Search Results (53 total)

The (W,Q²) dependence of the ratio of inclusive electron scattering cross sections for ¹⁵N/¹²C was determined in the kinematic ranges 0.8<W<2 GeV and 0.2<Q²<1 GeV² using 2.285 GeV electrons and the CLAS detector at Jefferson Lab. The ratios exhibit only slight resonance structure, in agreement with a simple phenomenological model and an extrapolation of deep-inelastic scattering ratios to low Q². Ratios of ⁴He/¹²C using 1.6 to 2.5 GeV electrons were measured with very high statistical precision and were used to correct for He in the N and C targets. The (W,Q²) dependence of the ⁴He/¹²C ratios is in good agreement with that of the phenomenological model and exhibit significant resonance structure centered at W=0.94,1.23, and 1.5 GeV.

The first measurements of the polarized structure function σ_LT^' for the reaction ¹H(e→,e^'K⁺)Λ in the nucleon resonance region are reported. Measurements are included from threshold up to W=2.05 GeV for central values of Q² of 0.65 and 1.00 GeV², and nearly the entire kaon center-of-mass angular range. σ_LT^' is the imaginary part of the longitudinal-transverse response and is expected to be sensitive to interferences between competing intermediate s-channel resonances, as well as resonant and nonresonant processes. The results for σ_LT^' are comparable in magnitude to previously reported results from CLAS for σ_LT, the real part of the same response. An intriguing sign change in σ_LT^' is observed in the high Q² data at W≈1.9 GeV. Comparisons to several existing model predictions are shown.

Differential cross sections for the reaction γp→pπ⁰ have been measured with the CEBAF Large Acceptance Spectrometer (CLAS) and a tagged photon beam with energies from 0.675 to 2.875 GeV. The results reported here possess greater accuracy in the absolute normalization than previous measurements. They disagree with recent CB-ELSA measurements for the process at forward scattering angles. Agreement with the SAID and MAID fits is found below 1 GeV. The present set of cross sections has been incorporated into the SAID database, and exploratory fits have been extended to 3 GeV. Resonance couplings have been extracted and compared to previous determinations.

New cross sections for the reaction ep→e^'ηp are reported for total center-of-mass energy W=1.5-2.3 GeV and invariant squared momentum transfer Q²=0.13-3.3 GeV². This large kinematic range allows the extraction of new information about response functions, photocouplings, and ηN coupling strengths of baryon resonances. A sharp structure is seen at W~1.7 GeV. The shape of the differential cross section is indicative of the presence of a P-wave resonance that persists to high Q². Improved values are derived for the photocoupling amplitude for the S₁₁(1535) resonance. The new data greatly expand the Q² range covered, and an interpretation of all data with a consistent parametrization is provided.

We report measurements of the exclusive electroproduction of K⁺Λ and K⁺Σ⁰ final states from a proton target using the Continuous Electron Beam Accelerator Facility (CEBAF) large-acceptance spectrometer (CLAS) detector at the Thomas Jefferson National Accelerator Facility. The separated structure functions σ_T,σ_L,σ_TT, and σ_LT were extracted from the Φ- and ε-dependent differential cross sections taken with electron beam energies of 2.567, 4.056, and 4.247 GeV. This analysis represents the first σ_L/σ_T separation with the CLAS detector, and the first measurement of the kaon electroproduction structure functions away from parallel kinematics. The data span a broad range of momentum transfers from 0.5≤Q²≤2.8GeV² and invariant energy from 1.6≤W≤2.4 GeV, while spanning nearly the full center-of-mass angular range of the kaon. The separated structure functions reveal clear differences between the production dynamics for the Λ and Σ⁰ hyperons. These results provide an unprecedented data sample with which to constrain current and future models for the associated production of strangeness, which will allow for a better understanding of the underlying resonant and nonresonant contributions to hyperon production.

Spin transfer from circularly polarized real photons to recoiling hyperons has been measured for the reactions γ→+p→K++Λ→ and γ→+p→K++Σ→0. The data were obtained using the CEBAF Large Acceptance Spectrometer (CLAS) detector at the Jefferson Lab for center-of-mass energies W between 1.6 and 2.53 GeV, and for -0.85<cosθK+c.m.<+0.95. For the Λ, the polarization transfer coefficient along the photon momentum axis, Cz, was found to be near unity for a wide range of energy and kaon production angles. The associated transverse polarization coefficient Cx is smaller than Cz by a roughly constant difference of unity. Most significantly, the total Λ polarization vector, including the induced polarization P, has magnitude consistent with unity at all measured energies and production angles when the beam is fully polarized. For the Σ0 this simple phenomenology does not hold. All existing hadrodynamic models are in poor agreement with these results.

New measurements of the spin structure functions of the proton and deuteron g₁^p(x,Q²) and g₁^d(x,Q²) in the nucleon resonance region are compared with extrapolations of target-mass-corrected next-to-leading-order (NLO) QCD fits to higher energy data. Averaged over the entire resonance region (W<2 GeV), the data and QCD fits are in good agreement in both magnitude and Q² dependence for Q²>1.7 GeV²/c². This “global” duality appears to result from cancellations among the prominent “local” resonance regions: in particular strong σ_3/2 contributions in the Δ(1232) region appear to be compensated by strong σ_1/2 contributions in the resonance region centered on 1.5 GeV. These results are encouraging for the extension of NLO QCD fits to lower W and Q² than have been used previously.

Inclusive electron scattering off the deuteron has been measured to extract the deuteron structure function F₂ with the CEBAF Large Acceptance Spectrometer (CLAS) at the Thomas Jefferson National Accelerator Facility. The measurement covers the entire resonance region from the quasielastic peak up to the invariant mass of the final-state hadronic system W≃2.7 GeV with four-momentum transfers Q² from 0.4 to 6 (GeV/c)². These data are complementary to previous measurements of the proton structure function F₂ and cover a similar two-dimensional region of Q² and Bjorken variable x. Determination of the deuteron F₂ over a large x interval including the quasielastic peak as a function of Q², together with the other world data, permit a direct evaluation of the structure function moments for the first time. By fitting the Q² evolution of these moments with an OPE-based twist expansion we have obtained a separation of the leading twist and higher twist terms. The observed Q² behavior of the higher twist contribution suggests a partial cancelation of different higher twists entering into the expansion with opposite signs. This cancelation, found also in the proton moments, is a manifestation of the “duality” phenomenon in the F₂ structure function.

High-statistics cross sections for the reactions γ+p→K++Λ and γ+p→K++Σ0 have been measured using CLAS at Jefferson Lab for center-of-mass energies W between 1.6 and 2.53 GeV, and for -0.85<cosθK+c.m.<+0.95. In the K+Λ channel we confirm a resonance-like structure near W=1.9 GeV at backward kaon angles. The position and width of this structure change with angle, indicating that more than one resonance is likely playing a role. The K+Λ channel at forward angles and all energies is well described by a t-channel scaling characteristic of Regge exchange, whereas the same scaling applied to the K+Σ0 channel is less successful. Several existing theoretical models are compared to the data, but none provide a good representation of the results.

The ratios of inclusive electron scattering cross sections of ⁴He, ¹²C, and ⁵⁶Fe to ³He have been measured at 1<x_B<3. At Q²>1.4  GeV², the ratios exhibit two separate plateaus, at 1.5<x_B<2 and at x_B>2.25. This pattern is predicted by models that include 2- and 3-nucleon short-range correlations (SRC). Relative to A=3, the per-nucleon probabilities of 3-nucleon SRC are 2.3, 3.1, and 4.4 times larger for A=4, 12, and 56. This is the first measurement of 3-nucleon SRC probabilities in nuclei.

The ep→e^'π⁺n reaction was studied in the first and second nucleon resonance regions in the 0.25 GeV²<Q²<0.65 GeV² range by use of the CLAS detector at Thomas Jefferson National Accelerator Facility. For the first time, to our knowledge, the absolute cross sections were measured, covering nearly the full angular range in the hadronic center-of-mass frame. We extracted the structure functions σ_TL,σ_TT, and the linear combination σ_T+εσ_L by fitting the ϕ dependence of the measured cross sections and compared them with the MAID and Sato-Lee models.

Differential cross sections for the reaction γp→η^′p have been measured with the CLAS spectrometer and a tagged photon beam with energies from 1.527 to 2.227 GeV. The results reported here possess much greater accuracy than previous measurements. Analyses of these data suggest for the first time the coupling of the η^′N channel to both the S₁₁(1535) and P₁₁(1710) resonances, known to couple strongly to the ηN channel in photoproduction on the proton, and the importance of J=3/2 resonances in the process.

The polarized longitudinal-transverse structure function σ_LT^' measures the interference between real and imaginary amplitudes in pion electroproduction and can be used to probe the coupling between resonant and nonresonant processes. We report new measurements of σ_LT^' in the N(1440)1 / 2⁺ (Roper) resonance region at Q²=0.40 and 0.65GeV² for both the π⁰p and π⁺n channels. The experiment was performed at Jefferson Lab with the CEBAF Large Acceptance Spectrometer (CLAS) using longitudinally polarized electrons at a beam energy of 1.515 GeV. Complete angular distributions were obtained and are compared to recent phenomenological models. The σ_LT^'(π⁺n) channel shows a large sensitivity to the Roper-resonance multipoles M_1- and S_1- and provides new constraints on models of resonance formation.

Beam-helicity asymmetries for the two-pion-photoproduction reaction γ→p→pπ⁺π^- have been studied for the first time in the resonance region for center-of-mass energies between 1.35 and 2.30 GeV. The experiment was performed at Jefferson Lab with the CEBAF Large Acceptance Spectrometer using circularly polarized tagged photons incident on an unpolarized hydrogen target. Beam-helicity-dependent angular distributions of the final-state particles were measured. The large cross-section asymmetries exhibit strong sensitivity to the kinematics and dynamics of the reaction. The data are compared with the results of various phenomenological model calculations, and show that these models currently do not provide an adequate description for the behavior of this new observable.

The electromagnetic decays of the Σ⁰(1385) and Λ(1520) hyperons were studied in photon-induced reactions γp→K⁺Λ(1116)γ in the Large Acceptance Spectrometer detector at the Thomas Jefferson National Accelerator Facility. We report the first observation of the radiative decay of the Σ⁰(1385) and a measurement of the Λ(1520) radiative decay width. For the Σ⁰(1385)→Λ(1116)γ transition, we measured a partial width of 479±120(stat)_-100⁺⁸¹(sys)  keV, larger than all of the existing model predictions. For the Λ(1520)→Λ(1116)γ transition, we obtained a partial width of 167±43(stat)_-12⁺²⁶(sys)  keV.

We report on the first measurement of exclusive Ξ^-(1321) hyperon photoproduction in γp→K⁺K⁺Ξ^- for 3.2<E_γ<3.9 GeV. The final state is identified by the missing mass in p(γ,K⁺K⁺)X measured with the CLAS detector at Jefferson Laboratory. We have detected a significant number of the ground state Ξ^-(1321)1 / 2⁺ and have estimated the total cross section for its production. We also have strong evidence for the first excited state Ξ^-(1530)3 / 2⁺. Photoproduction provides a copious source of Ξ's. We discuss the possibilities of a search for the recently proposed Ξ₅^- and Ξ₅⁺ pentaquarks.

We investigate the transition from the nucleon-meson to the quark-gluon description of the strong interaction using the photon energy dependence of the d(γ,p)n differential cross section for photon energies above 0.5 GeV and center-of-mass proton angles between 30° and 150°. A possible signature for this transition is the onset of cross-section s^-11 scaling with the total energy squared, s, at some proton transverse momentum P_T. The results show that the scaling has been reached for proton transverse momentum above about 1.1  GeV/c. This may indicate that the quark-gluon regime is reached above this momentum.

The three-body photodisintegration of ³He has been measured with the CLAS detector at Jefferson Lab, using tagged photons of energies between 0.35 GeV and 1.55  GeV. The large acceptance of the spectrometer allowed us for the first time to cover a wide momentum and angular range for the two outgoing protons. Three kinematic regions dominated by either two- or three-body contributions have been distinguished and analyzed. The measured cross sections have been compared with results of a theoretical model, which, in certain kinematic ranges, have been found to be in reasonable agreement with the data.

Two-proton correlations at small relative momentum q were studied in the eA(³He,⁴He,C,Fe)→e^′ppX reaction at E₀=4.46  GeV using the CLAS detector at Jefferson Lab. The enhancement of the correlation function at small q was found to be in accordance with theoretical expectations. Sizes of the emission region were extracted, and proved to be dependent on A and on the proton momentum. The size of the two-proton emission region for He was measured in eA reactions for the first time.

The polarized longitudinal-transverse structure function σ_LT^′ has been measured using the p(e⃗,e^′π⁺)n reaction in the Δ(1232) resonance region at Q²=0.40 and 0.65  GeV². No previous σ_LT^′ data exist for this reaction channel. The kinematically complete experiment was performed at the Jefferson Lab with the CEBAF large acceptance spectrometer using longitudinally polarized electrons at an energy of 1.515  GeV. A partial-wave analysis of the data shows generally better agreement with recent phenomenological models of pion electroproduction compared to the previously measured π⁰p channel. A fit to both π⁰p and π⁺n channels using a unitary isobar model suggests the unitarized Born terms provide a consistent description of the nonresonant background. The t-channel pion pole term is important in the π⁰p channel through a rescattering correction, which could be model dependent.

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.

Nearly complete angular distributions of the two-body deuteron photodisintegration differential cross section have been measured using the CEBAF Large Acceptance Spectrometer detector and the tagged photon beam at the Thomas Jefferson National Accelerator Facility. The data cover photon energies between 0.5 and 3.0  GeV and center-of-mass proton scattering angles 10°–160°. The data show a persistent forward-backward angle asymmetry over the explored energy range, and are well described by the nonperturbative quark gluon string model.

We report the first evidence for a nonzero beam-spin azimuthal asymmetry in the electroproduction of positive pions in the deep-inelastic kinematic region. Data for the reaction ep→e^′π⁺X have been obtained using a polarized electron beam of 4.3 GeV with the CEBAF Large Acceptance Spectrometer at the Thomas Jefferson National Accelerator Facility. The amplitude of the sin φ modulation increases with the momentum of the pion relative to the virtual photon, z. In the range z=0.5–0.8 the average amplitude is 0.038±0.005±0.003 for a missing mass M_X>1.1 GeV and 0.037±0.007±0.004 for M_X>1.4 GeV.