Search Results (58 total)

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 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.

An experiment measuring electroproduction of hypernuclei has been performed in hall A at Jefferson Lab on a ¹²C target. In order to increase counting rates and provide unambiguous kaon identification two superconducting septum magnets and a ring imaging Cherenkov detector were added to the hall A standard equipment. An unprecedented energy resolution of less than 700 keV FWHM has been achieved. Thus, the observed _Λ¹²B spectrum shows for the first time identifiable strength in the core-excited region between the ground-state s-wave Λ peak and the 11 MeV p-wave Λ peak.

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.

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.

Differential cross sections for the reaction γp→K^*0Σ⁺ are presented in the photon energy range of 1.7 to 3.0 GeV. The K^*0 was detected by its decay products, K⁺π^-, in the Continuous Electron Beam Accelerator Facility's large acceptance spectrometer (CLAS) detector at the Thomas Jefferson National Accelerator Facility. These data are the first K^*0 photoproduction cross sections ever published over a broad range of angles. Comparison with a theoretical model based on the vector and tensor K^*-quark couplings shows good agreement with the data, except at forward angles, suggesting that the role of scalar κ meson exchange should be investigated.

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.

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.

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 longitudinal target-spin asymmetry A_UL for the exclusive electroproduction of high-energy photons was measured for the first time in ep→→e^′pγ. The data have been accumulated at JLab with the CLAS spectrometer using 5.7 GeV electrons and a longitudinally polarized NH₃ target. A significant azimuthal angular dependence was observed, resulting from the interference of the deeply virtual Compton scattering and Bethe-Heitler processes. The amplitude of the sin⁡ϕ moment is 0.252±0.042^stat±0.020^sys. Theoretical calculations are in good agreement with the magnitude and the kinematic dependence of the target-spin asymmetry, which is sensitive to the generalized parton distributions H˜ and H.

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.

For the first time, the reaction γd→ΛnK⁺ has been analyzed in order to search for the exotic pentaquark baryon Θ⁺(1540). The data were taken at Jefferson Laboratory, using the Hall-B tagged-photon beam of energy between 0.8 and 3.6 GeV and the CEBAF Large Acceptance Spectrometer (CLAS). No statistically significant structures were observed in the nK⁺ invariant-mass distribution. The upper limit on the γd→ΛΘ⁺ integrated cross section has been calculated and found to be between 5 and 25 nb, depending on the production model assumed. The upper limit on the differential cross section is also reported.

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.).