Search Results (74 total)

We present results on the electroexcitation of the low mass resonances Δ(1232)P₃₃, N(1440)P₁₁, N(1520)D₁₃, and N(1535)S₁₁ in a wide range of Q². The results were obtained in the comprehensive analysis of data from the Continuous Electron Beam Accelerator Facility (CEBAF) large acceptance spectrometer (CLAS) detector at the Thomas Jefferson National Accelerator Facility (JLab) on differential cross sections, longitudinally polarized beam asymmetries, and longitudinal target and beam-target asymmetries for π electroproduction off the proton. The data were analyzed using two conceptually different approaches—fixed-t dispersion relations and a unitary isobar model—allowing us to draw conclusions on the model sensitivity of the obtained electrocoupling amplitudes. The amplitudes for the Δ(1232)P₃₃ show the importance of a meson-cloud contribution to quantitatively explain the magnetic dipole strength, as well as the electric and scalar quadrupole transitions. They do not show any tendency of approaching the pQCD regime for Q²≤6 GeV². For the Roper resonance, N(1440)P₁₁, the data provide strong evidence that this state is a predominantly radial excitation of a three-quark (3q) ground state. Measured in pion electroproduction, the transverse helicity amplitude for the N(1535)S₁₁ allowed us to obtain the branching ratios of this state to the πN and ηN channels via comparison with the results extracted from η electroproduction. The extensive CLAS data also enabled the extraction of the γ^*p→N(1520)D₁₃ and N(1535)S₁₁ longitudinal helicity amplitudes with good precision. For the N(1535)S₁₁, these results became a challenge for quark models and may be indicative of large meson-cloud contributions or of representations of this state that differ from a 3q excitation. The transverse amplitudes for the N(1520)D₁₃ clearly show the rapid changeover from helicity-3/2 dominance at the real photon point to helicity-1/2 dominance at Q²>1 GeV², confirming a long-standing prediction of the constituent quark model.

The two-body photodisintegration of ⁴He into a proton and a triton has been studied using the CEBAF Large-Acceptance Spectrometer (CLAS) at the Thomas Jefferson National Accelerator Facility. Real photons produced with the Hall-B bremsstrahlung-tagging system in the energy range from 0.35 to 1.55 GeV were incident on a liquid ⁴He target. This is the first measurement of the photodisintegration of ⁴He above 0.4 GeV. The differential cross sections for the γ⁴He→pt reaction were measured as a function of photon-beam energy and proton-scattering angle and are compared with the latest model calculations by J.-M. Laget. At 0.6-1.2 GeV, our data are in good agreement only with the calculations that include three-body mechanisms, thus confirming their importance. These results reinforce the conclusion of our previous study of the three-body breakup of ³He that demonstrated the great importance of three-body mechanisms in the energy region 0.5-0.8 GeV.

We report measurements of the beam spin asymmetry in deeply virtual Compton scattering (DVCS) at an electron beam energy of 4.8 GeV using the CLAS detector at the Thomas Jefferson National Accelerator Facility. The DVCS beam spin asymmetry has been measured in a wide range of kinematics, 1.0<Q²<2.8 (GeV/c)², 0.12<x_B<0.48, and 0.1<-t<0.8 (GeV/c)², using the reaction e→p→e^'pX. The number of H(e,e^'γp) and H(e,e^'π⁰p) events are separated in each (Q²,x_B,t) bin by a fit to the line shape of the H(e,e^'p)X M_x² distribution. The validity of the method was studied in detail using experimental and simulated data. It was shown that with the achieved missing mass squared resolution and the available statistics, the separation of DVCS–Bethe-Heitler and π⁰ events can reliably be done with less than 5% uncertainty. Also, the Q² and t dependences of the sinϕ moments of the asymmetry are extracted and compared with theoretical calculations.

Semi-inclusive π⁺ electroproduction on protons has been measured with the CLAS detector at Jefferson Lab. The measurement was performed on a liquid-hydrogen target using a 5.75 GeV electron beam. The complete five-fold differential cross sections were measured over a wide kinematic range including the complete range of azimuthal angles between hadronic and leptonic planes, ϕ, enabling us to separate the ϕ-dependent terms. Our measurements of the ϕ-independent term of the cross section at low Bjorken x were found to be in fairly good agreement with pQCD calculations. Indeed, the conventional current fragmentation calculation can account for almost all of the observed cross section, even at small π⁺ momentum. The measured center-of-momentum spectra are in qualitative agreement with high-energy data, which suggests a surprising numerical similarity between the spectator diquark fragmentation in the present reaction and the antiquark fragmentation measured in e⁺e^- collisions. We have observed that the two ϕ-dependent terms of the cross section are small. Within our precision the cos⁡2ϕ term is compatible with zero, except for the low-z region, and the measured cos⁡ϕ term is much smaller in magnitude than the sum of the Cahn and Berger effects.

We report a measurement of the differential cross section for the γn→π^-p process from the CLAS detector at Jefferson Laboratory in Hall B for photon energies between 1.0 and 3.5 GeV and pion center-of-mass (c.m.) angles (θ_c.m.) between 50° and 115°. We confirm a previous indication of a broad enhancement around a c.m. energy (sqrt[s]) of 2.1 GeV at θ_c.m.=90° in the scaled differential cross section s⁷dσ / dt and a rapid falloff in a center-of-mass energy region of about 400 MeV following the enhancement. Our data show an angular dependence of this enhancement as the suggested scaling region is approached for θ_c.m. from 70° to 105°.

Differential cross sections for the reaction γp→nπ⁺ have been measured with the CEBAF Large Acceptance Spectrometer (CLAS) and a tagged photon beam with energies from 0.725 to 2.875 GeV. Where available, the results obtained here compare well with previously published results for the reaction. Agreement with the SAID and MAID analyses is found below 1 GeV. The present set of cross sections has been incorporated into the SAID database, and exploratory fits have been made up to 2.7 GeV. Resonance couplings have been extracted and compared to previous determinations. With the addition of these cross sections to the world data set, significant changes have occurred in the high-energy behavior of the SAID cross-section predictions and amplitudes.

The neutron elastic magnetic form factor was extracted from quasielastic electron scattering on deuterium over the range Q²=1.0–4.8  GeV² with the CLAS detector at Jefferson Lab. High precision was achieved with a ratio technique and a simultaneous in situ calibration of the neutron detection efficiency. Neutrons were detected with electromagnetic calorimeters and time-of-flight scintillators at two beam energies. The dipole parametrization gives a good description of the data.

A search for exotic mesons in the π⁺π⁺π^- system photoproduced by the charge exchange reaction γp→π⁺π⁺π^-(n) was carried out by the CLAS Collaboration at Jefferson Lab. A tagged-photon beam with energies in the 4.8 to 5.4 GeV range, produced through bremsstrahlung from a 5.744 GeV electron beam, was incident on a liquid-hydrogen target. A partial wave analysis was performed on a sample of 83 000 events, the highest such statistics to date in this reaction at these energies. The main objective of this study was to look for the photoproduction of an exotic J^PC=1^-+ resonant state in the 1 to 2 GeV mass range. Our partial wave analysis shows production of the a₂(1320) and the π₂(1670) mesons, but no evidence for the a₁(1260), nor the π₁(1600) exotic state at the expected levels. An upper limit of 13.5 nb is determined for the exotic π₁(1600) cross section, less than 2% of the a₂(1320) production.

We report on the results of the first measurement of exclusive f₀(980) meson photoproduction on protons for E_γ=3.0–3.8  GeV and -t=0.4–1.0  GeV². Data were collected with the CLAS detector at the Thomas Jefferson National Accelerator Facility. The resonance was detected via its decay in the π⁺π^- channel by performing a partial wave analysis of the reaction γp→pπ⁺π^-. Clear evidence of the f₀(980) meson was found in the interference between P and S waves at M_π⁺π^-∼1  GeV. The S-wave differential cross section integrated in the mass range of the f₀(980) was found to be a factor of about 50 smaller than the cross section for the ρ meson. This is the first time the f₀(980) meson has been measured in a photoproduction experiment.

This paper reports on the most comprehensive data set obtained on differential and fully integrated cross sections for the process ep→e^'pπ⁺π^-. The data were collected with the CLAS detector at Jefferson Laboratory. Measurements were carried out in the as yet unexplored kinematic region of photon virtuality 0.2<Q²<0.6 GeV² and invariant mass of the final hadron system W from 1.3 to 1.57 GeV. For the first time, nine independent one-fold differential cross sections were determined in each bin of W and Q² covered by the measurements. A phenomenological analysis of the data allowed us to establish the most significant mechanisms contributing to the reaction. The nonresonant mechanisms account for a major part of cross sections. However, we find sensitivity to s-channel excitations of low-mass nucleon resonances, especially to the N(1440)P₁₁ and N(1520)D₁₃ states in kinematic dependencies of the one-fold differential cross sections.

The helicity amplitudes of the electroexcitation of the Roper resonance are extracted for 1.7<Q²<4.5  GeV² from recent high precision JLab-CLAS cross section and longitudinally polarized beam asymmetry data for π⁺ electroproduction on protons at W=1.15-1.69 GeV. The analysis is made using two approaches, dispersion relations and a unitary isobar model, which give consistent Q² behavior of the helicity amplitudes for the γ^*p→N(1440)P₁₁ transition. It is found that the transverse helicity amplitude A_1/2, which is large and negative at Q²=0, becomes large and positive at Q²≃2  GeV², and then drops slowly with Q². The longitudinal helicity amplitude S_1/2, which was previously found from CLAS e→p→epπ⁰,enπ⁺ data to be large and positive at Q²=0.4,0.65  GeV², drops with Q². Available model predictions for γ^*p→N(1440)P₁₁ allow us to conclude that these results provide strong evidence in favor of N(1440)P₁₁ as a first radial excitation of the 3q ground state. The results of the present paper also confirm the conclusion of our previous analysis for Q²<1 GeV² that the presentation of N(1440)P₁₁ as a q³G hybrid state is ruled out.

The exclusive channel p→(e→,e^'p)π⁰ was studied in the first and second nucleon resonance regions in the Q² range from 0.187 to 0.770 GeV² at Jefferson Lab using the CEBAF Large Acceptance Spectrometer. Longitudinal target and beam-target asymmetries were extracted over a large range of center-of-mass angles of the π⁰ and compared to the unitary isobar model MAID, the dynamic model by Sato and Lee, and the dynamic model DMT. A strong sensitivity to individual models was observed, in particular for the target asymmetry and in the higher invariant mass region. This data set, once included in the global fits of the above models, is expected to place strong constraints on the electrocoupling amplitudes A_1/2 and S_1/2 for the Roper resonance N(1400)P₁₁ and the N(1535)S₁₁ and N(1520)D₁₃ states.

Electroproduction of exclusive ϕ vector mesons has been studied with the CLAS detector in the kinematic range 1.4≤Q²≤3.8 GeV²,0.0≤t^'≤3.6 GeV², and 2.0≤W≤3.0 GeV. The scaling exponent for the total cross section as 1/(Q²+M_ϕ²)ⁿ was determined to be n=2.49±0.33. The slope of the four-momentum transfer t^' distribution is b_ϕ=0.98±0.17 GeV^-2. Under the assumption of s-channel helicity conservation, we determine the ratio of longitudinal to transverse cross sections to be R=0.86±0.24. A two-gluon exchange model is able to reproduce the main features of the data.

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 light vector mesons (ρ,ω, and ϕ) were produced in deuterium, carbon, titanium, and iron targets in a search for possible in-medium modifications to the properties of the ρ meson at normal nuclear densities and zero temperature. The vector mesons were detected with the CEBAF Large Acceptance Spectrometer (CLAS) via their decays to e⁺e^-. The rare leptonic decay was chosen to reduce final-state interactions. A combinatorial background was subtracted from the invariant mass spectra using a well-established event-mixing technique. The ρ-meson mass spectrum was extracted after the ω and ϕ signals were removed in a nearly model-independent way. Comparisons were made between the ρ mass spectra from the heavy targets (A>2) with the mass spectrum extracted from the deuterium target. With respect to the ρ-meson mass, we obtain a small shift compatible with zero. Also, we measure widths consistent with standard nuclear many-body effects such as collisional broadening and Fermi motion.

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 beam-spin asymmetries in the hard exclusive electroproduction of photons on the proton (e→p→epγ) were measured over a wide kinematic range and with high statistical accuracy. These asymmetries result from the interference of the Bethe-Heitler process and of deeply virtual Compton scattering. Over the whole kinematic range (x_B from 0.11 to 0.58, Q² from 1 to 4.8  GeV², -t from 0.09 to 1.8  GeV²), the azimuthal dependence of the asymmetries is compatible with expectations from leading-twist dominance, A≃asin⁡ϕ/(1+ccos⁡ϕ). This extensive set of data can thus be used to constrain significantly the generalized parton distributions of the nucleon in the valence quark sector.

The beam spin asymmetry (BSA) in the exclusive reaction e→p→epπ⁰ was measured with the CEBAF 5.77 GeV polarized electron beam and Large Acceptance Spectrometer (CLAS). The x_B,Q²,t, and ϕ dependences of the π⁰ BSA are presented in the deep inelastic regime. The asymmetries are fitted with a sinϕ function and their amplitudes are extracted. Overall, they are of the order of 0.04–0.11 and roughly independent of t. This is the signature of a nonzero longitudinal-transverse interference. The implications concerning the applicability of a formalism based on generalized parton distributions, as well as the extension of a Regge formalism at high photon virtualities, are discussed.

We examine the results of two measurements by the CLAS collaboration, one of which claimed evidence for a Θ⁺ pentaquark, while the other found no such evidence. The unique feature of these two experiments was that they were performed with the same experimental setup. Using a Bayesian analysis, we find that the results of the two experiments are in fact compatible with each other, but that the first measurement did not contain sufficient information to determine unambiguously the existence of a Θ⁺. Further, we suggest a means by which the existence of a new candidate particle can be tested in a rigorous manner.

The exclusive electroproduction process e→p→e^'nπ⁺ was measured in the range of the photon virtuality Q²=1.7-4.5 GeV², and the invariant mass range for the nπ⁺ system of W=1.15-1.7 GeV using the CEBAF Large Acceptance Spectrometer. For the first time, these kinematics are probed in exclusive π⁺ production from protons with nearly full coverage in the azimuthal and polar angles of the nπ⁺ center-of-mass system. The nπ⁺ channel has particular sensitivity to the isospin ½ excited nucleon states, and together with the pπ⁰ final state will serve to determine the transition form factors of a large number of resonances. The largest discrepancy between these results and present modes was seen in the σ_LT^' structure function. In this experiment, 31,295 cross section and 4,184 asymmetry data points were measured. Because of the large volume of data, only a reduced set of structure functions and Legendre polynomial moments can be presented that are obtained in model-independent fits to the differential cross sections.

The photoproduction of vector mesons on various nuclei has been studied using the CLAS detector at Jefferson Laboratory. The vector mesons, ρ, ω, and ϕ, are observed via their decay to e⁺e^-, in order to reduce the effects of final-state interactions in the nucleus. Of particular interest are possible in-medium effects on the properties of the ρ meson. The ρ mass spectrum is extracted from the data on various nuclei, ²H, C, Fe, and Ti. We observe no significant mass shift and some broadening consistent with expected collisional broadening for the ρ meson.

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.

Photoproduction of the cascade resonances has been investigated in the reactions γp→K⁺K⁺(X) and γp→K⁺K⁺π^-(X). The mass splitting of the ground state (Ξ^-,Ξ⁰) doublet is measured to be 5.4±1.8 MeV/c², consistent with existing measurements. The differential (total) cross sections for the Ξ^- have been determined for photon beam energies from 2.75 to 3.85 (4.75) GeV and are consistent with a production mechanism of Y^*→K⁺Ξ^- through a t-channel process. The reaction γp→K⁺K⁺π^-[Ξ⁰] has also been investigated to search of excited cascade resonances. No significant signal of excited cascade states other than the Ξ^-(1530) is observed. The cross-section results of the Ξ^-(1530) have also been obtained for photon beam energies from 3.35 to 4.75 GeV.