A central focus of our research program are several precision experiments that use parity-violating electron scattering to probe the structure of nucleons (the proton and the neutron). Parity-violation arises due to the quantum interference between the exchange processes of the photon (electromagnetic) and of the Z0 boson (neutral-current weak). 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.
Qweak (JLab Expt. 02-020)
The Standard Model of electroweak interactions has been confirmed with impressive precision in a variety of experiments, ranging in energies from the eV scale in atomic parity violation to a few hundred GeV in electron-positron collisions at LEP and the SLC. Low-energy experiments continue to play an important role in testing the Standard Model, measuring its parameters, and in searching for possible physics which may lie beyond the Standard Model. Low-energy electroweak observables are sensitive to new physics which does not sit on the Z0-resonance. The Qweak experiment uses parity-violating electron scattering from the proton at very low momentum transfers (where strange quark effects are small) to measure the "weak charge" of the nucleon, Qweak, (the vector coupling of the Z0 to the nucleon) to high precision.
In the Qweak experiment, the proton's weak charge Qweak = 1 - 4 sin2θW is measured to a 4% precision using elastic ep scattering at Q2 = 0.025 (GeV/c)2 employing 180 μA of 85% polarized beam incident on a 35 cm liquid hydrogen target. A dedicated high-acceptance toroidal magnetic spectrometer detects the scattered electrons.
The Standard Model makes a firm prediction for the value of Qweak, based on the running of the weak mixing angle from the Z0-pole down to low energies, corresponding to a 10 sigma effect in our experiment. Any significant deviation from the Standard Model prediction at low Q2 is therefore a signal of new physics, whereas agreement places new and significant constraints on possible Standard Model extensions, such as supersymmetry (SUSY).
G0 (JLab Expt. 00-006)
The G0 experiment measured 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 allows for a "Rosenbluth-type" separation of the magnetic and electric form factors arising from strange quarks. Quasi-elastic scattering from a deuterium target is used to provide access to the axial vector form factor (and hence the nucleon anapole moment). Using inelastic scattering events we access the neutral weak N-Delta form factor. A large dedicated magnetic spectrometer was constructed for this experiment; our group played a major role major in the experiment, focusing on the scintillation detector systems at the heart of the apparatus. The experiment was completed in Hall C at Jefferson Lab in the Fall of 2002.
HAPPEX-II (JLab Expt. 99-115)
This experiment measures 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 yields 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 was completed in the Spring of 2003.
HAPPEX-4He (JLab Expt. 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 measure at the same momentum transfer as HAPPEX-II, again using the Hall A spectrometers. This experiment provides a precise measurement of the strangeness radius of the proton; in combination with the HAPPEX-II results, we are able to determine the strange magnetic moment of the proton as well. The experiment was completed immediately following HAPPEX-II, in the Summer of 2003.
PREX (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 Rp of 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. At this momentum transfer Q2 of 7.9 x 10-3 GeV2, strange quark effects are negligible. A 3% determination of the small (0.5 ppm) asymmetry yields a 1% measurement of Rn/Rp. The experiment was completed in the Spring of 2010.