  We explore the nature of the Bose condensation transition in driven open
quantum systems, such as exciton-polariton condensates. Using a functional
renormalization group approach formulated in the Keldysh framework, we
characterize the dynamical critical behavior that governs decoherence and an
effective thermalization of the low frequency dynamics. We identify a critical
exponent special to the driven system, showing that it defines a new dynamical
universality class. Hence critical points in driven systems lie beyond the
standard classification of equilibrium dynamical phase transitions. We show how
the new critical exponent can be probed in experiments with driven cold atomic
systems and exciton-polariton condensates.
