666 research outputs found
Optical second harmonic generation probe of two-dimensional ferroelectricity
Optical second harmonic generation (SHG) is used as a noninvasive probe of
two-dimensional (2D) ferroelectricity in Langmuir-Blodgett (LB) films of
copolymer vinylidene fluoride with trifluorethylene. The surface 2D
ferroelectric-paraelectric phase transition in the topmost layer of LB films
and a thickness independent (almost 2D) transition in the bulk of these films
are observed in temperature studies of SHG.Comment: 9 pages, 2 figures, Optics Letters, in prin
A complementary compact laser based neutron source
Several experiments of neutron generation using high intensity laser sources,
with a power exceeding 10^19W/cm^2 via TNSA (Target Normal Sheath Acceleration)
or other similar methods, have been performed in the past years in different
laboratories. However, so far there is no one running neutron source based on
such a technology. In the framework of the Conceptual Report Design of a new
accelerator in the Eupraxia project we are studying the possibility to have a
laser-based neutron source, not only by TNSA but also from self-injection
schemes. We focus our attention on the applications in cultural heritage
studies as well also on the complementary role that such a source can have in
the framework of large facilities devoted to radiation production.Comment: 4 pages, two figures, 3rd European Advanced Accelerators Concept
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A nonhuman primate model of inherited retinal disease.
Inherited retinal degenerations are a common cause of untreatable blindness worldwide, with retinitis pigmentosa and cone dystrophy affecting approximately 1 in 3500 and 1 in 10,000 individuals, respectively. A major limitation to the development of effective therapies is the lack of availability of animal models that fully replicate the human condition. Particularly for cone disorders, rodent, canine, and feline models with no true macula have substantive limitations. By contrast, the cone-rich macula of a nonhuman primate (NHP) closely mirrors that of the human retina. Consequently, well-defined NHP models of heritable retinal diseases, particularly cone disorders that are predictive of human conditions, are necessary to more efficiently advance new therapies for patients. We have identified 4 related NHPs at the California National Primate Research Center with visual impairment and findings from clinical ophthalmic examination, advanced retinal imaging, and electrophysiology consistent with achromatopsia. Genetic sequencing confirmed a homozygous R565Q missense mutation in the catalytic domain of PDE6C, a cone-specific phototransduction enzyme associated with achromatopsia in humans. Biochemical studies demonstrate that the mutant mRNA is translated into a stable protein that displays normal cellular localization but is unable to hydrolyze cyclic GMP (cGMP). This NHP model of a cone disorder will not only serve as a therapeutic testing ground for achromatopsia gene replacement, but also for optimization of gene editing in the macula and of cone cell replacement in general
Search for a new gauge boson in the Experiment (APEX)
We present a search at Jefferson Laboratory for new forces mediated by
sub-GeV vector bosons with weak coupling to electrons. Such a
particle can be produced in electron-nucleus fixed-target scattering and
then decay to an pair, producing a narrow resonance in the QED trident
spectrum. Using APEX test run data, we searched in the mass range 175--250 MeV,
found no evidence for an reaction, and set an upper limit of
. Our findings demonstrate that fixed-target
searches can explore a new, wide, and important range of masses and couplings
for sub-GeV forces.Comment: 5 pages, 5 figures, references adde
Hard Photodisintegration of a Proton Pair
We present a study of high energy photodisintegration of proton-pairs through
the gamma + 3He -> p+p+n channel. Photon energies from 0.8 to 4.7 GeV were used
in kinematics corresponding to a proton pair with high relative momentum and a
neutron nearly at rest. The s-11 scaling of the cross section, as predicted by
the constituent counting rule for two nucleon photodisintegration, was observed
for the first time. The onset of the scaling is at a higher energy and the
cross section is significantly lower than for deuteron (pn pair)
photodisintegration. For photon energies below the scaling region, the scaled
cross section was found to present a strong energy-dependent structure not
observed in deuteron photodisintegration.Comment: 7 pages, 3 figures, for submission to Phys. Lett.
Electroexcitation of the at low momentum transfer
We report on new p measurements at the
resonance at the low momentum transfer region. The mesonic
cloud dynamics is predicted to be dominant and rapidly changing in this
kinematic region offering a test bed for chiral effective field theory
calculations. The new data explore the low dependence of the resonant
quadrupole amplitudes while extending the measurements of the Coulomb
quadrupole amplitude to the lowest momentum transfer ever reached. The results
disagree with predictions of constituent quark models and are in reasonable
agreement with dynamical calculations that include pion cloud effects, chiral
effective field theory and lattice calculations. The reported measurements
suggest that improvement is required to the theoretical calculations and
provide valuable input that will allow their refinements
Low Q^2 measurements of the proton form factor ratio
We present an updated extraction of the proton electromagnetic form factor
ratio, mu_p G_E/G_M, at low Q^2. The form factors are sensitive to the spatial
distribution of the proton, and precise measurements can be used to constrain
models of the proton. An improved selection of the elastic events and reduced
background contributions yielded a small systematic reduction in the ratio mu_p
G_E/G_M compared to the original analysis.Comment: 12 pages, 5 figures, archival paper for proton form factor extraction
from Jefferson Lab "LEDEX" experimen
The Proton Elastic Form Factor Ratio at Low Momentum Transfer
High precision measurements of the proton elastic form factor ratio have been
made at four-momentum transfers, Q^2, between 0.2 and 0.5 GeV^2. The new data,
while consistent with previous results, clearly show a ratio less than unity
and significant differences from the central values of several recent
phenomenological fits. By combining the new form-factor ratio data with an
existing cross-section measurement, one finds that in this Q^2 range the
deviation from unity is primarily due to GEp being smaller than the dipole
parameterization.Comment: 5 pages, 2 figure
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