MASS SPLITTINGS AND GHOST TADPOLES

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 NN̄→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 A2 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. © 1969 The American Physical Society.