Photodisintegration of 4^4He into p+t

The two-body photodisintegration of $^4$He into a proton and a triton has been studied using the CEBAF Large-Acceptance Spectrometer (CLAS) at Jefferson Laboratory. 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 $^4$He target. This is the first measurement of the photodisintegration of $^4$He above 0.4 GeV. The differential cross sections for the $\gamma$$^4$He$\to pt$ reaction have been 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 $^3$He that demonstrated the great importance of three-body mechanisms in the energy region 0.5-0.8 GeV .Comment: 13 pages submitted in one tgz file containing 2 tex file and 22 postscrip figure

Differential cross sections and spin density matrix elements for the reaction gamma p -> p omega

High-statistics differential cross sections and spin density matrix elements for the reaction gamma p -> p omega have been measured using the CLAS at Jefferson Lab for center-of-mass (CM) energies from threshold up to 2.84 GeV. Results are reported in 112 10-MeV wide CM energy bins, each subdivided into cos(theta_CM) bins of width 0.1. These are the most precise and extensive omega photoproduction measurements to date. A number of prominent structures are clearly present in the data. Many of these have not previously been observed due to limited statistics in earlier measurements

Exclusive ρ0\rho^0 electroproduction on the proton at CLAS

The $e p\to e^\prime p \rho^0$ reaction has been measured, using the 5.754 GeV electron beam of Jefferson Lab and the CLAS detector. This represents the largest ever set of data for this reaction in the valence region. Integrated and differential cross sections are presented. The $W$, $Q^2$ and $t$ dependences of the cross section are compared to theoretical calculations based on $t$-channel meson-exchange Regge theory on the one hand and on quark handbag diagrams related to Generalized Parton Distributions (GPDs) on the other hand. The Regge approach can describe at the $\approx$ 30% level most of the features of the present data while the two GPD calculations that are presented in this article which succesfully reproduce the high energy data strongly underestimate the present data. The question is then raised whether this discrepancy originates from an incomplete or inexact way of modelling the GPDs or the associated hard scattering amplitude or whether the GPD formalism is simply inapplicable in this region due to higher-twists contributions, incalculable at present.Comment: 29 pages, 29 figure

Single pi+ Electroproduction on the Proton in the First and Second Resonance Regions at 0.25GeV^2 < Q^2 < 0.65GeV^2 Using CLAS

The ep -> e'pi^+n reaction was studied in the first and second nucleon resonance regions in the 0.25 GeV^2 < Q^2 < 0.65 GeV^2 range using the CLAS detector at Thomas Jefferson National Accelerator Facility. For the first time the absolute cross sections were measured covering nearly the full angular range in the hadronic center-of-mass frame. The structure functions sigma_TL, sigma_TT and the linear combination sigma_T+epsilon*sigma_L were extracted by fitting the phi-dependence of the measured cross sections, and were compared to the MAID and Sato-Lee models.Comment: Accepted for publication in PR

π0\pi^0 photoproduction on the proton for photon energies from 0.675 to 2.875 GeV

Differential cross sections for the reaction $\gamma p \to p \pi^0$ 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.Comment: 18 pages, 48 figure

Search for the Θ+\Theta^+ pentaquark in the reaction γd→pK−K+n\gamma d \to p K^- K^+ n

A search for the \thp in the reaction $\gamma d \to pK^-K^+n$ was completed using the CLAS detector at Jefferson Lab. A study of the same reaction, published earlier, reported the observation of a narrow \thp 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 \thp production in the mass range of 1.52 to 1.56 GeV/c$^2$ for the $\gamma d \to pK^-\Theta^+$ reaction is 0.3 nb (95% CL). This upper limit depends on assumptions made for the mass and angular distribution of \thp production. Using \lamstar production as an empirical measure of rescattering in the deuteron, the cross section upper limit for the elementary $\gamma n \to K^-\Theta^+$ reaction is estimated to be a factor of 10 higher, {\it i.e.}, $\sim 3$ nb (95% CL).Comment: 5 figures, submitted to PRL, revised for referee comment

First Measurement of Transferred Polarization in the Exclusive e p --> e' K+ Lambda Reaction

The first measurements of the transferred polarization for the exclusive ep --> e'K+ Lambda reaction have been performed in Hall B at the Thomas Jefferson National Accelerator Facility using the CLAS spectrometer. A 2.567 GeV electron beam was used to measure the hyperon polarization over a range of Q2 from 0.3 to 1.5 (GeV/c)2, W from 1.6 to 2.15 GeV, and over the full center-of-mass angular range of the K+ meson. Comparison with predictions of hadrodynamic models indicates strong sensitivity to the underlying resonance contributions. A non-relativistic quark model interpretation of our data suggests that the s-sbar quark pair is produced with spins predominantly anti-aligned. Implications for the validity of the widely used 3P0 quark-pair creation operator are discussed.Comment: 6 pages, 4 figure

Regulation of Heterochromatin Assembly on Unpaired Chromosomes during Caenorhabditis elegans Meiosis by Components of a Small RNA-Mediated Pathway

Many organisms have a mechanism for down regulating the expression of non-synapsed chromosomes and chromosomal regions during meiosis. This phenomenon is thought to function in genome defense. During early meiosis in Caenorhabditis elegans, unpaired chromosomes (e.g., the male X chromosome) become enriched for a modification associated with heterochromatin and transcriptional repression, dimethylation of histone H3 on lysine 9 (H3K9me2). This enrichment requires activity of the cellular RNA-directed RNA polymerase, EGO-1. Here we use genetic mutation, RNA interference, immunofluorescence microscopy, fluorescence in situ hybridization, and molecular cloning methods to identify and analyze three additional regulators of meiotic H3K9me2 distribution: CSR-1 (a Piwi/PAZ/Argonaute protein), EKL-1 (a Tudor domain protein), and DRH-3 (a DEAH/D-box helicase). In csr-1, ekl-1, and drh-3 mutant males, we observed a reduction in H3K9me2 accumulation on the unpaired X chromosome and an increase in H3K9me2 accumulation on paired autosomes relative to controls. We observed a similar shift in H3K9me2 pattern in hermaphrodites that carry unpaired chromosomes. Based on several assays, we conclude that ectopic H3K9me2 accumulates on paired and synapsed chromosomes in these mutants. We propose alternative models for how a small RNA-mediated pathway may regulate H3K9me2 accumulation during meiosis. We also describe the germline phenotypes of csr-1, ekl-1, and drh-3 mutants. Our genetic data suggest that these factors, together with EGO-1, participate in a regulatory network to promote diverse aspects of development

First measurement of target and double spin asymmetries for polarized e- polarized p --> e p pi0 in the nucleon resonance region above the Delta(1232)

The exclusive channel polarized proton(polarized e,e prime p)pi0 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 (CLAS). Longitudinal target and beam-target asymmetries were extracted over a large range of center-of-mass angles of the pi0 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.Comment: 13 pages, 13 figure

Ratios of 15N/12C and 4He/12C inclusive electroproduction cross sections in the nucleon resonance region

The (W,Q2)-dependence of the ratio of inclusive electron scattering cross sections for 15N/12C was determined in the kinematic range 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 DIS 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 the phenomenological model, and exhibit significant resonance structure centered at W=0.94, 1.23 and 1.5 GeV.Comment: 13 pages, 2 figures. Significantly shortened version. Results unchanged. Small additions for Phys. Rev.