Superallowed Beta Decay of Nuclei with A≥62: The Limiting Effect of Weak Gamow-Teller Branches

The most precise value of V_ud, which is obtained from superallowed nuclear β decay, leads to a violation of Cabibbo-Kobayashi-Maskawa unitarity by 2.2σ. Experiments are underway on two continents to test and improve this result through decay studies of odd-odd N = Z nuclei with A≥62. We show, in a series of illustrative shell-model calculations, that numerous weak Gamow-Teller branches are expected to compete with the superallowed branch in each of these nuclei. Though the total Gamow-Teller strength is significant, many of the individual branches will be unobservably weak. Thus, new techniques must be developed if reliable ft values are to be obtained with 0.1% precision for the superallowed branches.