Direct Instanton Effects in Current-Current Correlators
  We compute the effect of small-size instantons on the coefficient function of the chiral condensate in the operator product expansion of current-current correlators. Furthermore, we also compute the instanton corrections associated with four-quark and six-quark operators in the factorization approximation. We discuss the phenomenological implications of our result. 
Paolo Nason and Matteo Palassini