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4.5 Fundamental Interactions
experiments can lead to the discovery of new physics
beyond the SM. At low momentum transfer, as is the
case in APNC experiments, new particles predicted for
instance in supersymmetric or in technicolor models
generate additional electron-quark PNC interactions. Q w
can be sensitive to new corrections, both weak isospinconserving
and isospin-breaking, and to extra Z bosons,
providing more stringent bounds than direct searches
at high energy colliders.
On the experimental side, one has to consider
that effects are very small and the analysis of APNC
experiments is complicated by the uncertainties in
both atomic and nuclear-structure theory. Weiman
and collaborators measured the amplitude of the
parity non-conserving transition between the 6S and
7S states of 133 Cs, the only naturally occurring cesium
isotope. Taking into account recent improved atomic
calculations, the value of the weak charge deduced
from this is Q w exp = 73.16(29) exp (20) th in perfect agreement
with the SM prediction of Q w exp = 73.16 ± 0.03 (see also
Figure 4). In this context, it is important to make new
precise measurements of the APNC effect with high-Z
atoms (e.g. francium), since the parity non conserving
effect increases faster than Z 3 . Francium trapping is
currently carried out at LNL (TRAPPRAD experiment)
and in preparation at TRIUMF (Canada) and RCNP/
CYRIC (Japan). Recently a strong APNC effect has been
observed in Ytterbium but, due to the complicated atomic
structure, this atom is less suitable for SM tests.
Figure 4. Overview of determinations of the weak mixing angle,
θ W , at different energies, compared to theoretical expectations (full
line). With respect to the present chapter the value at low energies
that was obtained from atomic parity violation experiments with
cesium is especially important. Red points indicate expected
sensitivities of planned atomic parity violation and electron
scattering experiments.
Another important field of investigation is the test of
single ions (like Ba + , Ra + ) confined in radiofrequency
traps: in particular, the Ra + experiment has the potential
to improve significantly the cesium result. A proof
of principle for several important aspects of a parity
non conservation experiment with one single Ba + ion
has been obtained by Fortson et al. in Seattle. At KVI
Groningen such an experiment with one single Ra + ion
is being set up now; the production, trapping and laser
spectroscopy has recently been achieved for several
Ra + isotopes.
In this perspective, techniques to produce and trap
large amounts of radioactive atoms and ions are crucial:
important steps have already been done in this field in
the past few years, and the upgrade of KVI and LNL
facilities in the trapping sector would be very important
in this context.
Uncertainties in atomic structure, which play an
important role for the extraction of the weak charge,
may be considerably reduced by studying parity violation
along a chain of isotopes. In this way the dependence
on the atomic theory contribution of the parity violating
amplitude measured in an APNC experiment will cancel
out in the ratio of two measurements performed on
two different isotopes of the same element, provided
that the atomic contribution does not change appreciably
along that isotopic chain. In particular, it would be
very valuable to extend the measurements which have
proved successful for natural cesium, i.e. 133 Cs, to some
of its numerous radioactive isotopes, and in future to
a series of Fr isotopes as well. Alternatively one can
study parity violation in heavy highly charged ions where
the electron-correlation and QED effects can be well
accounted for by perturbation theory. Again the strong
increase of the parity effect with increasing Z and the
near degeneracy of atomic levels with opposite parity
would be an asset.
Interpretation of PNC effects will also require, once a
certain precision level is reached, to take into account
nuclear structure effects that enter through the density
distributions of nucleons. Complementary experiments
to precisely measure these will be an important issue.
Finally, another important goal to be pursued is the
measurement of the nuclear anapole moment. Up to
now, this has been detected only for 133 Cs (an even
neutron-number isotope), and it would be important to
measure it for another isotope (in particular one with an
odd neutron-number) as well.
Parity violation in electron scattering
Parity-violating electron (or neutrino) scattering (PVeS)
experiments have developed into a high-precision tool to
determine the weak neutral-current electron–electron and
electron–quark coupling constants (see also the chapter
on Hadron Physics). The parity-violating observables are
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