Search Results (231 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.

By using 1.8×10⁶ DD̅ pairs, we have measured B(D⁰→π^-e⁺ν_e)=0.299(11)(9)%, B(D⁺→π⁰e⁺ν_e)=0.373(22)(13)%, B(D⁰→K^-e⁺ν_e)=3.56(3)(9)%, and B(D⁺→K̅ ⁰e⁺ν_e)=8.53(13)(23)% and have studied the q² dependence of the form factors. By combining our results with recent lattice calculations, we obtain |V_cd|=0.217(9)(4)(23) and |V_cs|=1.015(10)(11)(106).

Using a sample of 1.8 million DD̅ mesons collected at the ψ(3770) with the CLEO-c detector, we study the semileptonic decays D⁰→π^-e⁺ν_e, D⁺→π⁰e⁺ν_e, D⁰→K^-e⁺ν_e, and D⁺→K̅ ⁰e⁺ν_e. For the total branching fractions we find B(D⁰→π^-e⁺ν_e)=0.299(11)(9)%, B(D⁺→π⁰e⁺ν_e)=0.373(22)(13)%, B(D⁰→K^-e⁺ν_e)=3.56(3)(9)%, and B(D⁺→K̅ ⁰e⁺ν_e)=8.53(13)(23)%, where the first error is statistical and the second systematic. In addition, form factors are studied through fits to the partial branching fractions obtained in five q² ranges. By combining our results with recent unquenched lattice calculations, we obtain |V_cd|=0.217(9)(4)(23) and |V_cs|=1.015(10)(11)(106), where the final error is theoretical.

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.

We report the results of a search for ν_μ disappearance by the Main Injector Neutrino Oscillation Search [D. G. Michael (MINOS), Phys. Rev. Lett. 97, 191801 (2006).]. The experiment uses two detectors separated by 734 km to observe a beam of neutrinos created by the Neutrinos at the Main Injector facility at Fermi National Accelerator Laboratory. The data were collected in the first 282 days of beam operations and correspond to an exposure of 1.27×10²⁰ protons on target. Based on measurements in the Near Detector, in the absence of neutrino oscillations we expected 336±14 ν_μ charged-current interactions at the Far Detector but observed 215. This deficit of events corresponds to a significance of 5.2 standard deviations. The deficit is energy dependent and is consistent with two-flavor neutrino oscillations according to |Δm²|=2.74_-0.26^+0.44×10^-3  eV²/c⁴ and sin⁡²2θ>0.87 at 68% confidence level.

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.

Using the CLEO III detector, we measure absolute cross sections for e⁺e^-→hadrons at seven center-of-mass energies between 6.964 and 10.538 GeV. The values of R, the ratio of hadronic and muon pair production cross sections, are determined within 2% total root-mean-square uncertainty.

We report on a search for the radiative decay of Υ(1S) to the pseudoscalar mesons η and η^′ in (21.2±0.2)×10⁶ Υ(1S) decays collected with the CLEO III detector at the Cornell Electron Storage Ring. The η meson was reconstructed in the three modes η→γγ, η→π⁺π^-π⁰, or η→π⁰π⁰π⁰. The η^′ meson was reconstructed in the mode η^′→π⁺π^-η with η decaying through any of the above three modes, and also η^′→γρ⁰, where ρ⁰→π⁺π^-. Five out of the seven submodes are found to have very low backgrounds. In four of them we find no signal candidates and in one [Υ(1S)→γη^′, η^′→π⁺π^-η, η→π⁺π^-π⁰] there are two good signal candidates, which is insufficient evidence to claim a signal. The other two submodes (η→γγ and η^′→γρ) are background limited, and show no excess of events in their signal regions. We combine the results from different channels and obtain upper limits at the 90% C.L. which are B(Υ(1S)→γη)<1.0×10^-6 and B(Υ(1S)→γη^′)<1.9×10^-6. Our limits are an order of magnitude tighter than the previous ones and below the predictions made by some theoretical models.

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.

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.

We report on a study of exclusive radiative decays of the Υ(1S) resonance into the final states γπ⁰π⁰, γηη and γπ⁰η, using 1.13  fb^-1 of e⁺e^- annihilation data collected at sqrt[s]=9.46  GeV with the CLEO III detector operating at the Cornell Electron Storage Ring. In the channel γπ⁰π⁰, we measure the branching ratio for the decay mode Υ(1S)→γf₂(1270) to be (10.5±1.6(stat)_-1.8^+1.9(syst))×10^-5. We place upper limits on the product branching ratios for the isoscalar resonances f₀(1500) and f₀(1710) for the π⁰π⁰ and ηη decay channels. We also set an upper limit on the Υ(1S) radiative decay into π⁰η.