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due to the interference of photon and Z 0 exchange. From
the measurements, the weak mixing, or Weinberg, angle
at low momentum transfers can be extracted, providing
powerful constraints on TeV-scale physics, for example
additional neutral gauge bosons, SUSY, and leptoquarks.
In recent years, two experiments have been completed.
The Møller scattering experiment E158 at SLAC agrees
with the SM prediction for the weak mixing angle within
about one sigma. The NuTeV experiment, (anti)neutrino
scattering from iron, however, deviates from the SM by
about three sigma. The overall agreement of the APNC
and PVeS experiments with the SM prediction for the
running of the Weinberg angle from the Z 0 -pole down
to low energies ( Fig.4), is poor at the moment, and it is
apparent that more precise experiments are called for.
The Qweak experiment at JLab measures the parityviolating
asymmetry in polarised-electron scattering from
protons at Q 2 = 0.03 GeV 2 /c 2 . The goal is to extract the
weak charge of the proton, Q W
p
= 1−4sin 2 θ W with a combined
statistical and systematic error of 4%, providing
a 0.3% measurement of sin 2 θ W . After the completion of
the Qweak experiment and the realisation of the 12 GeV
JLab upgrade, two new PVeS experiments are planned.
A parity-violating Møller scattering experiment at 11 GeV
will measure the weak charge of the electron with much
better precision. A deep-inelastic electron–proton scattering
experiment will provide new information on the
parity-violating electron–quark interaction, in particular
the axial-vector quark couplings, which are very difficult
to measure with other low-energy techniques. This
experiment not only tests the electroweak sector of the
SM, but it will also provide information on novel aspects
of the QCD (parton) structure of nucleons.
Note that parity-violating elastic electron scattering
from nuclei (e.g. lead) is being pursued at JLab. Finally,
in the long-term future a high-energy polarised-electron
light-ion collider could provide new opportunities
to measure the weak-mixing angle to ultra-high precision.
Parity violation in hadronic systems
The standard model of particle physics provides a
well-established description of the fundamental weak
interaction between quarks but how it manifests in the
hadronic environment still remains enigmatic, e.g. parityviolating
asymmetries observed in non-leptonic decays
of hyperons still cannot be reconciled with theoretical
expectations. Current studies to shed more light on the
interplay between weak and strong interactions mostly
rely on parity violation as an experimental filter to discriminate
the much larger effects due to strong and
electromagnetic interactions. While peculiar enhancement
effects facilitate the observation of hadronic weak
interaction processes, they obscure the link to the
underlying theory framework, for which parity violating
asymmetries in few-nucleon systems without enhancement
offer much cleaner conditions.
A recent effective field theory approach provides a
model-independent framework to describe the weak
forces between nucleons and makes close contact with
QCD. However, so far only few observables in few body
systems have provided non-zero results, such as the longitudinal
analysing power in pp scattering at Bonn, PSI
and TRIUMF, and the triton asymmetry in polarised neutron
capture by 6 Li at the ILL. The prospect of obtaining
from effective field theory accurate predictions for parity
violating observables in hadronic few-body systems has
motivated strong new efforts in the US. However, these
experiments are very challenging in view of the smallness
of the expected asymmetry effects.
Time reversal and CP violation
in the quark sector
If one assumes CPT to be conserved, time reversal
violation and CP violation are equivalent. So far, CP violation
has been observed in the neutral K and B meson
systems, and is usually believed to be manifest also
in the huge baryon asymmetry of our universe. While
the SM, via a complex phase in the CKM matrix, perfectly
describes the observed CP violation in the K 0 and
B 0 systems, it completely fails, together with standard
cosmology, to explain the observed baryon asymmetry.
Another CP violating phase is present in the SM in QCD,
the so called θ-term, which is constrained by experiment
to be very small, i.e. less than ~10 -10 . Additional sources
of CP violation appear to be necessary and are naturally
found in most popular extensions of the SM offering a
variety of complex phases.
Searches for new CP violation, or equivalently for time
reversal violation, can be pursued by more precisely
investigating the known CP violating channels or by
concentrating on observables with negligible SM background,
thereby increasing the sensitivity to any kind of
new effect. Examples comprise time reversal violating β
decay correlations, effects in the neutral D-meson system
and permanent electric dipole moments (EDMs).
Electric dipole moments
Any permanent electric dipole moment of a fundamental
quantum system would violate invariance under time
reversal. Experimentally, so far, no finite value of any
such permanent EDM has been found and experiments
today are not yet sensitive enough to detect the small
EDMs due to the known SM contribution. They are,
however, probing and so far excluding many models
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