Perspectives of Nuclear Physics in Europe - European Science ...

ut also scintillating bolometers as well as experimental
techniques are also employed in experiments searching
for direct Dark Matter interactions. The latter experiments
are optimised to achieve a high discrimination
power between ionising and recoil events at a keV energy
deposition. Close co-operation between the communities
is recommended to resolve common experimental
challenges.
Quark mixing
Well before the observation of neutrino mixing it had
been established that quarks participate in weak interactions
with a mixture of their mass eigenstates. The
mixing is governed by the Cabibbo-Kobayashi-Maskawa
(CKM) matrix. In the past the unitarity of this matrix has
been questioned by a series of experiments, but recent
theoretical as well as experimental advances related to
the dominant V ud and V us matrix elements have confirmed
unitarity at the 6 x 10 -4 level.
This was mainly due to a shift of the value of V us
by about 2.5 standard deviations from the previously
adopted value, which has by now been confirmed by
several independent measurements and is now firmly
established. Further progress on V us not only requires
new and more precise branching ratio measurements in
K-decays, but also that theoretical uncertainties in calculating
SU(3) symmetry breaking effects in the axial-vector
couplings favouring K-decay experiments be reduced.
The value of the other matrix element of importance
for the unitarity test of the first row of the CKM matrix,
i.e. V ud , can be obtained from nuclear decays, from free
neutron decay and from pion decay, with the first one
presently providing the highest precision. As to the value
of V ud obtained from the corrected Ft-values (i.e. the
product of the partial half-life and the phase space factor
for a β transition, corrected for nucleus-dependent
radiative and nuclear structure corrections) for superallowed
pure Fermi transitions, recently much progress
was made both experimentally and theoretically: the
precision and control over isospin symmetry breaking
related nuclear structure effects, δ C , as well as the
nucleus independent radiative correction, Δ R , has been
significantly improved. On the experimental side several
new isotopes have been added to the set of nuclei
contributing to the determination of V ud . Further, the use
of Penning traps providing high precision mass values
(i.e. ISOLTRAP, JYFLTRAP and SHIPTRAP in Europe
and several similar setups in the US) has increased significantly
the reliability as well as the precision of Q EC
values (i.e. the energy difference between the ground
states of mother and daughter isotopes) for many of the
relevant transitions. As a result the Conserved Vector
Figure 2. Values of |V ud | from the 0 + -> 0 + super-allowed Fermi
transitions, neutron decay, pion beta decay and the beta transitions
of T=1/2 mirror nuclei.
Current hypothesis is now validated at the level of 1.3
x 10 -4 and |V ud | = 0.97425(22). Recent newly developed
trap operation modes will yield further improvements for
Q EC values. Further progress also requires that Penning
trap based mass measurements as well as high precision
half-life and branching ratio measurements be
performed for the recently added superallowed transitions,
which will at the same time help to improve the
theoretical calculations of the nuclear structure related
corrections.
An independent, though at present less sensitive,
test of CVC and CKM unitarity was recently provided
by combining the newly reported corrected Ft-values
for the superallowed β transitions between T = 1/2 mirror
nuclei with results from correlation measurements
performed with these nuclei. It was shown that several of
these mirror β transitions have the potential for providing
a precision on V ud that is competitive to the value from
the pure Fermi transitions, the highest sensitivity being
obtained for 35 Ar. Measurements leading to more precise
Ft-values as well as precise measurements of the β
asymmetry parameter and β-ν correlation coefficient for
mirror β transitions would therefore be of great value.
The neutron lifetime, τ n , and the β asymmetry parameter
A n or the beta-neutrino angular correlation coefficient
a n in neutron decay provide values of the axial-vector
weak coupling constant of the nucleon, g A . This not only
serves for determining many important semi-leptonic
weak cross sections, such as in stellar fusion, big-bang
nucleosynthesis and for neutrino detection, but also
allows one to extract a precise value for V ud , independent
of nuclear structure effects. Using the values for the neutron
lifetime and β asymmetry parameter recommended
by the Particle Data Group (2008) yields |V ud | = 0.9746(19),
in agreement with but still less precise than the value
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