Evidence for virtual Compton scattering from the proton

In virtual Compton scattering an electron is scattered off a nucleon such that the nucleon emits a photon. We show that these events can be selected experimentally, and present the first evidence for virtual Compton scattering from the proton in data obtained at the Stanford Linear Accelerator Center. The angular and energy dependence of the data is well described by a calculation that includes the coherent sum of electron and proton radiation

Momentum Transfer Dependence of Nuclear Transparency from the Quasielastic ^(12)C(e, e'p) Reaction

The cross section for quasielastic ^(12)C(e,e’p) scattering has been measured at momentum transfer Q^2=1, 3, 5, and 6.8 (GeV/c)^2. The results are consistent with scattering from a single nucleon as the dominant process. The nuclear transparency is obtained and compared with theoretical calculations that incorporate color transparency effects. No significant rise of the transparency with Q^2 is observed

Measurement of the Proton's Neutral Weak Magnetic Form Factor

We report the first measurement of the parity-violating asymmetry in elastic electron scattering from the proton. The asymmetry depends on the neutral weak magnetic form factor of the proton which contains new information on the contribution of strange quark-antiquark pairs to the magnetic moment of the proton. We obtain the value $G_M^Z= 0.34 \pm 0.09 \pm 0.04 \pm 0.05$ n.m. at $Q^2=0.1$ (GeV/c)${}^2$.Comment: 4 pages TEX, text available at http://www.krl.caltech.edu/preprints/OAP.htm

A High Power Hydrogen Target for Parity Violation Experiments

Parity-violating electron scattering measurements on hydrogen and deuterium, such as those underway at the Bates and CEBAF laboratories, require luminosities exceeding $10^{38}$cm$^{-2}$s$^{-1}$, resulting in large beam power deposition into cryogenic liquid. Such targets must be able to absorb 500 watts or more with minimal change in target density. A 40~cm long liquid hydrogen target, designed to absorb 500~watts of beam power without boiling, has been developed for the SAMPLE experiment at Bates. In recent tests with 40~$\mu$A of incident beam, no evidence was seen for density fluctuations in the target, at a sensitivity level of better than 1\%. A summary of the target design and operational experience will be presented.Comment: 13 pages, 9 postscript figure

Separated response functions in exclusive, forward pi(+/-) electroproduction on deuterium

Background: Measurements of forward exclusive meson production at different squared four-momenta of the exchanged virtual photon, Q(2), and at different four-momentum transfer, t, can be used to probe QCD\u27s transition from meson-nucleon degrees of freedom at long distances to quark-gluon degrees of freedom at short scales. Ratios of separated response functions in pi(-) and pi(+) electroproduction are particularly informative. The ratio for transverse photons may allow this transition to be more easily observed, while the ratio for longitudinal photons provides a crucial verification of the assumed pole dominance, needed for reliable extraction of the pion form factor from electroproduction data. Purpose: We perform the first complete separation of the four unpolarized electromagnetic structure functions L/T/LT/TT in forward, exclusive pi(+/-) electroproduction on deuterium above the dominant resonances. Method: Data were acquired with 2.6-5.2-GeV electron beams and the HMS + SOS spectrometers in Jefferson Lab Hall C at central Q(2) values of 0.6, 1.0, and 1.6 GeV2 at W = 1.95 GeV, and Q(2) = 2.45 GeV2 at W = 2.22 GeV. There was significant coverage in phi and is an element of, which allowed separation of sigma(L), T, LT, TT. Results: sigma(L) shows a clear signature of the pion pole, with a sharp rise at small -t. In contrast, sigma(T) is much flatter versus t. The longitudinal/transverse ratios evolve with Q(2) and t and at the highest Q(2) = 2.45 GeV2 show a slight enhancement for pi(-) production compared to pi(+). The pi(-)/pi(+) ratio for transverse photons exhibits only a small Q(2) dependence, following a nearly universal curve with t, with a steep transition to a value of about 0.25, consistent with s-channel quark knockout. The sigma(TT)/sigma(T) ratio also drops rapidly with Q(2), qualitatively consistent with s-channel helicity conservation. The pi(-)/pi(+) ratio for longitudinal photons indicates a small isoscalar contamination at W = 1.95 GeV, consistent with what was observed in our earlier determination of the pion form factor at these kinematics. Conclusions: The separated cross sections are compared to a variety of theoretical models, which generally describe sigma(L) but have varying success with sigma(T). Further theoretical input is required to provide a more profound insight into the relevant reaction mechanisms for longitudinal and transverse photons, such as whether the observed transverse ratio is indeed attributable to a transition from pion to quark knockout mechanisms and provide useful information regarding the twist-3 transversity generalized parton distribution, H-