John Hardy, Texas A & M U.
Superallowed Beta Decay: A Nuclear Probe of the Electroweak Standard Model
Physics Division Colloquium - 24 Mar 2000

The atomic nucleus provides a unique "laboratory" for studying properties of the electroweak force and probing the limitations of the Standard Model. To date, the strengths of superallowed 0+ --> 0+ beta-decay transitions have been determined with high precision for nine different short-lived nuclei, ranging from 10C to 54Co. Each result leads to an independent measure for the vector coupling constant, GV, and collectively the nine values can be used to test the conservation of weak vector current (CVC). The results support CVC to better than a few parts in 10,000 -- a clear success for the Standard Model! However, when the average value of GV, as determined in this way, is combined with data from decays of the muon and kaon to test another prediction of the Standard Model, the result is much more provocative. A test of the unitarity of the Cabibbo-Kobayash-Maskawa matrix fails by more than two standard deviations. This result, if confirmed by even more precise measurements in the future, could have far-reaching consequences.

The talk will describe the nuclear measurements; outline the experimental and theoretical limitations in their analysis; compare the results with neutron-decay measurements, which are less precise but consistent with the nuclear measurements; discuss the possible implications of non-unitarity; and indicate the direction to be taken in future measurements aimed at achieving a definitive result.

ANL Physics Division Colloquium Schedule