lost. We discuss the derivation of the (3+1)D dilute Glasma field-strength tensor, which relies on linearizing the Yang-Mills equations in the dilute approximation, i.e., assuming weak sources. The dilute Glasma energy-momentum tensor can efficiently be evaluated numerically on a lattice. Employing a generalized 3D McLerran-Venugopalan model, we discuss numerical results for the collisions of heavy ions at energies corresponding to experiments at RHIC and the LHC. We discover longitudinal flow that differs significantly from Bjorken flow and argue that this is a consequence of taking into account the longitudinal extension of nuclei. Furthermore, we find limiting fragmentation as a universal feature of the dilute Glasma analytically and numerically. Finally, we study the applicability of the dilute Glasma to proton-proton collisions and show the necessary modifications to reproduce experimental multiplicity distributions.