Measurement of the Deuteron Structure Function F₂ in the Resonance Region and Evaluation of its Moments

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 similar or equal to 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

Magnetic moment of the pentaquark Θ+(1540)\Theta^+(1540) with light-cone QCD sum rules

In this article, we study the magnetic moment of the pentaquark state $\Theta^+(1540)$ as diquark-diquark-antiquark ($[ud][ud]\bar{s}$) state in the framework of the light-cone QCD sum rules approach. The numerical results indicate the magnetic moment of the pentaquark state $\Theta^+(1540)$ is about $\mu_{\Theta^+}=-(0.49\pm 0.06)\mu_N$.Comment: 10 pages, 1 figure. The main contents of this article is included in hep-ph/0503007, this article will not be submitted to a journal for publicatio

Measurement of Deeply Virtual Compton Scattering Beam-Spin Asymmetries

The beam-spin asymmetries in the hard exclusive electroproduction of photons on the proton (→ep→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 (xB 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 ≃ asinϕ/(1+ccosϕ). This extensive set of data can thus be used to constrain significantly the generalized parton distributions of the nucleon in the valence quark sector

Differential Cross Sections for + p → K⁺ + Y for Λ and Σ⁰ Hyperons

High-statistics cross sections for the reactions + p → K⁺ + Λ and + 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 \u3c cos θ Kc.m. \u3c +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

Light Vector Mesons in the Nuclear Medium

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 ρmass spectra from the heavy targets (A\u3e2) 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

Beam-Helicity Asymmetries in Double-Charged-Pion Photoproduction on the Proton

Beam-helicity asymmetries for the two-pion-photoproduction reaction ⃗p → p π+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

Measurement of the x- and Q\u3csup\u3e2\u3c/sup\u3e -Dependence of the Asymmetry A1 on the Nucleon

We report results for the virtual photon asymmetry A1 on the nucleon from new Jefferson Lab measurements. The experiment, which used the CEBAF Large Acceptance Spectrometer and longitudinally polarized proton (15NH3) and deuteron (15ND3) targets, collected data with a longitudinally polarized electron beam at energies between 1.6 GeV and 5.7 GeV. In the present Letter, we concentrate on our results for A1 ( x , Q2 ) and the related ratio g1 / F1 ( x , Q2 ) in the resonance and the deep inelastic regions for our lowest and highest beam energies, covering a range in momentum transfer Q2 from 0.05 to 5.0 ( GeV /c )2 and in final-state invariant mass W up to about 3 GeV. Our data show detailed structure in the resonance region, which leads to a strong Q2 -dependence of A1 (x, Q2) for W below 2 GeV. At higher W, a smooth approach to the scaling limit, established by earlier experiments, can be seen, but A1 ( x, Q2) is not strictly Q2 -independent. We add significantly to the world data set at high x, up to x = 0.6 . Our data exceed the SU ( 6 ) -symmetric quark model expectation for both the proton and the deuteron while being consistent with a negative d-quark polarization up to our highest x. This data set should improve next-to-leading order (NLO) pQCD fits of the parton polarization distributions