QCD isospin breaking ChPT low-energy constants from the instanton vacuum

In the framework of the instanton vacuum model we evaluate the Chiral Perturbation Theory (ChPT) low-energy constants h_3,l_7. We found that in the instanton vacuum model the constant l_7 is very sensitive to the shape of the instanton and the instanton vacuum parameters. We evaluated the constant l_7 for two different zero-mode profiles and as a function of the average instanton size ρand inter-instanton distance R. Our result agrees with an old "order of magnitude" estimate of this constant from [1]. The obtained value of l_7 implies that the pure QCD contribution to the pion mass difference is small, ~1% of the observed experimental value.

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