The derivative of the topological susceptibility at zero momentum and an estimate of η′ mass in the chiral limit

The anomaly-anomaly correlator is studied using QCD sum rules. Using the matrix elements of anomaly between vacuum and pseudoscalars π, η and η′, the derivative of correlator χ′(0) is evaluated and found to be ≈1.82×10−3 GeV2. Assuming that χ′(0) has no significant dependence on quark masses, the mass of η′ in the chiral limit is found to be ≈723 MeV. The same calculation also yields for the singlet pseudoscalar decay constant in the chiral limit a value of ≈178 MeV.