Gluon condensation in nonperturbative flow equations

We employ nonperturbative flow equations for an investigation of the effective action in Yang-Mills theories. We compute the effective action Γ[B] for constant color magnetic fields B and examine Savvidy close-quote s conjecture of an unstable perturbative vacuum. Our results indicate that the absolute minimum of Γ[B] occurs for B=0. Gluon condensation is described by a nonvanishing expectation value of the regularized composite operator FμνFμν which agrees with phenomenological estimates. copyright 1997 The American Physical Society