Search Results (52 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.

Kaon electroproduction from light nuclei and hydrogen, using ¹H, ²H, ³He, ⁴He, and carbon targets has been measured at the Thomas Jefferson National Accelerator Facility. The quasifree angular distributions of Λ and Σ hyperons were determined at Q²=0.35 (GeV/c)² and W=1.91 GeV. Electroproduction on hydrogen was measured at the same kinematics for reference.

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 use the world's data on elastic electron-proton scattering and calculations of two-photon exchange effects to extract corrected values of the proton's electric and magnetic form factors over the full Q² range of the existing data. Our analysis combines the corrected Rosenbluth cross section and polarization transfer data, and is the first extraction of G_E and G_M including explicit two-photon exchange corrections and their associated uncertainties. In addition, we examine the angular dependence of the corrected cross sections to examine the possible nonlinearities of the cross section as a function of ɛ.

The extraction of the strangeness form factors from parity-violating elastic electron-proton scattering is sensitive to the electromagnetic form factors at low Q². We provide parametrizations for the form factors and uncertainties, including the effects of two-photon exchange corrections to the extracted electromagnetic form factors. We study effect of the correlations between different form factors, in particular as they impact the parity-violating asymmetry and the extraction of the strangeness form factors. We provide a prescription to extract the strangeness form factors from the asymmetry that provides an excellent approximation of the full two-photon correction. The corrected form factors are also appropriate as input for other low-Q analyses, although the effects of correlations and two-photon exchange corrections may be different.

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.

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.

The data analysis for the reaction ¹H(e,e^'π⁺)n, which was used to determine values for the charged pion form factor F_π for values of Q²= 0.6–1.6 GeV², has been repeated with careful inspection of all steps and special attention to systematic uncertainties. Also the method used to extract F_π from the measured longitudinal cross section was critically reconsidered. Final values for the separated longitudinal and transverse cross sections and the extracted values of F_π are presented.

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 report on a study of the longitudinal to transverse cross section ratio, R=σ_L/σ_T, at low values of x and Q², as determined from inclusive inelastic electron-hydrogen and electron-deuterium scattering data from Jefferson Laboratory Hall C spanning the four-momentum transfer range 0.06<Q²<2.8  GeV². Even at the lowest values of Q², R remains nearly constant and does not disappear with decreasing Q², as might be expected. We find a nearly identical behavior for hydrogen and deuterium.

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.

A large data set of charged-pion (π^±) electroproduction from both hydrogen and deuterium targets has been obtained spanning the low-energy residual-mass region. These data conclusively show the onset of the quark-hadron duality phenomenon, as predicted for high-energy hadron electroproduction. We construct several ratios from these data to exhibit the relation of this phenomenon to the high-energy factorization ansatz of electron-quark scattering and subsequent quark→pion production mechanisms.

The ¹H(e,e^′π⁺)n cross section was measured at four-momentum transfers of Q²=1.60 and 2.45  GeV² at an invariant mass of the photon nucleon system of W=2.22  GeV. The charged pion form factor (F_π) was extracted from the data by comparing the separated longitudinal pion electroproduction cross section to a Regge model prediction in which F_π is a free parameter. The results indicate that the pion form factor deviates from the charge-radius constrained monopole form at these values of Q² by one sigma, but is still far from its perturbative quantum chromodynamics prediction.

New nuclear structure function data from Jefferson Lab covering the higher-x and lower-Q² regime make it possible to extract the higher-order F₂ moments for iron and deuterium at low four-momentum transfer squared Q². These moments allow for an experimental investigation of the nuclear momentum sum rule and a direct comparison of the nonsinglet nucleon moment with lattice QCD results.

High energy lepton scattering has been the primary tool for mapping out the quark distributions of nucleons and nuclei. Data on the proton and deuteron have shown that there is a fundamental connection between the low and high energy regimes, referred to as quark-hadron duality. We present the results of similar studies to more carefully examine scaling, duality, and in particular the EMC effect in nuclei. We extract nuclear modifications to the structure function in the resonance region, and for the first time demonstrate that nuclear effects in the resonance region are identical to those measured in deep inelastic scattering. With the improved precision of the data at large x, we for the first time observe that the large-x crossover point appears to occur at lower x values in carbon than in iron or gold.

The effects of two-photon exchange corrections, suggested to explain the difference between measurements of the proton elastic electromagnetic form factors using the polarization transfer and Rosenbluth techniques, have been studied in elastic and inelastic scattering data. Such corrections could introduce ɛ-dependent nonlinearities in inelastic Rosenbluth separations, where ɛ is the virtual photon polarization parameter. It is concluded that such nonlinear effects are consistent with zero for elastic, resonance, and deep-inelastic scattering for all Q² and W² values measured.

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 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.

Significant discrepancies have been observed between proton form factors as measured by Rosenbluth separation and polarization transfer techniques. There are indications that this difference may be caused by corrections to the one-photon exchange approximation that are not taken into account in standard radiative correction procedures. In this paper, we constrain the two-photon amplitudes by combining data from Rosenbluth, polarization transfer, and positron-proton scattering measurements. This allows a rough extraction of these two-photon effects in elastic electron-proton scattering and provides an improved extraction of the proton electromagnetic form factors.

The _Λ^3,4H and _Λ⁴H hypernuclear bound states have been observed for the first time in kaon electroproduction on ^3,4He targets. The production cross sections have been determined at Q²=0.35   GeV² and W=1.91   GeV. For either hypernucleus the nuclear form factor is determined by comparing the angular distribution of the ^3,4He(e,e^′K⁺)_Λ^3,4H processes to the elementary cross section ¹H(e,éK⁺)Λ on the free proton, measured during the same experiment.

Electroproduction of the ω meson was investigated in the ¹H(e,e^′p)ω reaction. The measurement was performed at a four-momentum transfer Q²≈0.5  GeV². Angular distributions of the virtual photon-proton center-of-momentum cross sections have been extracted over the full angular range. These distributions exhibit a strong enhancement over t-channel parity exchange processes in the backward direction. According to a newly developed electroproduction model, this enhancement provides significant evidence of resonance formation in the γ*p→ωp reaction channel.

Recently, there has been a significant amount of activity to try and understand the discrepancy between Rosenbluth and polarization transfer measurements of the proton form factors. It has been suggested that the standard use of plane-wave Born approximation in extracting the form factors is insufficient, and that higher-order terms must also be included. Of the corrections missing in standard prescriptions, Coulomb distortion is the most well understood. In this paper, we examine the effect of Coulomb distortion on the extraction of the proton form factors.

We report on precision measurements of the elastic cross section for electron-proton scattering performed in Hall C at Jefferson Lab. The measurements were made at 28 distinct kinematic settings covering a range in momentum transfer of 0.4<Q²<5.5  (GeV∕c)². These measurements represent a significant contribution to the world’s cross section data set in the Q² range, where a large discrepancy currently exists between the ratio of electric to magnetic proton form factors extracted from previous cross section measurements and that recently measured via polarization transfer in Hall A at Jefferson Lab. This data set shows good agreement with previous cross section measurements, indicating that if a heretofore unknown systematic error does exist in the cross section measurements, then it is intrinsic to all such measurements.

The comparison of positron-proton and electron-proton elastic scattering cross sections is a sensitive test for the presence of two-photon exchange contributions. Thirty years ago, positron data were considered adequate to set tight limits on the size of two-photon corrections. More recently, these radiative corrections have again become a matter of great interest as a possible explanation for the discrepancy between Rosenbluth and polarization transfer measurements of the proton electromagnetic form factors. We have reexamined the electron and positron scattering data to see if they can accommodate two-photon effects of the size necessary to account for the Rosenbluth-polarization transfer discrepancy. The data are consistent with simple estimates of the two-photon contributions necessary to explain the discrepancy. In fact, they strongly favor a large ε-dependent correction to the positron to electron ratio, providing the first direct experimental evidence for a two-photon contribution to unpolarized lepton-proton scattering.