Workshop on the physics of nucleons and nuclei
Session 4: Neutron radii & weak interaction physics: beta-decay; n-capture; parity
violation
[G. Cates, T. Chupp, W. Donnelly, J. Engel, C. J. Horowitz, K. Kumar, J. Piekarewicz]
16-17 October 2006, Washington, DC
1) Parity violation in electron scattering allows precise, fundamental measurements of
nucleon and nuclear systems. These include probes of strange quarks, beyond standard
model physics, and isospin violation.
The excellent beam quality at Jefferson Laboratory, along with extensive experience in
parity measurements, opens up exciting opportunities for very accurate future
experiments.
2) The Parity Radius Experiment (PREX) at the Jefferson Laboratory aims to measure the
neutron radius of 208Pb to one percent using elastic parity violating electron scattering.
Parity violation provides a clean probe of the neutron density because the weak vector
charge of the neutron is much larger than that of the proton. The completion of this
experiment is important as PREX will provide significant nuclear structure information,
will aid in the determination of the equation of state of neutron-rich matter, will
determine the density dependence of the symmetry energy, and will constrain neutron
radii for atomic parity experiments. Further, PREX has important astrophysical
implications for a variety of neutron-star properties, such as radii, crust thickness,
composition, and cooling rate. A complimentary, albeit more model dependent,
determination of the density dependence of the symmetry energy from heavy-ion
collisions should be compared against results extracted from PREX. Finally, PREX will
provide a baseline for several properties to be studied at future radioactive beam
facilities. These include thicker neutron skins, the structure of the neutron-drip line, and
exotic modes of excitation, such as the Pygmy resonance.
3) Super-allowed nuclear beta decays measure the Vud quark mixing matrix element and
provide an important check on the unitarity of the CKM matrix. Furthermore, ft values
for a range of atomic number Z allow precise tests of isospin violating nuclear structure
corrections and determine isospin mixing in nuclei. One should study super allowed
beta decays in systems with larger isospin violating corrections and precisely determine
the relevant masses.
4) Quite apart from the additional opportunities for neutron density measurements in a
variety of nuclei, parity violating electron scattering can be used as a precision probe of
nucleon structure, such as searching for the manifestation of strange sea quarks to the
nucleon charge and magnetization distributions and for charge symmetry violation at the
partonic level. As a very important spinoff, these measurements then enable the nucleon
and the valence quarks to be used as a fundamental laboratory to test the limits of the
electroweak theory. Finally atomic parity experiments can also constrain neutron
densities and can measure anapole moments.
5) Parity violation in hadronic systems provides important insight into the structure of
nucleons and nuclei. Presently, the large Cesium anapole moment is mysterious given
expectations of a small parity violating pion-nucleon coupling. Therefore, it is important
to measure the pion coupling in parity violating n+p to d + gamma and to measure
anapole moments for other nuclei.
6) Electric dipole moments (EDM) violate CP and provide fundamental insight into
beyond standard model physics. This may be associated with the baryon asymmetry of
the universe. One should push EDM measurements in neutron and atomic systems along
with related nuclear and hadronic structure calculations. This includes measurements in
atoms such as radium and radon, which may have enhancements from nuclear octupole
deformations.
7) The neutrino response of nuclear matter is important for laboratory neutrino oscillation
experiments and in astrophysics. Electron scattering experiments provide important
information for the neutrino response. In addition the neutrino axial response may be
constrained from the axial response of parity violating electron scattering. Core collapse
supernovae involve neutrino interactions over a large range of densities, but perhaps most
importantly in low density neutron rich matter near the neutrino sphere. One should
explore the properties and modes of excitation of low density neutron rich matter with
radioactive beam experiments. This low density neutron rich matter may have new
modes of excitation such as the Pygmy resonance.