Physics Division Seminars bring us speakers on a variety of physics related subjects. Usually these are held in the Building 6008 large Conference Room, at 3:00 pm on the chosen day, but times and locations may vary. For more information, contact our seminar chairman,
Tel (Office): (865) 574-6124 (FAX): (865) 574-1268
Very precise measurements in nuclei can offer demanding tests of the Standard Model of particle physics. In particular, “superallowed” nuclear beta-decay between 0+ analogue states is a sensitive probe of the vector part of the weak interaction, and the measured strength (i.e. ft-value) of each such transition yields a direct measure of the vector coupling constant, GV. To date, the ft-values for nine 0+ --> 0+ transitions – the decays of 10C, 14O, 26mAl, 34Cl, 38mK, 42Sc, 46V, 50Mn and 54Co – have been measured with ~0.1% precision or better, and these results yield fully consistent values for GV. The resultant GV in turn yields an experimental value for Vud, the leading diagonal element of the quark mixing matrix, the Cabibbo-Kobayashi-Maskawa (CKM) matrix. Not only is this the most precise determination of Vud, it is the most precise result for any element in the CKM matrix.
The CKM matrix is a central pillar of the Standard Model and, although the model does not predict values for the matrix elements, it absolutely requires that the matrix be unitary. The experimental value for Vud obtained from superallowed beta-decay leads to the most demanding test available of CKM unitarity, a test which it fails by more than two standard deviations: viz. 0.9968 ± 0.0014. I shall outline the nuclear measurements made to date, compare them with less precise results for GV from neutron and pion beta decay, point out the sources of uncertainty in the unitarity test and indicate the directions being taken by current nuclear experiments that are designed to reduce some of these uncertainties. New results on 22Mg decay, obtained at the Texas A&M cyclotron, will serve to illustrate the challenges involved.
Refreshment at 2:40 Physics Division Colloquium