Effect of isospin mixing on superallowed Fermi beta decay

We study the effect of isospin impurity on the superallowed Fermi beta decay using the microscopic Hartree-Fock and random phase approximation (or Tamm-Dancoff approximation) model taking into account charge symmetry and independence breaking interactions. It is found that the isospin impurity of N=Z nuclei gives enhancement of the sum rule of Fermi transition probabilities. On the other hand, the superallowed transitions between odd-odd J=0 nuclei and even-even J=0 nuclei are quenched because of the cancellation of the isospin impurity effects of mother and daughter nuclei. An implication of the calculated Fermi transition rate on the unitarity of the Cabibbo-Kobayashi-Maskawa mixing matrix is also discussed.