Hadronic Physics Group
at the   Physics Department   of the   College of William and Mary
Parity Violation Experiments at Jefferson Lab

A central focus of our research is a program of several precision experiments that use parity-violating electron scattering to probe the structure of nucleons (proton, neutron). Parity violation arises due to the quantum interference between photon exchange (electromagnetic) and Z0-exchange (neutral-current weak) processes. These experiments are sensitive in particular to the contributions of strange quarks (a component of the sea of virtual quark-antiquark pairs) to the electromagnetic structure of the nucleon.

G0     JLab Expt. 00-006

The G0 experiment will measure the parity-violating asymmetry from elastic electron scattering on the proton at both forward and backward angles over a range of momentum transfers from 0.1 to 1.0 GeV2. This will allow a `Rosenbluth-type' separation of the magnetic and electric form factors arising from strange quarks. Quasi-elastic scattering from a deuterium target will also be used to provide access to the axial vector form factor (and hence the nucleon anapole moment). Inelastic scattering events will access the neutral weak N-Delta form factor. A large dedicated magnetic spectrometer has been constructed for this experiment; our group plays a major role major in the experiment, focusing on the scintillation detector systems at the heart of the apparatus. The experiment will begin commissioning in Hall C at Jefferson Lab in the Fall of this year (2002). Jeff Secrest and Sarah Phillips will write their PhD theses on different aspects of this experiment.

More detailed information on G0 can be found here.

HAPPEX-II    JLab Experiment 99-115

This experiment will measure parity-violating elastic scattering from the proton at a momentum transfer Q2 = 0.1 GeV2, at forward scattering angles, using the two high-resolution spectrometers in Hall A. This will yield a precise measurement of a linear combination of the strange electric and magnetic form factors, and is therefore complementary to the low-Q2 portion of the G0 experiment. The experiment is scheduled to run in the Spring of 2003.

HAPPEX-4He    JLab Experiment 00-114

Since 4He is a spinless target, electron scattering here is purely electric in nature (there are no magnetic or axial contributions). Therefore the parity-violating elastic asymmetry can be cleanly interpreted in terms of the strange electric form factor.

Armstrong (along with R. Michaels of JLab) is co-spokesperson of this experiment. We will measure at the same momentum transfer as HAPPEX-II, again using the Hall A spectrometers. This experiment will provide a precise measurement of the strangeness radius of the proton; in combination with the HAPPEX-II results, we will be able determine the strange magnetic moment of the proton as well. The experiment is scheduled to run immediately following HAPPEX-II, in the Summer of 2003. Bryan Moffit will base his PhD thesis on this measurement.

HAPPEX-Pb    JLab Expt. 00-003

The Pb-parity experiment has somewhat different physics goals than the rest of the Hall A parity program. Since the Z0 boson (unlike the photon) couples mainly to neutrons in a nucleus (the coupling to the proton is reduced in comparison due to an accidental cancelation), the parity-violating asymmetry can be used to determine the neutron radius Rn relative to the proton radius Rpof a heavy nucleus. The difference between Rn and Rp for a heavy nucleus is expected to be of the order of several percent, however this `neutron skin' has not been clearly observed in a stable nucleus.

The neutron radius is of interest for conventional nuclear structure physics (it is rather unsatisfactory that such a basic property of nuclei is so poorly determined), especially as a calibration point for theory and for application to the physics of neutron-rich radioactive beams and neutron-rich nuclei in astrophysics. However, there is an additional motivation, of particular interest to us, related to atomic parity violation (APV). Standard model tests using APV have an important systematic error due to the uncertainty in Rn. Although uncertainty in atomic theory (rather than in the nuclear structure term) presently dominates the systematics, intense theoretical work is underway, and these uncertainties are expected to be reduced. A measurement of Rn can pin down the nuclear structure terms for APV tests of the standard model.

The experiment will run at a Q2 of 7.9 x 10-3 GeV2. At this momentum transfer, strange quark effects are expected to be negligible.

The expected 3% determination of the small (0.5 ppm) asymmetry will yield a 1% measurement of Rn/Rp. This would, for the first time, definitively reveal the existence of a neutron skin, if it is of the predicted order of magnitude.

Back to the Hadronic Physics Group

Physics Department
Graduate Studies in Physics at William and Mary
Jefferson Lab

armd@physics.wm.edu
last updated: June 3 2001