Session J8 - Electromagnetic Interactions.
ORAL session, Sunday afternoon, April 29
Room Congressional B, Renaissance Hotel

[J8.001] Measurement of the Proton's G_Ep/G_Mp Ratio to Q^2=5.6 GeV^2 at JLab

The ratio of the electromagnetic form factors of the proton has been measured using the polarization transfer method in elastic \vecep\rightarrow e\vecp in two experiments at JLab: E93-027, and most recently, E99-007. The first experiment showed that the ratio G_Ep/G_Mp decreases monotonically with increasing Q^2, starting around 0.8 GeV^2 and up to 3.5 GeV^2. These results imply that G_Ep decreases significantly faster than the dipole form factor G_D in this Q^2-range. Non-relativistically this fact can be interpreted to indicate that the electric charge distribution in the proton extends to larger distances than the magnetization distribution. In the new experiment the Q^2-range is extended to 5.6 GeV^2, with intermediate values of 4.0 and 4.8 GeV^2, taking advantage of the high current and larger polarization now available at JLab; also a CH_2 analyzer was used instead of graphite. The event rate was optimized by matching the solid angle of the high resolution spectrometer, which detected the proton, and the electron detector, which was a new lead glass calorimeter with 3.56 m^2 frontal area assembled for this purpose. The new preliminary results will be presented and discussed.

[J8.002] Polarization Transfer in the D(\vec e\,,e^\prime\vec p\,) Reaction

The measurement of recoil polarization observables in the D(\vec e\,,e^\prime\vec p\,)n reaction provides a stringent test of the validity of deuteron models. In particular, various calculations suggest that polarization transfer at quasifree kinematics is expected to be free from effects that frustrate extraction of form factors in Rosenbluth L/T separations, most notably Final-State Interactions. Thus, this technique is widely used to extract the ratio of the neutron form factors, GEn/GMn, employing the analogous D(\vec e\,,e^\prime\vec p\,)p reaction. Comparing the D(\vec e\,,e^\prime\vec p\,)n and H(\vec e\,,e^\prime\vec p\,) polarization transfer reactions provides a benchmark test of such quasifree assumptions. The experiment was performed in Hall A at JLab, at a Q^2 of 0.43, 1.00, and 1.61 (GeV/c)^2, a comparable Q^2 range as used for the extraction of neutron form factors in JLab experiment E93-038. Results will be compared to various theoretical calculations. This work was supported in part by the U.S. Dept. of Energy and the U.S. National Science Foundation.

[J8.003] Neutron Electric Form Factor Measurement at TJNAF

The form factors of the neutron give information on fundamental properties of the nucleon and provide a critical testing ground for models based on QCD. In late 1998, TJNAF experiment E93-026 measured the spin-dependent part of the exclusive (e,e'n) scattering cross section at a four momentum transfer squared of Q^2=0.5 (GeV/c)^2. A longitudinally polarized electron beam was scattered from a polarized deuterated ammonia ( ^15ND_3) target and the quasi-elastically scattered electron was detected in coincidence with the knocked-out neutron. The data have been analyzed in terms of the spin-correlation parameter, or the electron-deuteron vector asymmetry of D(e,e'n), to determine the neutron electric form factor G_E^n. The result is consistent with data from existing polarized experiments and shows a good agreement with the Galster parameterization of G_E^n within our measurement uncertainty.

[J8.004] Extraction of g1p in the Resonance Region

An extensive program, designed to investigate the structure functions g1 and g2 in the resonance region, has been implemented at Jefferson Lab. Over three billion events were accumulated in the fall of 1998 by scattering the Jefferson Lab polarized electron beam off of solid dynamically polarized NH3 and ND3 targets. This presentation will focus on the extraction of g1 using only the inclusive proton data obtained during that period. Preliminary results of the virtual photon asymmetry, A1, as well the integral of g1p in the Q2 range from 0.3 – 2.0 Ge V^2 will be shown and compared with theoretical expectations.

[J8.005] Angular Distribution Measurements of the d(\gamma,p)n Differential Cross Section at High Energies

Measurements of the differential cross section of the d(\gamma,p)n reaction at center-of-mass angles of 30^\circ to 143^\circ were made at photon energies of 1.6 to 2.5 GeV in Hall A at the Thomas Jefferson National Accelerator Facility in Newport News, Virginia. These data, in combination with existing measurements, will provide a test of the symmetry of the shape of the angular distribution of the differential cross section about \theta_cm=90^\circ as a function of energy. Preliminary results will be presented.

[J8.006] Measurement of Proton Polarization in Neutral Pion Photo-production

Cross section data for H(\gamma,\pi^+)n and other photo reactions near \theta_cm = 90^\circ show surprisingly good agreement with the predictions of quark counting rules, starting at very low energies, near 2 GeV. We have measured spin observables for the \gamma p \rightarrow p \pi^0 reaction, both across much of the resonance region (E\gamma \approx 0.8 - 2.0 GeV), and up to much higher energies (E\gamma = 4.8 GeV) at center of mass angles between 45^\circ and 120^\circ. HHC provides an important test for short distance phenomena, such as QCD effects in exclusive reactions. The induced polarization and polarization transfer coefficients were determined using a circularly polarized bremsstrahlung beam and the Hall A focal plane polarimeter at the Thomas Jefferson National Accelerator Facility. Preliminary results for the energy dependence for the fixed \theta_cm will be compared to available theoretical predictions. This work was supported in part by the U.\ S.\ Department of Energy and National Science Foundation.

[J8.007] Dynamical calculations for d(e,e' \pi^+) reactions

Electroproduction of \pi^+ on the deuteron has been investigated by using the dynamical model developed by Sato and Lee(T.~Sato and T.-S.~H. Lee, Phys. Rev. C 54), 2660 (1996).. The final two-nucleon scattering and pion rescattering from the second nucleon are calculated using a unitary \pi NN model(T.-S. H. Lee and A. Matsuyama, Phys. Rev. C36), 1459 (1987). The effects due to NN, N\Delta interactions and \rho-exchange on the production mechanisms have also been investigated. The calculations have been performed for parallel kinematics. It has been found that the final NN scattering can have about a 10 % effect on the pion spectrum of Saclay data(R. Gilman et al., Phys. Rev. Lett. 64), 622 (1990), while other considered two-baryon mechanisms are negligibly small. The implications of our results in interpreting the new data from Jlab will be discussed.

[J8.008] Nuclear Transparency from Quasielastic A(e,e'p) Reactions up to Q^2 = 8 (GeV/c)^2

The quasielastic (e,e^\primep) reaction was studied on targets of deuterium, carbon, and iron up to a value of squared momentum transfer Q^2 of 8.1 (GeV)^2. The data are compared to calculations in the Plane-Wave Impulse Approximation, and a nuclear transparency is determined. The A- and Q^2-dependence of the nuclear transparency are investigated in a search for the onset of the Color Transparency phenomenon.

[J8.009] Study of a Possible Nuclear Dependence in R(x,Q^2) = \sigma_L / \sigma_T at Small x and Q^2

Recently, the Hermes Collaboration reported surprising evidence for an A dependence in the ratio of deep inelastic structure functions R(x,Q^2) = \sigma_L/\sigma_T at small x and Q^2. This nuclear dependence of R has been investigated by the E99-118 Collaboration at Jefferson Lab, using hydrogen, deuterium, aluminum, carbon, copper, and gold targets. An overview of the experiment, which was performed in Hall C in Summer 2000, will be presented along with preliminary results.

[J8.010] Measurement of R = \sigma_L / \sigma_T in the Nucleon Resonance Region

Jefferson Lab experiment E94-110 measured inclusive nucleon resonance electroproduction cross sections from hydrogen in Hall C in the missing mass region 1.0 < W^2 < 4.0 (GeV)^2, for four-momentum transfers 0.3 < Q^2 < 5.0 (GeV/c)^2. This experiment seeks to measure R=\sigma_L/\sigma_T, the ratio of the longitudinal to transverse cross sections, a fundamental quantity essentially unmeasured in this kinematic regime. The ratio R is extracted by the Rosenthbluth separation technique. These precision measurements of R will also be used as a first test of quark-hadron duality in the longitudinal channel. An overview of the experiment will be presented, as well as preliminary R results.

[J8.011] A high power liquid hydrogen target for the SLAC E158 experiment

A 154 cm long liquid hydrogen target has been built and installed in a scattering chamber in End Station A at SLAC, ready for measuring the parity violation in Moller scattering (E158). The target has a volume of 47 liters. It is normally operated at a temperature of 20 K, and a pressure of 25 psia. Inside the target loop, a pump keeps the hydrogen circulating at a speed of about 7 m/s, a heat exchanger uses the gaseous helium supplied from the SLAC CTI 4000 refrigerator as coolant to provide up to 1 kW cooling, and a 1 kW heater can vary the heat output to compensate for beam current fluctuations. The target cell contains eight mesh baffles which can promote transverse flow in the fluid, therefore the hydrogen density fluctuation caused by beam heating is minimized. The gas handling system can relieve the hydrogen gas automatically and safely during accidental or intentional target cryogen boil-off. The target is monitored/controlled by a PC/VXI/GPIB system running a LabVIEW program. Some results of the target performance will be presented.

[J8.012] Magnetic Verification of the G0 Superconducting Magnet System

The G0 Superconducting Magnet System (SMS) is a toroidal magnet composed of 8 superconducting coils placed symmetrically about a central beam axis. The entire assembly is iron-free, containing no iron pole faces nor return yokes, and is housed in a common liquid nitrogen heat shield and cryostat. As such, the spectrometer magnetic fields can be completely determined if sufficient knowledge of the current filaments in the coils is available. However, since the coils are not visible nor directly accessible when the SMS is cooled, measurements of external magnetic fields at ``key'' reference points must be used to discover or reconstruct the location of the current filaments to high precision. In order to accomplish this task, a Magnetic Verification Device has been constructed to scan the fringe-field regions of the G0 SMS to locate a series of zero-crossing points in specific components of the magnetic field. This Magnetic Verification Device -- which includes a 3D-motion gantry with 2 high precision 3-axis Hall Probes, and a host of temperature, inclination, and photo sensors -- will be described in detail.

Part J of program listing