Search Results (28 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.

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.

The beam-recoil double polarization Px′h and Pz′h and the recoil polarization Py′ were measured for the first time for the p(e→,e′p→)η reaction at a four-momentum transfer of Q2=0.1  GeV2/c2 and a center of mass production angle of θ=120° at the Mainz Microtron MAMI-C. With a center of mass energy range of 1500  MeV<W<1550  MeV the region of the S11(1535) and D13(1520) resonance was covered. The results are discussed in the framework of a phenomenological isobar model (Eta-MAID). While Px′h and Pz′h are in good agreement with the model, Py′ shows a significant deviation, consistent with existing photoproduction data on the polarized-target asymmetry.

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.

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 report the first precision measurement of the proton electric to magnetic form factor ratio from spin-dependent elastic scattering of longitudinally polarized electrons from a polarized hydrogen internal gas target. The measurement was performed at the MIT-Bates South Hall Ring over a range of four-momentum transfer squared Q² from 0.15 to 0.65  (GeV/c)². Significantly improved results on the proton electric and magnetic form factors are obtained in combination with existing cross-section data on elastic electron-proton scattering in the same Q² region.

We report new measurements of the parity-violating asymmetry A_PV in elastic scattering of 3 GeV electrons off hydrogen and ⁴He targets with ⟨θ_lab⟩≈6.0°. The ⁴He result is A_PV=(+6.40±0.23(stat)±0.12(syst))×10^-6. The hydrogen result is A_PV=(-1.58±0.12(stat)±0.04(syst))×10^-6. These results significantly improve constraints on the electric and magnetic strange form factors G_E^s and G_M^s. We extract G_E^s=0.002±0.014±0.007 at ⟨Q²⟩=0.077  GeV², and G_E^s+0.09G_M^s=0.007±0.011±0.006 at ⟨Q²⟩=0.109  GeV², providing new limits on the role of strange quarks in the nucleon charge and magnetization distributions.

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.

The mean square polarizability radii of the proton have been measured for the first time in a virtual-Compton-scattering experiment performed at the MIT-Bates out-of-plane scattering facility. Response functions and polarizabilities obtained from a dispersion analysis of the data at Q²=0.057  GeV²/c² are in agreement with O(p³) heavy baryon chiral perturbation theory. The data support the dominance of mesonic effects in the polarizabilities.

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.

The differential cross sections for the γn→π^-p and the γp→π⁺n processes were measured at Jefferson Lab. The photon energies ranged from 1.1 to 5.5 GeV, corresponding to center-of-mass energies from 1.7 to 3.4 GeV. The pion center-of-mass angles varied from 50^° to 110^°. The π^- and π⁺ photoproduction data both exhibit a global scaling behavior at high energies and high transverse momenta, consistent with the constituent counting rule prediction and the existing π⁺ data. The data suggest possible substructure of the scaling behavior, which might be oscillations around the scaling value. The data show an enhancement in the scaled cross section at center-of-mass energy near 2.2 GeV. The differential cross section ratios [dσ/dt(γn→π^-p)/dσ/dt(γp→π⁺n)] at high energies and high transverse momenta can be described by calculations based on one-hard-gluon-exchange diagrams.

We report new precise H(e,e^′p)π⁰ measurements at the Δ(1232) resonance at Q²=0.127  (GeV/c)² obtained at the MIT-Bates out-of-plane scattering facility which are particularly sensitive to the transverse electric amplitude (E2) of the γ^*N→Δ transition. The new data have been analyzed together with those of earlier measurements to yield precise quadrupole to dipole amplitude ratios: Re(E₁₊^3/2/M₁₊^3/2)=(-2.3±0.3_stat+syst±0.6_model)% and Re(S₁₊^3/2/M₁₊^3/2)=(-6.1±0.2_stat+syst±0.5_model)% for M₁₊^3/2=(41.4±0.3_stat+syst±0.4_model)(10^-3/m_π⁺). The derived amplitudes give credence to the conjecture of deformation in hadrons favoring, at low Q², the dominance of mesonic effects.

We report on measurements of the neutron spin asymmetries A_1,2ⁿ and polarized structure functions g_1,2ⁿ at three kinematics in the deep inelastic region, with x=0.33, 0.47, and 0.60 and Q²=2.7, 3.5, and 4.8  (GeV∕c)², respectively. These measurements were performed using a 5.7  GeV longitudinally polarized electron beam and a polarized ³He target. The results for A₁ⁿ and g₁ⁿ at x=0.33 are consistent with previous world data and, at the two higher-x points, have improved the precision of the world data by about an order of magnitude. The new A₁ⁿ data show a zero crossing around x=0.47 and the value at x=0.60 is significantly positive. These results agree with a next-to-leading-order QCD analysis of previous world data. The trend of data at high x agrees with constituent quark model predictions but disagrees with that from leading-order perturbative QCD (PQCD) assuming hadron helicity conservation. Results for A₂ⁿ and g₂ⁿ have a precision comparable to the best world data in this kinematic region. Combined with previous world data, the moment d₂ⁿ was evaluated and the new result has improved the precision of this quantity by about a factor of 2. When combined with the world proton data, polarized quark distribution functions were extracted from the new g₁ⁿ∕F₁ⁿ values based on the quark-parton model. While results for Δu∕u agree well with predictions from various models, results for Δd∕d disagree with the leading-order PQCD prediction when hadron helicity conservation is imposed.

New, high-precision measurements of the ³He(e,e^′p) reaction using the A1 Collaboration spectrometers at the Mainz microtron MAMI are presented. These were performed in antiparallel kinematics at energy transfers below the quasielastic peak, and at a central momentum transfer of 685 MeV/c. Cross sections and distorted momentum distributions were extracted and compared to theoretical predictions and existing data. The longitudinal and transverse behavior of the cross section was also studied. Sizable differences in the cross-section behavior from theoretical predictions based on the plane wave impulse approximation were observed in both the two- and three-body breakup channels. Full Faddeev-type calculations account for some of the observed excess cross-section, but significant differences remain.

We have measured the neutron spin asymmetry A₁ⁿ with high precision at three kinematics in the deep inelastic region at x=0.33, 0.47, and 0.60, and Q²=2.7, 3.5, and 4.8   (GeV/c)², respectively. Our results unambiguously show, for the first time, that A₁ⁿ crosses zero around x=0.47 and becomes significantly positive at x=0.60. Combined with the world proton data, polarized quark distributions were extracted. Our results, in general, agree with relativistic constituent quark models and with perturbative quantum chromodynamics (PQCD) analyses based on the earlier data. However they deviate from PQCD predictions based on hadron helicity conservation.

We have measured the nuclear transparency of the fundamental process γn→π^-p in ⁴He. These measurements were performed at Jefferson Lab in the photon energy range of 1.6–4.5 GeV and at θ_cm^π=70° and 90°. 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.

We have measured the differential cross section for the γn→π^-p and γp→π⁺n reactions at θ_c.m.=90° in the photon energy range from 1.1 to 5.5 GeV at Jefferson Lab (JLab). The data at E_γ≳3.3   GeV exhibit a global scaling behavior for both π^- and π⁺ photoproduction, consistent with the constituent counting rule and the existing π⁺ photoproduction data. Possible oscillations around the scaling value are suggested by these new data. The data show enhancement in the scaled cross section at a center-of-mass energy near 2.2 GeV. The cross section ratio of exclusive π^- to π⁺ photoproduction at high energy is consistent with the prediction based on one-hard-gluon-exchange diagrams.

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.

New data are presented on the p(e,e^′p)π⁰ reaction at threshold at a four-momentum transfer of Q²  =  0.05 GeV²/c². The data were taken with the three-spectrometer setup of the A1 Collaboration at the Mainz Microtron MAMI. The complete center of mass solid angle was covered up to a center of mass energy of 4 MeV above threshold. Combined with measurements at three different values of the virtual photon polarization ε, the structure functions σ_T, σ_L, σ_TT, and σ_TL are determined. The results are compared with calculations in heavy baryon chiral perturbation theory and with a phenomenological model. The measured cross section is significantly smaller than both predictions.

We recently used a prototype rf dipole magnet to study the spin flipping of a 669 MeV horizontally polarized electron beam stored in the presence of a nearly full Siberian snake in the new MIT-Bates storage ring. We flipped the spin by ramping the rf dipole's frequency through an rf-induced depolarizing resonance. After optimizing the frequency ramp parameters, we used multiple spin flipping to measure a spin-flip efficiency of 94.5±2.5%. The spin-flip efficiency was apparently limited by the field strength in the air-core prototype rf dipole magnet. This unexpectedly high efficiency indicates that very efficient spin flipping of the ring's stored polarized electron beam should be possible using the much stronger ferrite spin flipper, which is now being built by the University of Michigan's Spin Physics Center.

The recoil proton polarization has been measured in the p(e→,e^′p→)π⁰ reaction in parallel kinematics around W  =  1232  MeV, Q²  =  0.121  (GeV/c)², and ε  =  0.718 using the polarized cw electron beam of the Mainz Microtron. All three proton polarization components, P_x/P_e  =  (-11.4±1.3±1.4)%, P_y  =  (-43.1±1.3±2.2)%, and P_z/P_e  =  (56.2±1.5±2.6)%, could be measured simultaneously. The Coulomb quadrupole to magnetic dipole ratio, CMR  =  (-6.4±0.7_stat±0.8_syst)%, was determined from P_x in the framework of the Mainz Unitary Isobar Model. The consistency among the reduced polarizations and the extraction of the ratio of longitudinal-to-transverse response is discussed.