191 research outputs found
Fragment reattachment, reproductive status, and health indicators of the invasive colonial tunicate Didemnum vexillum with implications for dispersal
This manuscript is not subject to U.S. copyright. The definitive version was published in Biological Invasions 14 (2012): 2133-2140, doi:10.1007/s10530-012-0219-8.The invasive colonial tunicate Didemnum vexillum is now widespread in coastal and offshore waters of New England, USA. D. vexillum can inflict ecological and economic damage through biofouling and habitat modification. Natural and anthropogenic processes that fragment colonies of D. vexillum may be accelerating the spread of this invader. Reattachment success and fragment viability were confirmed in the laboratory after four weeks of suspension in experimental aquaria. The shape of suspended D. vexillum fragments progressed from flattened to globular spheres and then flattened again after reattachment to the substrate. Reproductive activity, confirmed by the presence of eggs and larvae, was observed for fragments suspended up to three weeks suggesting that D. vexillum is capable of reproducing while in a fragmented, suspended state. An index of colony health was used to monitor change in D. vexillum health while in suspension. Overall, colony health declined with time in suspension although colonies that appeared dead (black and gray in overall color) still contained a substantial number of healthy live zooids. These results suggest that activities that cause fragmentation can significantly facilitate the spread of D. vexillum. Coastal managers should consider reducing or eliminating, when practical, activities that return fragmented colonies of D. vexillum to the water. In-water cleaning of biofouling and dredging are likely expediting the spread of this invasive species unless biofouling can be contained and removed from the water.This research was funded by the NOAA Aquatic Invasive Species Program
photoproduction on the proton for photon energies from 0.725 to 2.875 GeV
Differential cross sections for the reaction have been
measured with the CEBAF Large Acceptance Spectrometer (CLAS) and a tagged
photon beam with energies from 0.725 to 2.875 GeV. Where available, the results
obtained here compare well with previously published results for the reaction.
Agreement with the SAID and MAID analyses is found below 1 GeV. The present set
of cross sections has been incorporated into the SAID database, and exploratory
fits have been made up to 2.7 GeV. Resonance couplings have been extracted and
compared to previous determinations. With the addition of these cross sections
to the world data set, significant changes have occurred in the high-energy
behavior of the SAID cross-section predictions and amplitudes.Comment: 18 pages, 10 figure
Measurement of Beam-Spin Asymmetries for Deep Inelastic Electroproduction
We report the first evidence for a non-zero beam-spin azimuthal asymmetry in
the electroproduction of positive pions in the deep-inelastic region. Data have
been obtained using a polarized electron beam of 4.3 GeV with the CLAS detector
at the Thomas Jefferson National Accelerator Facility (JLab). The amplitude of
the modulation increases with the momentum of the pion relative to
the virtual photon, , with an average amplitude of for range.Comment: 5 pages, RevTEX4, 3 figures, 2 table
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
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
Observation of an Exotic Baryon in Exclusive Photoproduction from the Deuteron
In an exclusive measurement of the reaction , a
narrow peak that can be attributed to an exotic baryon with strangeness
is seen in the invariant mass spectrum. The peak is at
GeV/c with a measured width of 0.021 GeV/c FWHM, which is largely
determined by experimental mass resolution. The statistical significance of the
peak is . The mass and width of the observed peak are
consistent with recent reports of a narrow baryon by other experimental
groups.Comment: 5 pages, 5 figure
Measurement of the Polarized Structure Function for in the Resonance Region
The polarized longitudinal-transverse structure function
has been measured in the resonance region at and 0.65
GeV. Data for the reaction were taken at Jefferson Lab
with the CEBAF Large Acceptance Spectrometer (CLAS) using longitudinally
polarized electrons at an energy of 1.515 GeV. For the first time a complete
angular distribution was measured, permitting the separation of different
non-resonant amplitudes using a partial wave analysis. Comparison with previous
beam asymmetry measurements at MAMI indicate a deviation from the predicted
dependence of using recent phenomenological
models.Comment: 5 pages, LaTex, 4 eps figures: to be published in PRC/Rapid
Communications. Version 2 has revised Q^2 analysi
Two-Nucleon Momentum Distributions Measured in 3He(e,e'pp)n
We have measured the 3He(e,e'pp)n reaction at 2.2 GeV over a wide kinematic
range. The kinetic energy distribution for `fast' nucleons (p > 250 MeV/c)
peaks where two nucleons each have 20% or less, and the third nucleon has most
of the transferred energy. These fast pp and pn pairs are back-to-back with
little momentum along the three-momentum transfer, indicating that they are
spectators. Experimental and theoretical evidence indicates that we have
measured distorted two-nucleon momentum distributions by striking the third
nucleon and detecting the spectator correlated pair.Comment: 6 pages, 5 figures, submitted to PR
Search for the pentaquark in the reaction
A search for the \thp in the reaction 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 for the 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 reaction is estimated to be
a factor of 10 higher, {\it i.e.}, nb (95% CL).Comment: 5 figures, submitted to PRL, revised for referee comment
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