Search Results (9 total)

We have measured the nuclear transparency of the A(e,e^′π⁺) process in ²H, ¹²C, ²⁷Al, ⁶³Cu, and ¹⁹⁷Au targets. These measurements were performed at the Jefferson Laboratory over a four momentum transfer squared range Q²=1.1 to 4.7  (GeV/c)². The nuclear transparency was extracted as the super-ratio of (σ_A/σ_H) from data to a model of pion-electroproduction from nuclei without π-N final-state interactions. The Q² and atomic number dependence of the nuclear transparency both show deviations from traditional nuclear physics expectations and are consistent with calculations that include the quantum chromodynamical phenomenon of color transparency.

High-precision measurements of the proton elastic form-factor ratio, μ_pG_E^p/G_M^p, have been made at four-momentum transfer, Q², values between 0.2 and 0.5  GeV². The new data, while consistent with previous results, clearly show a ratio less than unity and significant differences from the central values of several recent phenomenological fits. By combining the new form-factor ratio data with an existing cross-section measurement, one finds that in this Q² range the deviation from unity is primarily due to G_E^p being smaller than expected.

We investigated simultaneously the ¹²C(e,e^′p) and ¹²C(e,e^′pp) reactions at Q²=2  (GeV/c)², x_B=1.2, and in an (e, e^′p) missing-momentum range from 300 to 600  MeV/c. At these kinematics, with a missing momentum greater than the Fermi momentum of nucleons in a nucleus and far from the delta excitation, short-range nucleon-nucleon correlations are predicted to dominate the reaction. For (9.5±2)% of the ¹²C(e,e^′p) events, a recoiling partner proton was observed back-to-back to the ¹²C(e,e^′p) missing-momentum vector, an experimental signature of correlations.

We measured the angular dependence of the three recoil-proton polarization components in two-body photodisintegration of the deuteron at a photon energy of 2 GeV. These new data provide a benchmark for calculations based on quantum chromodynamics. Two of the five existing models have made predictions of polarization observables. Both explain the longitudinal polarization transfer satisfactorily. Transverse polarizations are not well described, but suggest isovector dominance.

We measured angular distributions of differential cross section, beam analyzing power, and recoil polarization for neutral pion electroproduction at Q²=1.0 (GeV/c)² in 10 bins of 1.17≤W≤1.35 GeV across the Δ resonance. A total of 16 independent response functions were extracted, of which 12 were observed for the first time. Comparisons with recent model calculations show that response functions governed by real parts of interference products are determined relatively well near the physical mass, W=M_Δ≈1.232 GeV, but the variation among models is large for response functions governed by imaginary parts, and for both types of response functions, the variation increases rapidly with W>M_Δ. We performed a multipole analysis that adjusts suitable subsets of ℓ_π≤2 amplitudes with higher partial waves constrained by baseline models. This analysis provides both real and imaginary parts. The fitted multipole amplitudes are nearly model independent—there is very little sensitivity to the choice of baseline model or truncation scheme. By contrast, truncation errors in the traditional Legendre analysis of N→Δ quadrupole ratios are not negligible. Parabolic fits to the W dependence around M_Δ for the multiple analysis gives values for Re(S₁₊/M₁₊)=(-6.61±0.18)% and Re(E₁₊/M₁₊)=(-2.87±0.19)% for the pπ⁰ channel at W=1.232 GeV and Q²=1.0 (GeV/c)² that are distinctly larger than those from the Legendre analysis of the same data. Similarly, the multipole analysis gives Re(S₀₊/M₁₊)=(+7.1±0.8)% at W=1.232 GeV, consistent with recent models, while the traditional Legendre analysis gives the opposite sign because its truncation errors are quite severe.

We measured angular distributions of recoil-polarization response functions for neutral pion electroproduction for W=1.23  GeV at Q²=1.0  (GeV/c)², obtaining 14 separated response functions plus 2 Rosenbluth combinations; of these, 12 have been observed for the first time. Dynamical models do not describe quantities governed by imaginary parts of interference products well, indicating the need for adjusting magnitudes and phases for nonresonant amplitudes. We performed a nearly model-independent multipole analysis and obtained values for Re (S₁₊/M₁₊)=-(6.84±0.15)% and Re (E₁₊/M₁₊)=-(2.91±0.19)% that are distinctly different from those from the traditional Legendre analysis based upon M₁₊ dominance and ℓ_π≤1 truncation.

We have studied the quasielastic ³He(e,e^′p)²H reaction in perpendicular coplanar kinematics, with the energy and the momentum transferred by the electron fixed at 840 MeV and 1502  MeV/c, respectively. The ³He(e,e^′p)²H cross section was measured for missing momenta up to 1000  MeV/c, while the A_TL asymmetry was extracted for missing momenta up to 660  MeV/c. For missing momenta up to 150  MeV/c, the cross section is described by variational calculations using modern ³He wave functions. For missing momenta from 150 to 750  MeV/c, strong final-state interaction effects are observed. Near 1000  MeV/c, the experimental cross section is more than an order of magnitude larger than predicted by available theories. The A_TL asymmetry displays characteristic features of broken factorization with a structure that is similar to that generated by available models.

Results of the Jefferson Lab Hall A quasielastic ³He(e,e^′p)pn measurements are presented. These measurements were performed at fixed transferred momentum and energy, q=1502  MeV/c and ω=840  MeV, respectively, for missing momenta p_m up to 1  GeV/c and missing energies in the continuum region, up to pion threshold; this kinematic coverage is much more extensive than that of any previous experiment. The cross section data are presented along with the effective momentum density distribution and compared to theoretical models.

The ratio of the electric and magnetic form factors of the proton G_Ep/G_Mp, which is an image of its charge and magnetization distributions, was measured at the Thomas Jefferson National Accelerator Facility (JLab) using the recoil polarization technique. The ratio of the form factors is directly proportional to the ratio of the transverse to longitudinal components of the polarization of the recoil proton in the elastic e→p→ep→ reaction. The new data presented span the range 3.5<Q²<5.6 GeV² and are well described by a linear Q² fit. Also, the ratio sqrt[Q²] F_2p/F_1p reaches a constant value above Q²  =  2 GeV².