The Correlator of Topological Charge Densities at low $Q^2$ in QCD

Abstract

The correlator of topological charge densities $χ(Q^2)$ in QCD is calculated in the domain of low $Q^2$. Basing on the Ward identities the low energy theorems are proved. The value of the first derivative $χ^{\prime}(0)$ found recently by QCD sum rules \cite{1} is used. The contributions of pseudoscalar quasi-Goldstone bosons as intermediate states in the correlator are calculated with the account of $η-π$ mixing.