Search Results (87 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 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 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.

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.

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.

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.

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 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.

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.