Electromagnetic contribution to the proton-neutron mass splitting.

We study the electromagnetic contribution to the proton-neutron mass splitting by combining lattice simulations and the modified Cottingham sum rule of Walker-Loud, Carlson, and Miller. This analysis yields an estimate of the isovector nucleón magnetic polarizability as a function of pion mass. The physical value, obtained by chiral extrapolation to the physical pion mass, is ßp-n = (-1.12 °0.40) x 10-4 fm³ which is in agreement with the empirical result, albeit with a somewhat smaller error. As a result, we find σp-nγ = 1.04 ± 0.11 MeV, which represents a significant improvement in precision. [ABSTRACT FROM AUTHOR]