Measurement of the. delta. /sup -/-. delta. /sup + +/ isobar electromagnetic mass difference

The ..delta../sup -/-..delta../sup + +/ isobar electromagnetic mass difference has been measured in a study of the charge-symmetric reactions ..pi../sup +/d..--> pi../sup +/..pi../sup -/pp and ..pi../sup -/d..--> pi../sup +/..pi../sup -/nn initiated by 555-MeV/c pions in a deuterium bubble chamber. The measured value of the mass difference is 5.9 +- 3.1 MeV. This measurement is more accurate than previous measurements by a factor of two. A new value is obtained for the ..sigma../sup asterisk0/ mass: ..sigma../sup asterisk0/ = 1384.3 +- 1.1 MeV.