Parity Violation in Atomic, Nuclear and Hadronic Systems

 

Workshop at ECT* in Trento, Italy

June 5-16, 2000

Organizers:
Charles J. Horowitz  (Indiana)
M. Ramsey-Musolf (Connecticut)
Willem T. H. van Oers (Manitoba)
Gerald A. Miller (Washington)
Peter Ring (Munich)

Workshop supported by the ECT* with additional funds from Jefferson Laboratory, Indiana University and Indiana University Cyclotron Facility.
Information on the workshop location is at the  ECT* Web Site
 

Parity violation provides a uniquely clean probe of complex strong interaction dynamics and allows important tests of the Standard Model.  There is a new opportunity to use parity violating elastic electron scattering from a heavy nucleus to accurately and model independently measure the neutron density.  This could have many implications for atomic parity experiments, nuclear structure and nuclear astrophysics.  New results on proton-proton parity violation are being disseminated. The first week of this workshop will focus on parity violating electron scattering, atomic PNC and the nuclear structure related to neutron densities.  The second week will focus on parity violation in nuclei and in nucleon scattering.

For more information:
Email: charlie@iucf.indiana.edu 1 (812) 855-2959, or
vanoers@physics.umanitoba.ca
 
 

<<<< NEW >>>>>
Summary of First Week
Summary of Second Week

Postscript files for Some talks

List of Participants including E-mails
 

Schedule (Subject to change):
  First Week, June 5-10, 2000
 Second Week, June 12-16, 2000
 
 

List of Topics:

Scientific Proposal

Parity violation can be used to probe many aspects of the Standard Model and nuclear
and nucleon structure.  First, parity violation in atoms and electron scattering
will be discussed and then parity violation with hadronic probes.  These two sections
share many important ideas including the crucial role of nuclear structure and anapole
moments.  Anapole moments involve hadronic weak interactions but are measured with atomic
or electron probes.
 

The first section of the workshop will bring together atomic, parity violating
electron scattering and nuclear structure physicists to exploit a new possibility
for measuring neutron densities.  The common theme is neutron densities:
how they are calculated, how they are measured using hadronic and weak
probes and how this information can be used for atomic parity
experiments, extrapolation to exotic nuclei in radioactive beams and
astrophysics, etc.  The workshop will assess our present knowledge of
neutron densities and the improvements expected from an accurate
measurement.  In addition, related parity violating issues will be
discussed.
 

The neutron radius of a heavy nucleus can be measured to 1\% using
parity violating electron scattering (because the $Z^0$ couples primarily
to neutrons).   This would be the only accurate and model independent
measurement of the size of a large hadronic system.  Because of the
neutron skin, the size does not follow from the charge radius.
Such a measurement will have many implications for atomic parity violation
(PV), low energy tests of the standard model, nuclear structure, nuclear
astrophysics and the physics of radioactive beams.
 

It is important to test the standard model at low energy.  There is an
apparent disagreement between the measurement of parity violation in atomic
Cs and the standard model.   As the precision of the atomic experiments
improve they will need increasingly accurate nuclear structure information
on neutron densities.   The most precise standard model test may involve the
combination of an atomic measurement and PV electron scattering to
constrain the nuclear structure.  Atomic measurements suffer from atomic
theory uncertainties.  This motivates measurements of ratios of PV in
different isotopes.  While minimizing the atomic theory uncertainties,
this requires more information on neutron radii of different isotopes.
 

The present understanding of neutron densities in medium and heavy nuclei
is based on both non-relativistic and relativistic mean field models.
Recent advances in effective field theories may allow the construction
of these mean field models in a more systematic fashion.  This could
allow one to determine the present uncertainty in neutron densities and
which parts of the effective interaction, such as the surface symmetry
energy, will be constrained by a neutron measurement.  This constraint
could be important in the extrapolation to exotic neutron rich nuclei for
astrophysics or radioactive beams.
 

Some goals of this first section include: the introduction of atomic and
electron scattering PV issues to nuclear structure physicists and vice versa,
to assess interest in and potential impact of a PV neutron density measurement,
to help optimize possible experiments including the choice of target
(208Pb, 138Ba,...) and to improve the knowledge of neutron densities.
 

The next section will focus on hadronic probes of parity violation.
There are to date, several recently disseminated experimental
results. First, the TRIUMF proton-proton parity violation experiment has
obtained a  result for the longitudinal analyzing power A_z at an energy
(221.3~MeV) where only the weak rho-nucleon coupling constant plays a role. The
measured value for A_z places an important constraint on the weak rho-nucleon
coupling constant. Together with the low energy results from the Paul Scherrer
Institute and the University of Bonn, constraints can now be imposed on both
the weak rho-nucleon and omega-nucleon coupling constants. Secondly, the
measurements of the anapole moments have led to deductions of the weak
pion-nucleon coupling constant. However, there appears to be an inconsistency
between the anapole moments for 133Cs and 205Tl. But even more
importantly the value of the weak pion-nucleon coupling constant deduced from
the anapole moment of 133Cs does not agree with the weak pion-nucleon
coupling constant deduced from the value of the circular polarization of
1.081~MeV gamma-rays of the decay of the well-known parity mixed doublet in
18F, for which the nuclear structure is relatively well known. One notes
that there are several experiments, which have measured the circular
polarization of the 1.081~MeV gamma-rays, giving results in mutual agreement.
An interesting way to reconcile the different experimental results is to
postulate that the weak meson-nucleon coupling constants depend on the nuclear
medium. Thirdly, there now exists a large body of parity violating longitudinal
analyzing power data obtained in scattering of epithermal neutrons from a large
range of nuclei throughout the periodic table. Very large parity violating
effects have been observed; but extraction of the weak meson-nucleon coupling
constants appears a daunting task. And fourthly, there is the old, but still
unexplained, experiment performed at the ZGS of Argonne National Laboratory,
measuring the longitudinal analyzing power in scattering protons from a water
target, which has given a result more than ten times larger than what is
expected from simple scaling arguments.

Several new parity violation experiments are currently in preparation: a
measurement of the parity violating longitudinal analyzing power A_z in
proton-proton scattering at 450~MeV at TRIUMF and at a few GeV at COSY (in a
novel cooled stored beam environment); a measurement of the parity violating
asymmetry in the capture of longitudinally polarized epithermal neutrons by
hydrogen at LANSCE; a measurement of the parity violating spin rotation of cold
neutrons passing through hydrogen (and also helium) at ILL and NIST.

The second part of the Workshop is to bring perspective to this
subfield of fundamental symmetries.
The general purpose of such studies is in using the weak interaction to learn
about more difficult aspects of the strong interaction including the derivation
of the meson-nucleon coupling constants, the nucleon-nucleon potential, and the
precise treatment of the many-body problem.  Some specific questions to be
addressed are: