The four independent baryonic and two mesonic electromagnetic (E.M.) mass differences are evaluated on the basis of the unitary symmetry model. An approximation scheme is devised and its relation to a dispersion-theoretic treatment by Cottingham is pointed out. Four contributions to the E.M. mass differences are taken into account: the elastic form-factor part, processes which involve the two isovector transitions (${\pi }^0-\eta$) and (${\Sigma }^0-\Lambda$), and what we call the self-induced mass differences. Unitary symmetry is used to provide values for quantities not given by experiments, e.g., form factors for strange particles and coupling constants. An earlier correct result for the pion mass difference is confirmed because it is shown that the chief contribution is due to the elastic form-factor part. All the observed baryonic mass differences can be approximately reproduced. The sign of the kaon mass difference can be explained, but the quantitative predictions for the kaon are still unreliable.

• Received 13 July 1964

DOI:https://doi.org/10.1103/PhysRev.136.B1053