  The momentum distribution of particle production in heavy-ion collisions
encodes information about thermalization processes in the early-stage
quark-gluon plasma. We use kinetic theory to study the far-from-equilibrium
evolution of an expanding plasma with an anisotropic momentum-space
distribution. We identify slow and fast degrees of freedom in the
far-from-equilibrium plasma from the evolution of moments of this distribution.
At late times, the slow modes correspond to hydrodynamic degrees of freedom and
are naturally gapped from the fast modes by the inverse of the relaxation time,
$\tau_R^{-1}$. At early times, however, there are an infinite number of slow
modes with a gap inversely proportional to time, $\tau^{-1}$. From the
evolution of the slow modes we generalize the paradigm of the
far-from-equilibrium attractor to vector and tensor components of the
energy-momentum tensor, and even to higher moments of the distribution function
that are not part of the hydrodynamic evolution. We predict that initial-state
momentum anisotropy decays slowly in the far-from-equilibrium phase and may
persist until the relaxation time.
