ISOSPIN VIOLATION AND HYPERON SEMILEPTONIC DECAYS

Isospin violation induces a little mixing between the LAMBDA and -SIGM A0 baryons, and this in turn leads to small changes in matrix elements for weak current involving these particles. The corrections are small but not negligible. The relevant formulae to first order in the mixin g are listed in this note. The largest effect is in the ratio of rates of semileptonic decay of SIGMA+ and SIGMA- (to LAMBDA) which changes by about 6%. A modest improvement in the measurement of the branching ratio of SIGMA+ --> LAMBDAevBAR would allow the experimental determina tion of the mixing angle. The SU3 fitting of axial to vector ratios in the octet is also affected.