Abstract
The neutron-proton mass difference is expressed, on the assumption that it is purely electromagnetic in origin, in terms of partial-wave amplitudes for the process NN→γγ at zero total energy, and also in terms of the single n=0, M=0 O(4) amplitude for the same process. This representation allows quite directly a qualitative understanding of the sign of the n-p mass difference, and also allows, with more drastic assumptions, an approximate numerical evaluation of the mass difference based on experimentally determined parameters from the A_2 Regge trajectory. This numerical estimate is in reasonable agreement with experiment. The approach is easily generalized to other electromagnetic mass differences, and is shown to yield the tadpole model.
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