602 research outputs found
Superallowed Fermi beta decay and Coulomb mixing in nuclei
Superallowed nuclear beta decay provides a direct measure of
the weak vector coupling constant, \GV. We survey current world data on the
nine accurately determined transitions of this type, which range from the decay
of C to that of Co, and demonstrate that the results confirm
conservation of the weak vector current (CVC) but differ at the 98% confidence
level from the unitarity condition for the Cabibbo-Kobayashi-Maskawa (CKM)
matrix. We examine the reliability of the small calculated corrections that
have been applied to the data, and conclude that there are no evident defects
although the Coulomb correction, , depends sensitively on nuclear
structure and thus needs to be constrained independently. The potential
importance of a result in disagreement with unitarity, clearly indicates the
need for further work to confirm or deny the discrepancy. We examine the
options and recommend priorities for new experiments and improved calculations.
Some of the required experiments depend upon the availability of intense
radioactive beams. Others are possible with existing facilities.Comment: 12 pages, LaTeX with aipproc2.sty (included) and epsf.sty; Invited
talk at Nuclear Structure 98, Gatlinburg, Tennessee, Aug. 199
New limit on fundamental weak-interaction parameters from superallowed beta decay
A new critical survey of all world data on superallowed beta decays provides
demanding tests of, and tight constraints on, the weak interaction. In
confirmation of the conserved vector current (CVC) hypothesis, the vector
coupling coupling constant is demonstrated to be constant to better than 3
parts in 10^4, and any induced scalar current is limited to fs \leq 0.0013 in
electron rest-mass units. The possible existence of a fundamental scalar
current is similarly limited to |Cs/Cv| \leq 0.0013. The superallowed data also
determine the CKM matrix element to be Vud = 0.9738(4). With PDG values for Vus
and Vub, the top-row test of CKM unitarity yields Vud^2 + Vus^2 + Vub^2 =
0.9966(14); although, if a recent result on Ke3 decay is used exclusively to
obtain Vus, this sum becomes 0.9999(16). Limits on possible right-hand currents
are given.Comment: 4 pages, one postscript fil
CURRENTS AND THEIR COUPLINGS IN THE WEAK SECTOR OF THE STANDARD MODEL
Beta-decay and muon-capture experiments in nuclei are reviewed. The conserved
vector current hypothesis is confirmed through the observed constancy of the
vector coupling constant determined from the superallowed Fermi transitions and
from the measurement of the weak-magnetism term in mirror Gamow-Teller
transitions. The axial-vector and pseudoscalar coupling constants in the
nucleon are determined from neutron decay and muon capture on the proton
respectively. In finite nuclei, evidence for these coupling constants being
reduced relative to their free-nucleon values is discussed. Meson-exchange
currents are shown to be an important correction to the time-like part of the
axial current as evident in first-forbidden beta decays. Tests of the Standard
Model are discussed, as well as extensions beyond it involving right-hand
currents and scalar interactions.Comment: 67 pages, plain LaTex, uses worldsci.sty, two figures embedded in
manuscript as tex statements. A chapter for a book entitled 'The Nucleus as a
Laboratory for Studying Symmetries and Fundamental Interactions', eds. E.M.
Henley and W.C. Haxto
Superallowed 0+ --> 0+ nuclear beta decays: 2014 critical survey, with precise results for Vud and CKM unitarity
A new critical survey is presented of all half-life, decay-energy and
branching-ratio measurements related to 20 superallowed 0+ --> 0+ beta decays.
Included are 222 individual measurements of comparable precision obtained from
177 published references. Compared with our last review in 2008, we have added
results from 24 new publications and eliminated 9 references, the results from
which having been superseded by much more precise modern data. We obtain
world-average ft-values for each of the eighteen transitions that have a
complete set of data, then apply radiative and isospin-symmetry-breaking
corrections to extract corrected Ft values. Fourteen of these Ft values now
have a precision of order 0.1% or better. In the process of obtaining these
results we carefully evaluate the available calculations of the
isospin-symmetry-breaking corrections by testing the extent to which they lead
to Ft values consistent with conservation of the vector current (CVC). Only one
set of calculations satisfactorily meets this condition. The resultant average
Ft value, when combined with the muon liftime, yields the up-down quark-mixing
element of the Cabibbo-Kobayashi-Maskawa (CKM) matrix, Vud = 0.97417 +/-
0.00021. The unitarity test on the top row of the matrix becomes |Vud|^2 +
|Vus|^2 + |Vub|^2 = 0.99978 +/- 0.00055 if the Particle Data Group recommended
value for Vus is used. However, recent lattice QCD calculations, not included
yet in the PDG evaluation, have introduced some inconsistency into kaon-decay
measurements of Vus and Vus/Vud. We examine the impact of these new results on
the unitarity test and conclude that there is no evidence of any statistically
significant violation of unitarity. Finally, from the Ft-value data we also set
limits on the possible existence of scalar interactions.Comment: 29 pages, 8 figures. arXiv admin note: substantial text overlap with
arXiv:0812.120
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