Attractors Without Scaling: Adiabatic Hydrodynamization With and Without Inelastic Scattering

Abstract

We study the process of hydrodynamization in kinetic theories of gluons undergoing boost-invariant expansion using the Adiabatic Hydrodynamization (AH) framework. We study both number-conserving and non-conserving theories, and find that including number non-conserving inelastic scattering processes restores many qualitative features of hydrodynamization in QCD EKT despite the simplicity of our model. In particular, introducing inelastic scattering results in a more realistic hydrodynamization time. With or without number non-conservation, we find that first-order hydrodynamics becomes applicable at the same time that a unique ground state emerges in the dynamical evolution of the one-particle distribution function. Furthermore, we find a set of low-effective-energy attractor modes which evolve adiabatically long before hydrodynamization, and find that the emergence of a gap between these ground state modes and the excited modes coincides with the time at which the system falls onto an attractor surface. Strikingly, this is the case even in the absence of pre-thermal scaling of the gluon distribution function, which has previously been strongly associated with pre-thermal attractor behavior. Finally, motivated by a generic feature we observe in the spectrum, we show that as the system hydrodynamizes, the rapid decoupling of non-hydrodynamic modes in boost-invariant kinetic theory can be understood with the AH framework in a model-independent fashion.