Search Results (21 total)

The present experiment exploits the interference between the deeply virtual Compton scattering (DVCS) and the Bethe-Heitler processes to extract the imaginary part of DVCS amplitudes on the neutron and on the deuteron from the helicity-dependent D(e→,e^′γ)X cross section measured at Q²=1.9  GeV² and x_B=0.36. We extract a linear combination of generalized parton distributions (GPDs) particularly sensitive to E_q, the least constrained GPD. A model dependent constraint on the contribution of the up and down quarks to the nucleon spin is deduced.

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.

An experiment measuring electroproduction of hypernuclei has been performed in hall A at Jefferson Lab on a ¹²C target. In order to increase counting rates and provide unambiguous kaon identification two superconducting septum magnets and a ring imaging Cherenkov detector were added to the hall A standard equipment. An unprecedented energy resolution of less than 700 keV FWHM has been achieved. Thus, the observed _Λ¹²B spectrum shows for the first time identifiable strength in the core-excited region between the ground-state s-wave Λ peak and the 11 MeV p-wave Λ peak.

A high-resolution (σ_instr.=1.5 MeV) search for narrow states (Γ<10 MeV) with masses of M_x≈1500–1850 MeV in ep→e^′K⁺X,e^′K^-X, and e^′π⁺X electroproduction at small angles and low Q² was performed. These states would be candidate partner states of the reported Θ⁺(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 0.8% and 4.9% of the Λ(1520) production cross section, depending on the channel and the assumed mass and width of the state.

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.

Cross-section values for Compton scattering on the proton were measured at 25 kinematic settings over the range s=5–11 and -t=2–7  GeV² with a statistical accuracy of a few percent. The scaling power for the s dependence of the cross section at fixed center-of-mass angle was found to be 8.0±0.2, strongly inconsistent with the prediction of perturbative QCD. The observed cross-section values are in fair agreement with the calculations using the handbag mechanism, in which the external photons couple to a single quark.

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 present the first measurements of the e→p→epγ cross section in the deeply virtual Compton scattering (DVCS) regime and the valence quark region. The Q² dependence (from 1.5 to 2.3  GeV²) of the helicity-dependent cross section indicates the twist-2 dominance of DVCS, proving that generalized parton distributions (GPDs) are accessible to experiment at moderate Q². The helicity-independent cross section is also measured at Q²=2.3  GeV². We present the first model-independent measurement of linear combinations of GPDs and GPD integrals up to the twist-3 approximation.

We present the first measurement of the Q² dependence of the neutron spin structure function g₂ⁿ at five kinematic points covering 0.57  (GeV/c)²≤Q²≤1.34  (GeV/c)² at x≃0.2. Though the naive quark-parton model predicts g₂=0, nonzero values occur in more realistic models of the nucleon which include quark-gluon correlations, finite quark masses, or orbital angular momentum. When scattering from a noninteracting quark, g₂ⁿ can be predicted using next-to-leading order fits to world data for g₁ⁿ. Deviations from this prediction provide an opportunity to examine QCD dynamics in nucleon structure. Our results show a positive deviation from this prediction at lower Q², indicating that contributions such as quark-gluon interactions may be important. Precision data obtained for g₁ⁿ are consistent with next-to-leading order fits to world data.

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.

Compton scattering from the proton was investigated at s=6.9  GeV² and t=-4.0  GeV² via polarization transfer from circularly polarized incident photons. The longitudinal and transverse components of the recoil proton polarization were measured. The results are in disagreement with a prediction of perturbative QCD based on a two-gluon exchange mechanism, but agree well with a prediction based on a reaction mechanism in which the photon interacts with a single quark carrying the spin of the proton.

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.

We report the results of a new Rosenbluth measurement of the proton electromagnetic form factors at Q² values of 2.64, 3.20, and 4.10  GeV². Cross sections were determined by detecting the recoiling proton, in contrast to previous measurements which detected the scattered electron. Cross sections were determined to 3%, with relative uncertainties below 1%. The ratio μ_pG_E/G_M was determined to 4%–8% and showed μ_pG_E/G_M≈1. These results are consistent with, and much more precise than, previous Rosenbluth extractions. They are inconsistent with recent polarization transfer measurements of similar precision, implying a systematic difference between the techniques.

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.

We have measured the proton recoil polarization in the ⁴He(e→ ,e^′p→)⁴H reaction at Q²=0.5, 1.0, 1.6, and 2.6   (GeV/c)². The measured ratio of polarization transfer coefficients differs from a fully relativistic calculation, favoring the inclusion of a medium modification of the proton form factors predicted by a quark-meson coupling model. In addition, the measured induced polarizations agree reasonably well with the fully relativistic calculation indicating that the treatment of final-state interactions is under control.

The first complete measurements of the angular distributions of the two-body deuteron photodisintegration differential cross section at photon energies above 1.6 GeV were performed at the Thomas Jefferson National Accelerator Facility. The results show a persistent forward-backward asymmetry up to E_γ=2.4 GeV, the highest-energy measured in this experiment. The Hard Rescattering and the Quark-Gluon string models are in fair agreement with the results.

We present measurements of the recoil proton polarization for the ¹H(γ→,p→)π⁰ reaction for θ_c.m.^π=60°–135° and for photon energies up to 4.1 GeV. These are the first data in this reaction for polarization transfer with circularly polarized photons. Various theoretical models are compared with the results. No evidence for hadron helicity conservation is observed. Models that employ factorization are not favored. It appears from the strong angular dependence of the induced polarization at photon energies of 2.5 and 3.1 GeV that a relatively high spin resonance or background amplitude might exist in this energy region.

The ²H(e,e^′p)n cross section was measured in Hall A of the Thomas Jefferson National Accelerator Facility near the top of the quasielastic peak (x_Bj=0.964) at a four-momentum transfer squared, Q²=0.665   (GeV/c)² (ω=0.368  GeV, W=2.057  GeV), and for recoil momenta up to 550  MeV/c. The measured cross section deviates by 1–2σ from a state-of-the-art calculation at low recoil momenta. At high recoil momenta the cross section is well described by the same calculation; however, in this region, final-state interactions and interaction currents are predicted to be large, and alternative choices of nucleon-nucleon potential and nucleon current operator may result in significant spread in the calculations.

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.

We present measurements of the ratio of the proton elastic electromagnetic form factors, μ_pG_Ep/G_Mp. The Jefferson Lab Hall A Focal Plane Polarimeter was used to determine the longitudinal and transverse components of the recoil proton polarization in ep elastic scattering; the ratio of these polarization components is proportional to the ratio of the two form factors. These data reproduce the observation of Jones et al. [Phys. Rev. Lett. 84, 1398 (2000)], that the form factor ratio decreases significantly from unity above Q²=1 GeV².

We present measurements of the recoil proton polarization for the d(γ→,p→)n reaction at θ_c.m.  =  90° for photon energies up to 2.4 GeV. These are the first data in this reaction for polarization transfer with circularly polarized photons. The induced polarization p_y vanishes above 1 GeV, contrary to meson-baryon model expectations, in which resonances lead to large polarizations. However, the polarization transfer C_x does not vanish above 1 GeV, inconsistent with hadron helicity conservation. Thus, we show that the scaling behavior observed in the d(γ,p)n cross sections is not a result of perturbative QCD. These data should provide important tests of new nonperturbative calculations in the intermediate energy regime.