  The Kardar-Parisi-Zhang (KPZ) equation of nonlinear stochastic growth in d
dimensions is studied using the mapping onto a system of directed polymers in a
quenched random medium. The polymer problem is renormalized exactly in a
minimally subtracted perturbation expansion about d = 2. For the KPZ roughening
transition in dimensions d > 2, this renormalization group yields the dynamic
exponent z* = 2 and the roughness exponent chi* = 0, which are exact to all
orders in epsilon = (2 - d)/2. The expansion becomes singular in d = 4, which
is hence identified with the upper critical dimension of the KPZ equation. The
implications of this perturbation theory for the strong-coupling phase are
discussed. In particular, it is shown that the correlation functions and the
coupling constant defined in minimal subtraction develop an essential
singularity at the strong-coupling fixed point.
