Tests of the standard model from superallowed Fermi β-decay studies:: The 74Rb β-decay

Precise measurements of the intensities of superallowed Fermi 0(+) --> 0(+)beta-decays have provided a powerful test of the CVC hypothesis at the level of 3 x 10(-4) and also led to a result in disagreement with unitarity for the CKM matrix at the 98% confidence level. It is essential to address possible trivial explanations for the apparent non-unitarity such as uncertainties in the isospin symmetry-breaking corrections. We have carefully studied the (74)Rb --> (74)Kr beta-decay in order to measure the non-analog beta-decay branching to the 0(+) state at 508 keV in (74)Kr. We have determined that this branching is < 3 X 10(-4), far smaller than any published theoretical estimate. We also show that high-precision, complete spectroscopy, measuring the major beta-branches to excited 0(+) and 1(+) states, must be performed if one is to obtain a meaningful branching ratio to the excited 0(+) state and concomitantly deal, in a substantial way, with the possibility of 0(+) feeding to an array of 1(+) states.