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
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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:
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: