  In Phys. Rev. Lett. 114 (2015) 6, 061601, we reported on a new universality
class for longitudinally expanding systems, encompassing strongly correlated
non-Abelian plasmas and $N$-component self-interacting scalar field theories.
Using classical-statistical methods, we showed that these systems share the
same self-similar scaling properties for a wide range of momenta in a limit
where particles are weakly coupled but their occupancy is high. Here we
significantly expand on our previous work and delineate two further
self-similar regimes. One of these occurs in the deep infrared (IR) regime of
very high occupancies, where the nonequilibrium dynamics leads to the formation
of a Bose-Einstein Condensate. The universal IR scaling exponents and the
spectral index characterizing the isotropic IR distributions are described by
an effective theory derived from a systematic large-$N$ expansion at
next-to-leading order. Remarkably, this effective theory can be cast as a
vertex-resummed kinetic theory. The other novel self-similar regime occurs
close to the hard physical scale of the theory, and sets in only at later
times. We argue that the important role of the infrared dynamics ensures that
key features of our results for scalar and gauge theories cannot be reproduced
consistently in conventional kinetic theory frameworks.
