  We use non-equilibrium dynamical mean-field theory to demonstrate the
existence of a critical interaction in the real-time dynamics of the Hubbard
model after an interaction quench. The critical point is characterized by fast
thermalization and separates weak-coupling and strong-coupling regimes in which
the relaxation is delayed due to prethermalization on intermediate timescales.
This dynamical phase transition should be observable in experiments on trapped
fermionic atoms.
