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T.E.O. Ericson, A. Ivanov:
"Dispersive e.m. corrections to π Ν scattering lengths";
Physics Letters B, 634 (2006), 39 - 47.

We investigate the dispersive contribution by radiative processes such as π−p→nγ and π−p→∆γ to the πN scattering lengths of charged
pions in the heavy baryon limit. They give a large isospin violating contribution in the corresponding isoscalar scattering length, but only a small violation in the isovector one. These terms contribute 7.1(3)% to the 1s level shift of pionic hydrogen and give a chiral constant F2 π f1 = −28.0(8) MeV.

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