John J. LeRose

Nuclear Transparency with the gamma + n -> pi- + p Process in 4He

We have measured the nuclear transparency of the fundamental process gamman-->pi(-)p in He-4. These measurements were performed at Jefferson Lab in the photon energy range of 1.6-4.5 GeV and at theta(cm)(pi)=70degrees and 90degrees. These measurements are the first of their kind in the study of nuclear transparency in photoreactions. They also provide a benchmark test of Glauber calculations based on traditional models of nuclear physics. The transparency results suggest deviations from the traditional nuclear physics picture. The momentum transfer dependence of the measured nuclear transparency is consistent with Glauber calculations that include the quantum chromodynamics phenomenon of color transparency.

High-resolution hypernuclear spectroscopy electron scattering at JLab, Hall A

The characteristics of the Jefferson LAB electron beam, together with those of the experimental equipments, offer a unique opportunity to study hypernuclear spectroscopy via electromagnetic induced reactions. Experiment 94-107 started a systematic study on 1p-shell targets, 12C, 9Be and 16O. We present the results from 12C, 16O and very preliminary results from 9Be. For 12C for the first time measurable strength in the core-excited part of the spectrum between the ground state and the pΛ state was shown in for the first time. A high-quality 16ΛN spectrum was produced for the first time with sub-MeV Energy resolution. A very precise BΛ value for 16ΛN, calibrated against the elementary (e, e′K+) reaction on hydrogen, has also been obtained. Final data on 9Be will be available soon. The missing energy resolution is the best ever obtained in hypernuclear production experiments.

A Search for Sigma^0_5, N^0_5 and Theta^++ Pentaquark States

A high-resolution ({sigma}{sub instr.} = 1.5 MeV) search for narrow states ({Lambda} < 10 MeV) with masses of M{sub x} {approx} 1500-1850 MeV in ep {yields} e K{sup +} X, e K{sup -} X and e {pi}{sup +} X electroproduction at small angles and low Q{sup 2} was performed. These states would be candidate partner states of the reported {Theta}{sup +}(1540) pentaquark. No statistically significant signal was observed in any of the channels at 90% C.L. Upper limits on forward production were determined to be between 0.7% and 4.2% of the {Lambda}(1520) production cross section, depending on the channel and the assumed mass and width of the state.

Precise Extraction of the Neutron Magnetic Form Factor from Quasi-elastic 3He(pol)(e(pol),e') at Q^2 = 0.1-0.6 (GeV/c)^2

We have measured the spin-dependent transverse asymmetry, A_T, in quasi-elastic inclusive electron scattering from polarized 3He with high precision at Q^2 = 0.1 to 0.6 (GeV/c)^2. The neutron magnetic form factor, GMn, was extracted at Q^2 = 0.1 and 0.2 (GeV/c)^2 using a non-relativistic Faddeev calculation that includes both final-state interactions (FSI) and meson-exchange currents (MEC). In addition, GMn was extracted at Q^2 = 0.3 to 0.6 (GeV/c)^2 using a Plane Wave Impulse Approximation calculation. The accuracy of the modeling of FSI and MEC effects was tested and confirmed with a precision measurement of the spin-dependent asymmetry in the breakup threshold region of the 3He(pol)(e(pol),e) reaction. The total relative uncertainty of the extracted GMn data is approximately 3%. Close agreement was found with other recent high-precision GMn data in this Q^2 range.

Polarization transfer in the d(epol,e' ppol)n reaction up to Q^2=1.61 (GeV/c)^2

The recoil proton polarization was measured in the d(epol,e ppol)n reaction in Hall A of the Thomas Jefferson National Accelerator Facility (JLab). The electron kinematics were centered on the quasielastic peak (x{sub Bj} {approx} 1) and included three values of the squared four-momentum transfer, Q{sup 2}=0.43, 1.00 and 1.61 (GeV/c){sup 2}. For Q{sup 2}=0.43 and 1.61 (GeV/c){sup 2}, the missing momentum, p{sub m}, was centered at zero while for Q{sup 2}=1.00 (GeV/c){sup 2} two values of p{sub m} were chosen: 0 and 174 MeV/c. At low p{sub m}, the Q{sup 2} dependence of the longitudinal polarization, P{sub z}, is not well described by a state-of-the-art calculation. Further, at higher p{sub m}, a 3.5 sigma discrepancy was observed in the transverse polarization, P{sub x}. Understanding the origin of these discrepancies is important in order to confidently extract the neutron electric form factor from the analogous d(epol,e npol)p experiment.

Quasi-elastic electron scattering fromU238

Rosenbluth separations of electron scattering response functions for uranium targets were made with data using measurements from five laboratory angles (60/sup 0/, 90/sup 0/, 134.5/sup 0/, 140/sup 0/, and 160/sup 0/) at three-momentum transfers ranging from 280 to 500 MeV/c. The separations were made to determine if previously reported quenching of the longitudinal response function persisted in targets with large atomic number. The results are compared to relativistic Fermi gas calculations and calculations following the formalism of Rosenfelder. The measured transverse response functions agree quite well with the Rosenfelder-type calculations and almost as well with the relativistic Fermi gas calculations. The measured longitudinal response functions show significant quenching at low q. This quenching diminishes with increasing q but is never quite overcome.

Quasi-elastic electron scattering from /sup 238/U

Rosenbluth separations of electron scattering response functions for uranium targets were made with data using measurements from five laboratory angles (60/sup 0/, 90/sup 0/, 134.5/sup 0/, 140/sup 0/, and 160/sup 0/) at three-momentum transfers ranging from 280 to 500 MeV/c. The separations were made to determine if previously reported quenching of the longitudinal response function persisted in targets with large atomic number. The results are compared to relativistic Fermi gas calculations and calculations following the formalism of Rosenfelder. The measured transverse response functions agree quite well with the Rosenfelder-type calculations and almost as well with the relativistic Fermi gas calculations. The measured longitudinal response functions show significant quenching at low q. This quenching diminishes with increasing q but is never quite overcome.

Quasi-elastic electron scattering from U 238

Rosenbluth separations of electron scattering response functions for uranium targets were made with data using measurements from five laboratory angles (60/sup 0/, 90/sup 0/, 134.5/sup 0/, 140/sup 0/, and 160/sup 0/) at three-momentum transfers ranging from 280 to 500 MeV/c. The separations were made to determine if previously reported quenching of the longitudinal response function persisted in targets with large atomic number. The results are compared to relativistic Fermi gas calculations and calculations following the formalism of Rosenfelder. The measured transverse response functions agree quite well with the Rosenfelder-type calculations and almost as well with the relativistic Fermi gas calculations. The measured longitudinal response functions show significant quenching at low q. This quenching diminishes with increasing q but is never quite overcome.

pi. /sup +/ photoproduction from /sup 13/C

Differential cross section measurements were made for /sup 13/C(..gamma..,..pi../sup +/)/sup 13/B (g.s.) at 90/sup 0/ (lab) for pion energies of 18, 29, and 42 MeV, and for /sup 13/C(..gamma..,..pi../sup +/)/sup 13/B (E/sub x/ = 3.5 MeV) at 42 MeV. The ground-state results are compared to several distorted-wave impulse approximation calculations and to Helm model calculations. There are significant discrepancies between experiment and theory and among the theoretical results.