Nuclear mismatch in super-allowed Fermi beta-decay

If super-allowed Fermi beta-decay is to be consistent with the conserved vector current and with unitarity of the Cabibbo-Kobayashi-Maskawa matrix it is necessary to understand a mismatch, as between the parent and daughter members of the isotriplets involved, that is worth several tenths of a percent in the transition rates. Present theoretical estimates of this mismatch fall short of what is needed by a factor of order 2 or more. It has been suggested that this shortfall is due to inadequate allowance for the effects of the spectator nucleons, but the present study shows that such effects are worth only a few hundredths of one percent.