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 $γ^{*} N\to πN$ 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 $γ\vec{N} \to πN$ 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.

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