It is demonstrated that there is a dynamic isospin breaking effect in the near threshold $\gamma^{ *} N\to \pi N$ reaction due to the mass difference of the up and down quarks, which also causes isospin breaking in the $\pi N$ system. The photopion reaction is affected through final state $\pi N$ interactions (formally implemented by unitarity and time reversal invariance). It is also demonstrated that the near threshold $\gamma \vec{ N} \to \pi N$ reaction is a practical reaction to measure isospin breaking in the $\pi N$ system, which was first predicted by Weinberg about 20 years ago but has never been experimentally tested.
Categories: hep-ph