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

New time reversal invariant scalar
and tensor interactions
As to time reversal invariant scalar (S) and tensor (T)
interactions, the βν-correlation coefficient a (which is
sensitive to S and T) and the β asymmetry and neutrino
asymmetry parameter, A and B, respectively (which are
both mainly sensitive to T) provide the best prospects.
Over the past years significant progress has been made
in the precision of all three observables.
Atom and ion traps have by now become a standard for
measuring the βν-correlation. Several measurements
have already reported a precision of about 0.5% while
several other experiments are in progress or in preparation.
Whereas this type of activity was started in North
America with atom traps, Europe is catching up now with
both atom and ion traps being used. For example RF and
Penning trap based ion trap experiments are ongoing
at GANIL (the LPC-TRAP experiment) and ISOLDE (the
WITCH experiment), respectively, while a neutral atom
trap based experiment is being set up at KVI-Groningen.
In free neutron decay an experiment using a retardation
spectrometer (aSPECT) is well on its way to reach a
similar precision.
Recently, significant progress has also been made in
the determination of the β asymmetry parameter, in
both trap based and low temperature nuclear orientation
experiments with nuclei, as well as in neutron decay.
Monte Carlo simulations have played a crucial role in this.
Finally, new measurements of the neutrino and recoil
ion asymmetries have become available as well, both
in neutron decay and nuclear β decay.
Further progress in the search for time reversal invariant
S and T currents at low energies would highly benefit
from further improvements of the simulation codes with
respect to scattering of keV to MeV β particles, as well
as from new and more precise methods to determine the
degree of nuclear polarisation. Increased yields as can
be expected e.g. at DESIR, HIE-ISOLDE an EURISOL
for nuclei, and at ILL, FRM-II, PSI and the European
Spallation Source for neutrons, are a further important
asset. Further, methods should be developed to store
polarised isotopes also in ion traps (Paul and Penning
traps), as this would significantly extend the number of
accessible isotopes. In neutron decay the planned PERC
facility will allow for higher precision in the determination
of all three relevant correlation coefficients, i.e. a, A and
B. With all these improvements precisions at the 10 -3
level and beyond are within reach. Also in this case the
efforts in the US mentioned in the previous section will
provide important contributions.
4.5.3 Discrete Symmetries
The three discrete symmetries in Nature are:
• charge conjugation C (i.e. exchanging particles by
their antiparticles),
• parity change P (i.e. the mirror inversion of space
coordinates), and
• time reversal T.
CPT invariance is one of the most important symmetries
of Nature. It states that these three operations
when performed simultaneously do not change the
measurable physical properties of a system. A number of
experiments to improve bounds on discrete symmetries
is going on worldwide, which all have the potential to
produce surprises.
Parity
Atomic parity violation
Atomic parity non conservation (APNC) experiments are
a fundamental tool in testing our understanding of the
electroweak interaction. The observation of parity non
conservation in atoms (and then in the deep inelastic
electron-deuteron scattering) led to the discovery of
the weak electron-nucleon interaction due to neutral
currents. Recent APNC results have reached a level of
precision to provide powerful constraints on new physics
beyond the SM.
Parity violation in atomic systems arises from the interference
between the parity conserving electromagnetic
interaction and the parity violating weak interaction.
Although most of the binding energy of the atomic electrons
comes from their attractive Coulomb interaction
with the Z protons in the nucleus, the weak electronnucleus
interaction mediated by the exchange of neutral
gauge bosons Z 0 induces a small correction in the binding
energy and parity of the electronic wave functions.
This correction results in an electric dipole amplitude
between two electronic states that in the absence of
parity violating effects would have the same parity, i.e.
forbidden transitions.
The dominant (and nuclear spin-independent) part
of the parity non conserving Hamiltonian depends on
the weak charge Q w , which contains the SM coupling
constants. The weak charge is extracted from the experimental
data combined with accurate calculations of the
electronic wave functions. The observation of deviations
between the values measured in the laboratory
and the predictions of the SM in high precision APNC
Perspectives of Nuclear Physics in Europe – NuPECC Long Range Plan 2010 | 161