  Quantum systems in extreme conditions can exhibit universal behavior far from
equilibrium associated to nonthermal fixed points with a wide range of topical
applications from early-universe inflaton dynamics and heavy-ion collisions to
strong quenches in ultracold quantum gases. So far, most studies have relied on
a mapping of the quantum dynamics onto a classical-statistical theory that can
be simulated on a computer. However, the mapping is based on a weak-coupling
limit, while phenomenological applications often require moderate interaction
strengths. We report on the observation of nonthermal fixed points directly in
quantum field theory beyond the weak-coupling limit. For the example of a
relativistic scalar O(N)-symmetric quantum field theory, we numerically solve
the nonequilibrium dynamics employing a 1/N expansion to next-to-leading order,
which does not rely on a small coupling parameter. Starting from two different
sets of overoccupied and of strong-field initial conditions, we find that
nonthermal fixed points are not restricted to parameter ranges suitable for
classical-statistical simulations, but extend also to couplings of order one.
While the infrared behavior is found to be insensitive to the differences in
the initial conditions, we demonstrate that transport phenomena to higher
momenta depend on the presence or absence of a symmetry-breaking field
expectation value.
