184 research outputs found
Generalized Parton Distributions and Nucleon Resonances
We discuss the relations between Generalized Parton Distributions (GPDs) and
nucleon resonances. We first briefly introduce the concept of "transition"
GPDs. Then we discuss a straightforward application to the modelization of the
N- magnetic transition form factor. Finally, we discuss the
experimental aspects of the subject and present first preliminary experimental
investigations in this field.Comment: 6 pages, 4 figures, Nstar02 conference proceedin
Development of portable NMR polarimeter system for polarized HD target
A portable NMR polarimeter system has been developed to measure the
polarization of a polarized Hydrogen-Deuteride (HD) target for hadron
photoproduction experiments at SPring-8. The polarized HD target is produced at
the Research Center for Nuclear Physics (RCNP), Osaka university and is
transported to SPring-8. The HD polarization should be monitored at both
places. We have constructed the portable NMR polarimeter system by replacing
the devices in the conventional system with the software system with PCI
eXtensions for Instrumentation (PXI). The weight of the NMR system is downsized
from 80 kg to 7 kg, and the cost is reduced to 25%. We check the performance of
the portable NMR polarimeter system. The signal-to-noise (S/N) ratio of the NMR
signal for the portable system is about 50% of that for the conventional NMR
system. This performance of the portable NMR system is proved to be compatible
with the conventional NMR system for the polarization measurement.Comment: 6 page, 8 figures, 2011/Mar/9 Replace Author
Vector meson photoproduction studied in its radiative decay channel
We provide an analysis of vector meson photoproduction in the channel of the
vector meson decaying into a pseudoscalar meson plus a photon, i.e. . It is shown that non-trivial kinematic correlations arise from the
measurement of the angular distributions in the overall c.m. system
in comparison with those in the vector-meson-rest frame. In terms of the vector
meson density matrix elements, the implication of such kinematic correlations
in the measurement of polarization observables is discussed. For the
meson production, due to its relatively large branching ratios for
, additional events from this channel may enrich the
information about the reaction mechanism and improve the statistics of the
recent measurement of polarized beam asymmetries by the GRAAL Collaboration.
For , , and , we expect
that additional information about the spin structure of the vector meson
production vertex can be derived.Comment: Revtex, 14 pages, 2 eps figures; Version accepted by PR
Distillation of hydrogen isotopes for polarized HD target
We have developed a cryogenic distillation system to purify
Hydrogen-Deuteride (HD) gas for a polarized HD target in LEPS experiments at
SPring-8. A small amount of ortho-H (0.01%) in the HD gas plays an
important role in efficiently polarizing the HD target. Since there are
15% impurities of H and D in commercially available HD gases, it
is inevitable that the HD gas is purified up to 99.99%. The distillation
system has a cryogenic pot (1721 K) containing many small stainless steel
cells called Heli-pack. Commercial HD gas with an amount of 5.2 mol is fed into
the pot. We carried out three distillation runs by changing temperatures (17.5
K and 20.5 K) and gas extraction speeds (1.3 ml/min and 5.2 ml/min). The
extracted gas was analyzed by using a gas analyzer system combining a
quadrupole mass spectrometer with a gas chromatograph. The HD gas of 1 mol with
a purity better than 99.99% has been successfully obtained. The effective NTS
(Number of Theoretical Stages), which is an indicator of the distillator
performances, is obtained as 37.20.6. This value is in reasonable
agreement with a designed value of 37.9. The HD target is expected to be
efficiently polarized under a well-controlled condition by doping an optimal
amount of ortho-H to the purified HD gas.Comment: 7 pages, 8 figures, 2 tables, updated 2011-12-1
Deeply virtual and exclusive electroproduction of omega mesons
The exclusive omega electroproduction off the proton was studied in a large
kinematical domain above the nucleon resonance region and for the highest
possible photon virtuality (Q2) with the 5.75 GeV beam at CEBAF and the CLAS
spectrometer. Cross sections were measured up to large values of the
four-momentum transfer (-t < 2.7 GeV2) to the proton. The contributions of the
interference terms sigma_TT and sigma_TL to the cross sections, as well as an
analysis of the omega spin density matrix, indicate that helicity is not
conserved in this process. The t-channel pi0 exchange, or more generally the
exchange of the associated Regge trajectory, seems to dominate the reaction
gamma* p -> omega p, even for Q2 as large as 5 GeV2. Contributions of handbag
diagrams, related to Generalized Parton Distributions in the nucleon, are
therefore difficult to extract for this process. Remarkably, the high-t
behaviour of the cross sections is nearly Q2-independent, which may be
interpreted as a coupling of the photon to a point-like object in this
kinematical limit.Comment: 15 pages,19 figure
Search for pentaquark in high statistics measurement of at CLAS
The exclusive reaction was studied in the
photon energy range between 1.6-3.8 GeV searching for evidence of the exotic
baryon . The decay to requires the assignment of
strangeness 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 . No evidence for the
pentaquark was found. Upper limits were set on the production cross section as
function of center-of-mass angle and mass. The 95% CL upper limit on the
total cross section for a narrow resonance at 1540 MeV was found to be 0.8 nb.Comment: Submitted to Physical Review Letter
Q^2 Dependence of the S_{11}(1535) Photocoupling and Evidence for a P-wave resonance in eta electroproduction
New cross sections for the reaction are reported for total
center of mass energy =1.5--2.3 GeV and invariant squared momentum transfer
=0.13--3.3 GeV. This large kinematic range allows extraction of new
information about response functions, photocouplings, and coupling
strengths of baryon resonances. A sharp structure is seen at 1.7 GeV.
The shape of the differential cross section is indicative of the presence of a
-wave resonance that persists to high . Improved values are derived for
the photon coupling amplitude for the (1535) resonance. The new data
greatly expands the range covered and an interpretation of all data with
a consistent parameterization is provided.Comment: 31 pages, 9 figure
Light Vector Mesons in the Nuclear Medium
The light vector mesons (, , and ) were produced in
deuterium, carbon, titanium, and iron targets in a search for possible
in-medium modifications to the properties of the meson at normal nuclear
densities and zero temperature. The vector mesons were detected with the CEBAF
Large Acceptance Spectrometer (CLAS) via their decays to . 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 meson mass spectrum was
extracted after the and signals were removed in a nearly
model-independent way. Comparisons were made between the mass spectra
from the heavy targets () with the mass spectrum extracted from the
deuterium target. With respect to the -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.Comment: 15 pages, 18 figures, 3 table
Measurement of Deeply Virtual Compton Scattering with a Polarized Proton Target
The longitudinal target-spin asymmetry A_UL for the exclusive
electroproduction of high energy photons was measured for the first time in
p(e,e'p\gamma). The data have been accumulated at Jefferson Lab with the CLAS
spectrometer using 5.7 GeV electrons and a longitudinally polarized NH_3
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(phi) 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-tilde.Comment: Modified text slightly, added reference
A Precise Measurement of the Neutron Magnetic Form Factor GMn in the Few-GeV2 Region
The neutron elastic magnetic form factor GMn has been extracted from
quasielastic electron scattering data on deuterium with the CEBAF Large
Acceptance Spectrometer (CLAS) at Jefferson Lab. The kinematic coverage of the
measurement is continuous from Q2=1 GeV2 to 4.8 GeV2. High precision was
achieved by employing a ratio technique in which many uncertainties cancel, and
by a simultaneous in-situ calibration of the neutron detection efficiency, the
largest correction to the data. Neutrons were detected using the CLAS
electromagnetic calorimeters and the time-of-flight scintillators. Data were
taken at two different electron beam energies, allowing up to four
semi-independent measurements of GMn to be made at each value of Q2. The dipole
parameterization is found to provide a good description of the data over the
measured Q2 range.Comment: 14 pages, 5 figures, revtex4, submitted to Physical Review Letters,
Revised version has changes recommended by journal referee
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