Superallowed beta decay studies at TRIUMF -nuclear structure and fundamental symmetries

Precision measurement of the β-decay half-life, Q-value, and branching ratio between nuclear analog states of J^π = 0⁺ and T = 1 can provide critical and fundamental tests of the Standard Model's description of electroweak interactions. A program has been initiated at TRIUMF-ISAC to measure the ft values of these superallowed beta transitions. Two Tz = 0, A > 60 cases, ⁷⁴Rb and ⁶²Ga, are presented. These are particularly relevant because they can provide critical tests of the calculated nuclear structure and isospin-symmetry breaking corrections that are predicted to be larger for heavier nuclei, and because they demonstrate the advance in the experimental precision on ft at TRIUMF-ISAC from 0.26% for ⁷⁴Rb in 2002 to 0.05% for ⁶²Ga in 2006. The high precision world data on experimental ft and corrected Ft values are discussed and shown to be consistent with CVC at the 10^-4 level, yielding an average Ft = 3073.70(74) s. This Ft leads to Vud = 0.9737(4) for the up-down element of the Standard Model's CKM matrix. With this value and the Particle Data Group's 2006 values for Vus and V ub, the unitarity condition for the CKM matrix is met. Additional measurements and calculations are needed, however, to reduce the uncertainties in that evaluation, That objective is the focus of the continuing program on superallowed-beta decay at TRIUMF-ISAC.