Renormalization of 4-Quark Operators and QCD-Sum Rules
  We compute the renormalization mismatch displayed in 1--loop approximation by classically equivalent 4-quark operators and coming from different possible definitions of the $\gamma_5$ matrix in dimensional regularization. The result is then employed to study the effect of the various treatments of $\gamma_5$ upon the size of radiative corrections to 4-quark condensates in the QCD sum rules for $\rho$ and $A_1$ mesons. We find that a fully anticommuting $\gamma_5$ which automatically respects non-anomalous chiral Ward-Slavnov identities leads to considerably smaller corrections and reduces theoretical uncertainty in the QCD prediction for the $\tau$ hadronic decay rate. 
L.-E. Adam and K.G. Chetyrkin