Light quark mass difference and isospin breaking in electromagnetic pion production

It is demonstrated that there is a dynamic isospin breaking effect in the near threshold γ∗N→πN reaction due to the mass difference of the up and down quarks, which also causes isospin breaking in the πN system. The photopion reaction is affected through final state πN interactions (formally implemented by unitarity and time reversal invariance). It is also demonstrated that the near threshold γN→πN reaction is a practical reaction to measure isospin breaking in the πN system, which was first predicted by Weinberg about 20 years ago but has never been experimentally tested.