According to the SM, the CKM is a unitary matrix; thus, adding the elements of any row in quadrature should result in unity. Given the experimental precision with which each of the elements of the CKM has been determined, the top row provides the most stringent test of unitarity.
(1.5)
has been determined from Ke3 decays (e.g.
K+ -> p 0 e+
n e) to be 0.2196 ± 0.0023 [PDG 96, Shr 78] . A more recent
re-analysis by Barker et al. modifies the value only slightly to 0.2199
± 0.0017 [Bar 92]. Slightly higher values of
have been determined from the study of hyperon decays (e.g. [Xi] -> [Lambda]
e n and [Sigma]- -> n e n ) [Gar 92] which leads
to an overall average of 0.2205 ± 0.0018 [PDG 96].
The value
has not yet been determined directly. Values for
and
however, have been measured and can be used to indicate of the magnitude of
.
The value
can be determined from the semileptonic decay of B mesons produced on the
[hammer](4S)
resonance. The current value, which combines the experimental and theoretical
uncertainties, is [PDG 96]:
=0.08
± 0.02 (1.6)
The value
can be obtained from B semileptonic decays to charmed mesons (
).
The current value of
is :
=0.041
± 0.003 (1.7)
Combining (1.6) and (1.7) yields a value for
on the order of 10-3 (about the same order uncertainty as in
)
and is therefore safe to ignore.
The dominant term in (1.5) is
which can be determined from both nuclear beta-decay and neutron decay
experiments. The results disagree somewhat as described below.