Search Results (78 total)

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.

The cross section and decay angular distributions for the coherent ϕ-meson photoproduction on the deuteron have been measured for the first time up to a squared four-momentum transfer t=(p_γ-p_ϕ)²=-2 GeV²/c², using the CLAS detector at the Thomas Jefferson National Accelerator Facility. The cross sections are compared with predictions from a rescattering model. In a framework of vector meson dominance, the data are consistent with the total ϕ-N cross section σ_ϕN at about 10 mb. If vector meson dominance is violated, a larger σ_ϕN is possible by introducing a larger t slope for the ϕN→ϕN process than that for the γN→ϕN process. The decay angular distributions of the ϕ are consistent with helicity conservation.

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.

The reaction ²H(e,e^′p)n has been studied with full kinematic coverage for photon virtuality 1.75<Q²<5.5  GeV². Comparisons of experimental data with theory indicate that for very low values of neutron recoil momentum (p_n<100  MeV/c) the neutron is primarily a spectator and the reaction can be described by the plane-wave impulse approximation. For 100<p_n<750  MeV/c, proton-neutron rescattering dominates the cross section, while Δ production followed by the NΔ→NN transition is the primary contribution at higher momenta.

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.

We report a new measurement of the exclusive electroproduction reaction γ^*p→π⁰p to explore the evolution from soft nonperturbative physics to hard processes via the Q² dependence of the magnetic (M₁₊), electric (E₁₊), and scalar (S₁₊) multipoles in the N→Δ transition. 9000 differential cross section data points cover W from threshold to 1.4  GeV/c², 4π center-of-mass solid angle, and Q² from 3 to 6  GeV²/c², the highest yet achieved. It is found that the magnetic form factor G_M^* decreases with Q² more steeply than the proton magnetic form factor, the ratio E₁₊/M₁₊ is small and negative, indicating strong helicity nonconservation, and the ratio S₁₊/M₁₊ is negative, while its magnitude increases with Q².

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 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.

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.

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.

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.

High-statistics cross sections and recoil polarizations for the reactions γ+p→K⁺+Λ and γ+p→K⁺+Σ⁰ have been measured at CLAS for center-of-mass energies between 1.6 and 2.3  GeV. In the K⁺Λ channel we confirm a resonance-like structure near W=1.9  GeV at backward kaon angles. Our data show more complex s- and u- channel behavior than previously seen, since structure is also present at forward angles, but not at central angles. The position and width change with angle, indicating that more than one resonance is playing a role. Large positive Λ polarization at backward angles, which is also energy dependent, is consistent with sizable s- or u-channel contributions. Presently available model calculations cannot explain these aspects of the data.