Search Results (33 total)

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.

The reaction γp→pK⁺K^- was studied at Jefferson Lab with photon energies from 1.8 to 3.8 GeV using a tagged photon beam. The goal was to search for a Θ⁺⁺ pentaquark, a narrow, doubly charged baryon state having strangeness S=+1 and isospin I=1, in the pK⁺ invariant mass spectrum. No statistically significant evidence of a Θ⁺⁺ was found. Upper limits on the total and differential cross section for the reaction γp→K^-Θ⁺⁺ were obtained in the mass range from 1.5 to 2.0  GeV/c², with an upper limit for a narrow resonance with a mass M_Θ⁺⁺=1.54  GeV/c² of about 0.15 nb, 95% C.L.. This result places a stringent upper limit on the Θ⁺⁺ width Γ_Θ⁺⁺<0.1  MeV/c².

The exclusive reactions γp→K̅ ⁰K⁺n and γp→K̅ ⁰K⁰p have been studied in the photon energy range 1.6–3.8 GeV, searching for evidence of the exotic baryon Θ⁺(1540) in the decays Θ⁺→nK⁺ and Θ⁺→pK⁰. Data were collected with the CLAS detector at the Thomas Jefferson National Accelerator Facility. The integrated luminosity was about 70  pb^-1. The reactions have been isolated by detecting the K⁺ and proton directly, the neutral kaon via its decay to K_S→π⁺π^- and the neutron or neutral kaon via the missing mass technique. The mass and width of known hyperons such as Σ⁺, Σ^- and Λ(1116) were used as a check of the mass determination accuracy and experimental resolution. Approximately 100 000 Λ^*(1520)’s and 150 000 ϕ’s were observed in the K̅ ⁰K⁺n and K̅ ⁰K⁰p final state, respectively. No evidence for the Θ⁺ pentaquark was found in the nK⁺ or pK_S invariant mass spectra. Upper limits were set on the production cross section of the reaction γp→K̅ ⁰Θ⁺ as functions of center-of-mass angle, nK⁺ and pK_S masses. Combining the results of the two reactions, the 95% C.L. upper limit on the total cross section for a resonance peaked at 1540 MeV was found to be 0.7 nb. Within most of the available theoretical models, this corresponds to an upper limit on the Θ⁺ width, Γ_Θ⁺, ranging between 0.01 and 7 MeV.

A search for the Θ⁺ in the reaction γd→pK^-K⁺n was completed using the CLAS detector at Jefferson Lab. A study of the same reaction, published earlier, reported the observation of a narrow Θ⁺ resonance. The present experiment, with more than 30 times the integrated luminosity of our earlier measurement, does not show any evidence for a narrow pentaquark resonance. The angle-integrated upper limit on Θ⁺ production in the mass range of 1.52–1.56  GeV/c² for the γd→pK^-Θ⁺ reaction is 0.3 nb (95% C.L.). This upper limit depends on assumptions made for the mass and angular distribution of Θ⁺ production. Using Λ(1520) production as an empirical measure of rescattering in the deuteron, the cross section upper limit for the elementary γn→K^-Θ⁺ reaction is estimated to be a factor of 10 higher, i.e., ∼3  nb (95% C.L.).

A pioneering experiment in Λ hypernuclear spectroscopy, undertaken at the Thomas Jefferson National Accelerator Facility (JLab), was recently reported. The experiment used the high precision, continuous electron beam at JLab, and a special arrangement of spectrometer magnets to measure the hypernuclear spectrum from C and ⁷Li targets using the (e,e'K⁺) reaction. The _Λ¹²B spectrum found in this investigation was previously published, but is reported here in more detail, with improved resolution. In addition, the results of a _Λ⁷He spectrum also obtained in the experiment, are shown. This latter spectrum indicates the need for a more detailed few-body calculation of the hypernucleus and the reaction process. The success of the experiment demonstrates the potential of the (e,e'K⁺) reaction for high resolution spectroscopy of hypernuclear spectra.

We report values for the neutron electric to magnetic form factor ratio, G_En/G_Mn, deduced from measurements of the neutron's recoil polarization in the quasielastic ²H(e→,e^'n→)¹H reaction, at three Q² values of 0.45, 1.13, and 1.45 (GeV/c)². The data at Q²=1.13 and 1.45 (GeV/c)² are the first direct experimental measurements of G_En employing polarization degrees of freedom in the Q²>1 (GeV/c)² region and stand as the most precise determinations of G_En for all values of Q².

The exclusive reaction γp→K̅ ⁰K⁺n was studied in the photon energy range between 1.6 and 3.8 GeV searching for evidence of the exotic baryon Θ⁺(1540)→nK⁺. The decay to nK⁺ requires the assignment of strangeness S=+1 to any observed resonance. Data were collected with the CLAS detector at the Thomas Jefferson National Accelerator Facility corresponding to an integrated luminosity of 70  pb^-1. No evidence for the Θ⁺ pentaquark was found. Upper limits were set on the production cross section as function of center-of-mass angle and nK⁺ mass. The 95% C.L. upper limit on the total cross section for a narrow resonance at 1540 MeV was found to be 0.8 nb.

We studied the reaction ¹²C(e,e^'p) in quasielastic kinematics at momentum transfers between 0.6 and 1.8 (GeV/c)² covering the single-particle region. From this the nuclear transparency factors are extracted using two methods. The results are compared to theoretical predictions obtained using a generalization of Glauber theory described in this paper. Furthermore, the momentum distribution in the region of the 1s-state up to momenta of 300 MeV/c is obtained from the data and compared to the correlated basis function theory and the independent-particle shell model.

This paper was published online on 20 May 2005 without several of the authors’ corrections incorporated. Equation (13) has been replaced. The captions of Figs. 16–18 have also been replaced. Typographical errors on pages 4, 6, 14, 15, 18, 19, 22, and 24 have all been corrected. The paper has been corrected as of 8 June 2005. The text is correct in the printed version of the journal.

The ratio of the proton elastic electromagnetic form factors, G_Ep/G_Mp, was obtained by measuring P_t and P_ℓ, the transverse and longitudinal recoil proton polarization components, respectively, for the elastic e→p→ep→reaction in the four-momentum transfer squared range of 0.5 to 3.5  GeV². In the single-photon exchange approximation, G_Ep/G_Mp is directly proportional to P_t/P_ℓ. The simultaneous measurement of P_t and P_ℓ in a polarimeter reduces systematic uncertainties. The results for G_Ep/G_Mp show a systematic decrease with increasing Q², indicating for the first time a definite difference in the distribution of charge and magnetization in the proton. The data have been reanalyzed and their systematic uncertainties have become significantly smaller than those reported previously.

We have carried out an (e,e^′p) experiment at high momentum transfer and in parallel kinematics to measure the strength of the nuclear spectral function S(k,E) at high nucleon momenta k and large removal energies E. This strength is related to the presence of short-range and tensor correlations, and was known hitherto only indirectly and with considerable uncertainty from the lack of strength in the independent-particle region. This experiment locates by direct measurement the correlated strength predicted by theory.

The generalized forward spin polarizabilities γ₀ and δ_LT of the neutron have been extracted for the first time in a Q² range from 0.1 to 0.9  GeV². Since γ₀ is sensitive to nucleon resonances and δ_LT is insensitive to the Δ resonance, it is expected that the pair of forward spin polarizabilities should provide benchmark tests of the current understanding of the chiral dynamics of QCD. The new results on δ_LT show significant disagreement with chiral perturbation theory calculations, while the data for γ₀ at low Q² are in good agreement with a next-to-leading-order relativistic baryon chiral perturbation theory calculation. The data show good agreement with the phenomenological MAID model.

The physics program in Hall A at Jefferson Lab commenced in the summer of 1997 with a detailed investigation of the ¹⁶O(e,e^′p) reaction in quasielastic, constant (q,ω) kinematics at Q²≈0.8  (GeV∕c)², q≈1  GeV∕c, and ω≈445  MeV. Use of a self-calibrating, self-normalizing, thin-film waterfall target enabled a systematically rigorous measurement. Five-fold differential cross-section data for the removal of protons from the 1p-shell have been obtained for 0<p_miss<350  MeV∕c. Six-fold differential cross-section data for 0<E_miss<120  MeV were obtained for 0<p_miss<340  MeV∕c. These results have been used to extract the A_LT asymmetry and the R_L, R_T, R_LT, and R_L+TT effective response functions over a large range of E_miss and p_miss. Detailed comparisons of the 1p-shell data with Relativistic Distorted-Wave Impulse Approximation (RDWIA), Relativistic Optical-Model Eikonal Approximation (ROMEA), and Relativistic Multiple-Scattering Glauber Approximation (RMSGA) calculations indicate that two-body currents stemming from meson-exchange currents (MEC) and isobar currents (IC) are not needed to explain the data at this Q². Further, dynamical relativistic effects are strongly indicated by the observed structure in A_LT at p_miss≈300  MeV∕c. For 25<E_miss<50  MeV and p_miss≈50  MeV∕c, proton knockout from the 1s_1∕2-state dominates, and ROMEA calculations do an excellent job of explaining the data. However, as p_miss increases, the single-particle behavior of the reaction is increasingly hidden by more complicated processes, and for 280<p_miss<340  MeV∕c, ROMEA calculations together with two-body currents stemming from MEC and IC account for the shape and transverse nature of the data, but only about half the magnitude of the measured cross section. For 50<E_miss<120  MeV and 145<p_miss<340  MeV∕c, (e,e^′pN) calculations which include the contributions of central and tensor correlations (two-nucleon correlations) together with MEC and IC (two-nucleon currents) account for only about half of the measured cross section. The kinematic consistency of the 1p-shell normalization factors extracted from these data with respect to all available ¹⁶O(e,e^′p) data is also examined in detail. Finally, the Q²-dependence of the normalization factors is discussed.

We report a virtual Compton scattering study of the proton at low c.m. energies. We have determined the structure functions P_LL-P_TT/ϵ and P_LT, and the electric and magnetic generalized polarizabilities (GPs) α_E(Q²) and β_M(Q²) at momentum transfer Q²=0.92 and 1.76  GeV². The electric GP shows a strong falloff with Q², and its global behavior does not follow a simple dipole form. The magnetic GP shows a rise and then a falloff; this can be interpreted as the dominance of a long-distance diamagnetic pion cloud at low Q², compensated at higher Q² by a paramagnetic contribution from πN intermediate states.

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.

We have measured the parity-violating electroweak asymmetry in the elastic scattering of polarized electrons from protons. Significant contributions to this asymmetry could arise from the contributions of strange form factors in the nucleon. The measured asymmetry is A=−15.05±0.98(stat)±0.56(syst)  ppm at the kinematic point ⟨θ_lab⟩=12.3° and ⟨Q²⟩=0.477  (GeV∕c)². Based on these data as well as data on electromagnetic form factors, we extract the linear combination of strange form factors G_E^s+0.392G_M^s=0.014±0.020±0.010, where the first error arises from this experiment and the second arises from the electromagnetic form factor data. This paper provides a full description of the special experimental techniques employed for precisely measuring the small asymmetry, including the first use of a strained GaAs crystal and a laser-Compton polarimeter in a fixed target parity-violation experiment.

Exclusive electroproduction of π⁰ mesons on protons in the backward hemisphere has been studied at Q²=1.0  GeV² by detecting protons in the forward direction in coincidence with scattered electrons from the 4  GeV electron beam in Jefferson Lab’s Hall A. The data span the range of the total (γ*p) center-of-mass energy W from the pion production threshold to W=2.0  GeV. The differential cross sections σ_T+ϵσ_L, σ_TL, and σ_TT were separated from the azimuthal distribution and are presented together with the MAID and SAID parametrizations.

The electric form factor of the neutron was determined from measurements of the d→(e→,e^′n)p reaction for quasielastic kinematics. Polarized electrons were scattered off a polarized deuterated ammonia (¹⁵ND₃) target in which the deuteron polarization was perpendicular to the momentum transfer. The scattered electrons were detected in a magnetic spectrometer in coincidence with neutrons in a large solid angle detector. We find G_Eⁿ=0.0526±0.0033(stat)±0.0026(sys) and 0.0454±0.0054±0.0037 at Q²=0.5 and 1.0  (GeV/c)², respectively.

We have measured the spin structure functions g₁ and g₂ of ³He in a double-spin experiment by inclusively scattering polarized electrons at energies ranging from 0.862 to 5.058 GeV off a polarized ³He target at a 15.5° scattering angle. Excitation energies covered the resonance and the onset of the deep inelastic regions. We have determined for the first time the Q² evolution of Γ₁(Q²)=∫₀¹g₁(x,Q²)dx, Γ₂(Q²)=∫₀¹g₂(x,Q²)dx, and d₂(Q²)=∫₀¹x²[2g₁(x,Q²)+3g₂(x,Q²)]dx for the neutron in the range 0.1≤Q²≤0.9  GeV² with good precision. Γ₁(Q²) displays a smooth variation from high to low Q². The Burkhardt-Cottingham sum rule holds within uncertainties and d₂ is nonzero over the measured range.

We report new measurements of the ratio of the electric form factor to the magnetic form factor of the neutron, G_Eⁿ/G_Mⁿ, obtained via recoil polarimetry from the quasielastic ²H(e→,e^′n→)¹H reaction at Q² values of 0.45, 1.13, and 1.45   (GeV/c)² with relative statistical uncertainties of 7.6% and 8.4% at the two higher Q² points, which points have never been achieved in polarization measurements.

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.

High-energy, cw electron beams at new accelerator facilities allow electromagnetic production and precision study of hypernuclear structure, and we report here on the first experiment demonstrating the potential of the (e,e^′K⁺) reaction for hypernuclear spectroscopy. This experiment is also the first to take advantage of the enhanced virtual photon flux available when electrons are scattered at approximately zero degrees. The observed energy resolution was found to be ≈900  keV for the _Λ¹²B spectrum, and is substantially better than any previous hypernuclear experiment using magnetic spectrometers. The positions of the major excitations are found to be in agreement with a theoretical prediction and with a previous binding energy measurement, but additional structure is also observed in the core excited region, underlining the future promise of this technique.

We present data on the inclusive scattering of polarized electrons from a polarized ³He target at energies from 0.862 to 5.06 GeV, obtained at a scattering angle of 15.5°. Our data include measurements from the quasielastic peak, through the nucleon resonance region, and beyond, and were used to determine the virtual photon cross-section difference σ_1/2-σ_3/2. We extract the extended Gerasimov-Drell-Hearn integral for the neutron in the range of four-momentum transfer squared Q² of 0.1–0.9   GeV².

The quasielastic (e,e^′p) reaction was studied on targets of deuterium, carbon, and iron up to a value of momentum transfer Q² of 8.1 (GeV/c)². A nuclear transparency was determined by comparing the data to calculations in the plane-wave impulse approximation. The dependence of the nuclear transparency on Q² and the mass number A was investigated in a search for the onset of the color transparency phenomenon. We find no evidence for the onset of color transparency within our range of Q². A fit to the world’s nuclear transparency data reflects the energy dependence of the free-proton–nucleon cross section.

The coherent ³He(e,e^′π⁺)³H reaction was measured at Q²=0.4 (GeV/c)² and W=1.6 GeV for two values of the virtual photon polarization, ε, allowing the separation of longitudinal and transverse cross sections. The results from the coherent process on ³He were compared to H(e,e^′π⁺)n data taken at the same kinematics. This marks the first direct comparison of these processes. At these kinematics (p_π=1.1 GeV/c), pion rescattering from the spectator nucleons in the ³He(e,e^′π⁺)³H process is expected to be small, simplifying the comparison to π⁺ production from the free proton.