Search Results (52 total)

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.

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 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.

Cross sections were measured for the near-threshold electrodisintegration of ³He at momentum transfer values of q=2.4, 4.4, and 4.7 fm^-1. From these and prior measurements the transverse and longitudinal response functions R_T and R_L were deduced. Comparisons are made against previously published and new nonrelativistic A=3 calculations using the best available nucleon-nucleon NN potentials. In general, for q<2 fm^-1 these calculations accurately predict the threshold electrodisintegration of ³He. Agreement at increasing q demands consideration of two-body terms, but discrepancies still appear at the highest momentum transfers probed, perhaps due to the neglect of relativistic dynamics, or to the underestimation of high-momentum wave-function components.

Quadrupole amplitudes in the γ^*N→Δ transition are associated with the issue of nucleon deformation. A search for these small amplitudes has been the focus of a series of measurements undertaken at Bates/MIT by the OOPS Collaboration. We report on results from H(e,e^′p)π⁰ data obtained at Q²=0.070 (GeV/c)² and invariant mass of W=1155 MeV using the out-of-plane detection technique with the OOPS spectrometers. The σ_LT and σ_T+εσ_L response functions were isolated. These results, along with those of previous measurements at W=1172 MeV and Q²=0.127 (GeV/c)², aim in elucidating the interplay between resonant and nonresonant amplitudes.

Measurements of the ²H(e→,e^′p)n reaction were performed with the out-of-plane magnetic spectrometers (OOPS) at the MIT-Bates Linear Accelerator. The longitudinal-transverse, f_LT and f_LT^′, and the transverse-transverse, f_TT, interference responses at a missing momentum of 210 MeV/c were simultaneously extracted in the dip region at Q²  =  0.15  (GeV/c)². In comparison to models of deuteron electrodisintegration, the data clearly reveal strong effects of relativity and final-state interactions and the importance of two-body meson-exchange currents and isobar configurations. We demonstrate that such effects can be disentangled by extracting these responses using the novel out-of-plane technique.

New electron scattering measurements have been made that extend data on the ³He elastic magnetic form factor up to Q²  =  42.6 fm^-2. These new data test theoretical conjectures regarding non-nucleonic effects in the three-body system. The very small cross sections, as low as 10^-40 cm²/sr, required the use of a high-pressure cryogenic gas target and a detector system with excellent background rejection capability. No existing theoretical calculation satisfactorily accounts for all the available data.

High-precision ¹H(e,e^′p)π⁰ measurements at Q²  =  0.126 (GeV/c)² are reported, which allow the determination of quadrupole amplitudes in the γ^*N→Δ transition; they simultaneously test the reliability of electroproduction models. The derived quadrupole-to-dipole ( I  =  3/2) amplitude ratios, R_SM  =  (-6.5±0.2_stat+sys±2.5_mod)% and R_EM  =  (-2.1±0.2_stat+sys±2.0_mod)%, are dominated by model error. Previous R_SM and R_EM results should be reconsidered after the model uncertainties associated with the method of their extraction are taken into account.

Tensor polarization observables ( t₂₀, t₂₁, and t₂₂) have been measured in elastic electron-deuteron scattering for six values of momentum transfer between 0.66 and 1.7 (GeV/c)². The experiment was performed at the Jefferson Laboratory in Hall C using the electron High Momentum Spectrometer, a specially designed deuteron magnetic channel and the recoil deuteron polarimeter POLDER. The new data determine to much larger Q² the deuteron charge form factors G_C and G_Q. They are in good agreement with relativistic calculations and disagree with perturbative QCD predictions.

The first measurements of f_LT^′, known as the fifth response function, have been made for the ²H(e→,e^′p) and ¹²C(e→,e^′p) reactions. This response is directly related to the imaginary part of the interference between the transverse and longitudinal nuclear electromagnetic currents. Its observation requires longitudinally polarized electron beams and out-of-plane detection, the latter made possible by the newly developed out-of-plane spectrometer system. The initial measurements were made by using a 560-MeV polarized electron beam and quasielastic kinematics at Q²=3.3 fm^-2. The development of the methodology for out-of-plane physics, and the analysis of the data from the initial experiments are described in detail. The measured fifth response and the related asymmetry in the coincidence cross section are in agreement, albeit with large statistical errors, with the theoretical predictions. Future extensions of the out-of-plane program are also discussed.

The A(Q²) structure function in elastic electron-deuteron scattering was measured at six momentum transfers Q² between 0.66 and 1.80 (GeV/c)² in Hall C at Jefferson Laboratory. The scattered electrons and recoil deuterons were detected in coincidence, at a fixed deuteron angle of 60.5°. These new precise measurements resolve discrepancies between older sets of data. They put significant constraints on existing models of the deuteron electromagnetic structure, and on the strength of isoscalar meson exchange currents.

We present a measurement of the induced proton polarization P_n in π⁰ electroproduction on the proton around the Δ resonance. The measurement was made at a central invariant mass and a squared four-momentum transfer of W=1231 MeV and Q²=0.126 GeV²/c², respectively. We measured a large induced polarization, P_n=-0.397±0.055±0.009. The data suggest that the scalar background is larger than expected from a recent effective Hamiltonian model.

The coincidence cross section and the interference structure function, R_LT, were measured for the ¹²C(e,e^′p) ¹¹B reaction at quasielastic kinematics and central momentum transfer of |q→|=400 MeV/c. The measurement was at an opening angle of θ_pq=11°, covering a range in missing energy of E_m=0 to 65 MeV. The R_LT structure function is found to be consistent with zero for E_m>50 MeV, confirming an earlier study which indicated that R_L vanishes in this region. The integrated strengths of the p- and s-shell are compared with a distorted wave impulse approximation (DWIA) calculation. The s-shell strength and shape are also compared with a Hartree Fock–random phase approximation (HF-RPA) calculation. The DWIA calculation does not succeed in giving a consistent description of both the cross section data and the extracted R_LT response for either shell. The HF-RPA calculation describes the data more consistently, which may be due to the inclusion of 2-body currents in this calculation.

The first measurements of the induced proton polarization P_n for the ¹²C(e,e^′p→) reaction are reported. The experiment was performed at quasifree kinematics for energy and momentum transfer (ω,q)≈ (294 MeV, 756 MeV/c) and sampled a missing momentum range of 0–250 MeV/c. The induced polarization arises from final-state interactions and for these kinematics is dominated by the real part of the spin-orbit optical potential. The distorted-wave impulse approximation provides good agreement with data for the 1p_3/2 shell. The data for the continuum suggest that both the 1s_1/2 shell and underlying ℓ>1 configurations contribute.

Recoil proton polarization observables were measured for both the p(e→,e^′p→) and d(e→,e^′p→)n reactions at two values of Q² using a newly commissioned proton focal plane polarimeter at the MIT-Bates Linear Accelerator Center. The hydrogen and deuterium spin-dependent observables D_ℓℓ and D_ℓt, the induced polarization P_n, and the form factor ratio G_E^p/G_M^p were measured under identical kinematics. The deuterium and hydrogen results are in good agreement with each other and with the plane-wave impulse approximation (PWIA).

Absolute differential cross sections for the threshold electrodisintegration of the deuteron with good resolution were measured at a laboratory scattering angle of 160° for five values of Q² ranging from 8.66 to 42.4 fm^-2. Comparisons of the data averaged over E_np from 0 to 3 MeV and from 0 to10 MeV are made with nonrelativistic meson exchange calculations. These calculations are sensitive to the nucleon electromagnetic form factors, nucleon-nucleon potential, and relativistic effects. The data are also compared with a hybrid quark-hadron model calculation that describes the deuteron as a six-quark cluster for the short range part of the interaction. Some of these calculations can describe the data reasonably well over certain ranges of Q²; however, none of these calculations can accurately describe the data over the entire measured Q² range.

We have separated the longitudinal ( f₀₀), transverse ( f₁₁), and longitudinal-transverse interference ( f₀₁) structure functions in the ²H(e,e^′p)n reaction at |q→|≈400 MeV/c and ω≈110 MeV. A nonrelativistic calculation which includes effects due to final state interactions, meson exchange currents, and isobar configurations agrees with the measured f₁₁ and f₀₁ but overpredicts f₀₀ by 25% ( 2σ). The data are also compared to the results of previous structure function measurements.

The reaction ³He(e,e^′d)p was measured at four-momentum transfer q  =  420 MeV, for proton recoil momentum P_r  =  22, 54, and 62 MeV/c, in parallel deuteron kinematics. Longitudinal (L) and transverse (T) response functions were obtained by means of Rosenbluth separations. The separated response functions are, respectively, found to be dominated by the contributions of isoscalar and isovector two-body currents. The L/T ratios are about 3.5. This is an order of magnitude lower than that for elastic scattering on a free deuteron, or what would be expected for a purely isoscalar transition, but approximately agrees with what is theoretically expected for isoscalar plus isovector contributions.

We have measured the coincidence d(e→,e′p) reaction in quasielastic scattering, detecting the proton in a noncoplanar geometry. The electron helicity asymmetry A_e and the imaginary part of the longitudinal-transverse interference structure function f_LT^′ have been determined at a four-momentum transfer Q²=3.3 fm^-2. The results are compared with theoretical calculations which use realistic potentials for the NN interaction.

We have measured a new electron scattering observable, the “fifth” structure function, f₀₁^′, which is the imaginary part of the transverse-longitudinal interference response. Its observation requires a longitudinally polarized beam and coincident, out-of-plane particle detection. f₀₁^′ arises from interference between reaction channels and provides an additional means for their disentanglement. In the quasielastic C¹²(e→,e′p) measurements reported here, f₀₁^′ is driven by the interference of the direct knockout and rescattering amplitudes.

The tensor polarization of the recoil deuteron in elastic electron-deuteron scattering has been measured at the Bates Linear Accelerator Center at three values of four-momentum transfer Q=3.78, 4.22, and 4.62 fm^-1, corresponding to incident electron energies of 653, 755, and 853 MeV. The scattered electrons and the recoil deuterons were detected in coincidence. The recoil deuterons were transported to a liquid hydrogen target to undergo a second scattering. The angular distribution of the d→-p scattering was measured using a polarimeter. The polarimeter was calibrated in an auxiliary experiment using a polarized deuteron beam at the Laboratoire National Saturne. A Monte Carlo procedure was used to generate interpolated calibration data because the energy spread in the deuteron energies in the Bates experiment spanned the range of deuteron energies in the calibration experiment. The extracted values of t₂₀ are compared to predictions of different theoretical models of the electromagnetic form factors of the deuteron: nonrelativistic and relativistic nucleon-meson dynamics, Skyrme model, quark models, and perturbative quantum chromodynamics. Along with the world data the structure functions A(Q) and B(Q) are used to separate the charge monopole and charge quadrupole form factors of the deuteron. A node in the charge monopole form factor is observed at Q=4.39±0.16 fm^-1.

Inclusive inelastic electron scattering cross sections for ³H and ³He were measured for excitation energies below 18 MeV. Longitudinal and transverse response functions were determined from these cross sections for six values of the three-momentum transfer q in the range 0.88 <q<2.87 fm^-1. A previously observed C0 multipole enhancement near threshold in the two-body channel for ³He is confirmed but is not observed for ³H. The experimental data are found to be in good agreement with two recent calculations of the longitudinal and transverse response functions. The first uses variational ground-state wave functions and the orthogonal correlated states method to describe the two- and three-body breakup channels. The second uses bound and continuum Faddeev wave functions obtained for a simple central potential. Agreement with the data is good for the first model and better for the second. The inclusion of final-state interactions (FSI) in the Faddeev continuum is found to be very important in these threshold breakup kinematics; in many cases inclusion of FSI changes the response functions by factors of two or three giving excellent agreement with the data. The transverse response functions are well reproduced, even though neither model includes meson exchange currents. Ratios of the response functions for the two nuclei are also well described.

The ²H(e,e’n)¹H quasielastic cross section was measured at Q² values of 0.109, 0.176, and 0.255 (GeV/c)². The neutron detection efficiency was determined by the associated particle technique with the ²H(γ,pn) reaction for each of the three neutron kinetic energies. These ²H(e,e’n) measurements of the coincidence cross sections are the first at low Q². The cross sections are sensitive primarily to the neutron magnetic form factor G_Mⁿ at these kinematics. The extracted G_Mⁿ values have smaller uncertainties than previous data and are consistent with the dipole parametrization at the two higher momentum transfers; at the lowest momentum transfer, the value of G_Mⁿ is ∼10% higher than the dipole value.

The threshold electrodisintegration of the deuteron was measured with good energy resolution to a maximum four-momentum transfer squared of 42 fm^-2. The data are compared with meson exchange calculations that show sensitivity to the choice of exchange-current form factor, the nucleon electromagnetic form factors, and the nucleon-nucleon potential. While conventional theories can give a reasonable description of the data, comparison of the data with quark cluster calculations shows poor agreement.