28 research outputs found
Production of highly-polarized positrons using polarized electrons at MeV energies
The Polarized Electrons for Polarized Positrons experiment at the injector of
the Continuous Electron Beam Accelerator Facility has demonstrated for the
first time the efficient transfer of polarization from electrons to positrons
produced by the polarized bremsstrahlung radiation induced by a polarized
electron beam in a high- target. Positron polarization up to 82\% have been
measured for an initial electron beam momentum of 8.19~MeV/, limited only by
the electron beam polarization. This technique extends polarized positron
capabilities from GeV to MeV electron beams, and opens access to polarized
positron beam physics to a wide community.Comment: 5 pages, 4 figure
A Precision Measurement of the Neutron Twist-3 Matrix Element : Probing Color Forces
Double-spin asymmetries and absolute cross sections were measured at large
Bjorken (0.25 0.90), in both the deep-inelastic and resonance
regions, by scattering longitudinally polarized electrons at beam energies of
4.7 and 5.9 GeV from a transversely and longitudinally polarized He target.
In this dedicated experiment, the spin structure function on He was
determined with precision at large , and the neutron twist-three matrix
element was measured at \left of 3.21 and 4.32
GeV/, with an absolute precision of about . Our results are
found to be in agreement with lattice QCD calculations and resolve the
disagreement found with previous data at \left = 5 GeV/.
Combining and a newly extracted twist-four matrix element, , the
average neutron color electric and magnetic forces were extracted and found to
be of opposite sign and about 30 MeV/fm in magnitude.Comment: Corrected a typo in the author list and Figure 1 legend. 6 pages, 2
figures, 2 table
The E00-110 experiment in Jefferson Lab's Hall A: Deeply Virtual Compton Scattering off the Proton at 6 GeV
We present final results on the photon electroproduction
() cross section in the deeply virtual Compton
scattering (DVCS) regime and the valence quark region from Jefferson Lab
experiment E00-110. Results from an analysis of a subset of these data were
published before, but the analysis has been improved which is described here at
length, together with details on the experimental setup. Furthermore,
additional data have been analyzed resulting in photon electroproduction cross
sections at new kinematic settings, for a total of 588 experimental bins.
Results of the - and -dependences of both the helicity-dependent and
helicity-independent cross sections are discussed. The -dependence
illustrates the dominance of the twist-2 handbag amplitude in the kinematics of
the experiment, as previously noted. Thanks to the excellent accuracy of this
high luminosity experiment, it becomes clear that the unpolarized cross section
shows a significant deviation from the Bethe-Heitler process in our kinematics,
compatible with a large contribution from the leading twist-2 DVCS term to
the photon electroproduction cross section. The necessity to include
higher-twist corrections in order to fully reproduce the shape of the data is
also discussed. The DVCS cross sections in this paper represent the final set
of experimental results from E00-110, superseding the previous publication.Comment: 48 pages, 32 figure
Virtual Compton Scattering and the Generalized Polarizabilities of the Proton at Q^2=0.92 and 1.76 GeV^2
Virtual Compton Scattering (VCS) on the proton has been studied at Jefferson
Lab using the exclusive photon electroproduction reaction (e p --> e p gamma).
This paper gives a detailed account of the analysis which has led to the
determination of the structure functions P_LL-P_TT/epsilon and P_LT, and the
electric and magnetic generalized polarizabilities (GPs) alpha_E(Q^2) and
beta_M(Q^2) at values of the four-momentum transfer squared Q^2= 0.92 and 1.76
GeV^2. These data, together with the results of VCS experiments at lower
momenta, help building a coherent picture of the electric and magnetic GPs of
the proton over the full measured Q^2-range, and point to their non-trivial
behavior.Comment: version 2: modified according to PRC Editor's and Referee's
recommendations. Archival paper for the E93-050 experiment at JLab Hall A. 28
pages, 23 figures, 5 cross-section tables. To be submitted to Phys.Rev.
Measurement of the neutron F2 structure function via spectator tagging with CLAS
We report on the first measurement of the F2 structure function of the
neutron from semi-inclusive scattering of electrons from deuterium, with
low-momentum protons detected in the backward hemisphere. Restricting the
momentum of the spectator protons to < 100 MeV and their angles to < 100
degrees relative to the momentum transfer allows an interpretation of the
process in terms of scattering from nearly on-shell neutrons. The F2n data
collected cover the nucleon resonance and deep-inelastic regions over a wide
range of Bjorken x for 0.65 < Q2 < 4.52 GeV2, with uncertainties from nuclear
corrections estimated to be less than a few percent. These measurements provide
the first determination of the neutron to proton structure function ratio
F2n/F2p at 0.2 < x < 0.8 with little uncertainty due to nuclear effects.Comment: 6 pages, 3 page
First Measurement of the Ti Cross Section at Jefferson Lab
To probe CP violation in the leptonic sector using GeV energy neutrino beams
in current and future experiments using argon detectors, precise models of the
complex underlying neutrino and antineutrino interactions are needed. The
E12-14-012 experiment at Jefferson Lab Hall A was designed to perform a
combined analysis of inclusive and exclusive electron scatterings on both argon
() and titanium () nuclei using GeV energy electron beams. The
measurement on titanium nucleus provides essential information to understand
the neutrino scattering on argon, large contribution to which comes from
scattering off neutrons. Here we report the first experimental study of
electron-titanium scattering as double differential cross section at beam
energy GeV and electron scattering angle deg,
measured over a broad range of energy transfer, spanning the kinematical
regions in which quasielastic scattering and delta production are the dominant
reaction mechanisms. The data provide valuable new information needed to
develop accurate theoretical models of the electromagnetic and weak cross
sections of these complex nuclei in the kinematic regime of interest to
neutrino experiments.Comment: 6 pages, 5 figures. Version published in Physical Review
Erratum to: Methods for evaluating medical tests and biomarkers
[This corrects the article DOI: 10.1186/s41512-016-0001-y.]
Comparative yield loss estimates due to blast in some upland rice cultivars.
Leaf and panicle blast severities and grain yield of some upland rice cultivars were measured in three successive years in field plots unprotected or protected with fungicides. The variation in disease severities in different plots was used to establish relationships between severity of leaf and panicle blast and yield. Linear multiple regression equations for each cultivar by year were developed to estimate the yield decrease in different cultivars per unit increase in disease. Leaf blast severities at maximum tillering or booting stage and panicle blast 25 days after heading accounted for variation in grain yield in most of the cultivars. General equations combining five early and eight medium-duration rice cultivars were developed. The estimated percentage losses in grain yield due to blastwere 2.7 and 1.5 for one percent increase in blast in the early and medium-duration cultivars, respectively
Differential photoproduction cross sections of the Σ0(1385), Î(1405), and Î(1520)
We report the exclusive photoproduction cross sections for the Sigma(1385),
Lambda(1405), and Lambda(1520) in the reactions gamma + p -> K+ + Y* using the
CLAS detector for energies from near the respective production thresholds up to
a center-of-mass energy W of 2.85 GeV. The differential cross sections are
integrated to give the total exclusive cross sections for each hyperon.
Comparisons are made to current theoretical models based on the effective
Lagrangian approach and fitted to previous data. The accuracy of these models
is seen to vary widely. The cross sections for the Lambda(1405) region are
strikingly different for the Sigma+pi-, Sigma0 pi0, and Sigma- pi+ decay
channels, indicating the effect of isospin interference, especially at W values
close to the threshold.Comment: 21 pages, 18 figures, to be submitted to PRC, revised to include a
missing referenc
Spectroscopy of Lambda-9Li by electroproduction
In the absence of accurate data on the free two-body hyperon-nucleon
interaction, the spectra of hypernuclei can provide information on the details
of the effective hyperon-nucleon interaction. Electroproduction of the
hypernucleus Lambda-9Li has been studied for the first time with sub-MeV energy
resolution in Hall A at Jefferson Lab on a 9Be target. In order to increase the
counting rate and to provide unambiguous kaon identification, two
superconducting septum magnets and a Ring Imaging CHerenkov detector (RICH)
were added to the Hall A standard equipment. The cross section to low-lying
states of Lambda-9Li is concentrated within 3 MeV of the ground state and can
be fitted with four peaks. The positions of the doublets agree with theory
while a disagreement could exist with respect to the relative strengths of the
peaks in the doublets. A Lambda separation energy of 8.36 +- 0.08 (stat.) +-
0.08 (syst.) MeV was measured, in agreement with an earlier experiment.Comment: 14 pages, 8 figure