483 research outputs found
Resonances and higher twist in polarized lepton-nucleon scattering
We present a detailed analysis of resonance contributions in the context of
higher twist effects in the moments of the proton spin structure function g_1.
For each of these moments, it is found that there exists a characteristic Q^2
region in which (perturbative) higher twist corrections coexist with
(non-perturbative) resonance contribution of comparable magnitude.Comment: 17 pages LaTe
Nucleon Polarizibilities for Virtual Photons
We generalize the sum rules for the nucleon electric plus magnetic
polarizability and for the nucleon spin-polarizability
, to virtual photons with . The dominant low energy cross
sections are represented in our calculation by one-pion-loop graphs of
relativistic baryon chiral perturbation theory and the -resonance
excitation. For the proton we find good agreement of the calculated
with empirical values obtained from integrating up
electroproduction data for . The proton spin-polarizability
switches sign around and it joins smoothly the
"partonic" curve, extracted from polarized deep-inelastic scattering, around
. For the neutron our predictions of and
agree reasonably well at with existing determinations.
Upcoming (polarized) electroproduction experiments will be able to test the
generalized polarizability sum rules investigated here.Comment: 12 pages, 5 figures, submittes to Nuclear Physics
Deep electrical resistivity structure of northwestern Costa Rica
First long-period magnetotelluric investigations were conducted in early 2008 in northwestern Costa Rica, along a profile that extends from the coast of the Pacific Ocean, traverses the volcanic arc and ends currently at the Nicaraguan border. The aim of this study is to gain insight into the electrical resistivity structure and thus fluid distribution at the continental margin where the Cocos plate subducts beneath the Caribbean plate. Preliminary two-dimensional models map the only moderately resistive mafic/ultramafic complexes of the Nicoya Peninsula (resistivity of a few hundred Ωm), the conductive forearc and the backarc basins (several Ωm). Beneath the backarc basin the data image a poor conductor in the basement with a clear termination in the south, which may tentatively be interpreted as the Santa Elena Suture. The volcanic arc shows no pronounced anomaly at depth, but a moderate conductor underlies the backarc with a possible connection to the upper mantle. A conductor at deep-crustal levels in the forearc may reflect fluid release from the downgoing slab
On astrophysical solution to ultra high energy cosmic rays
We argue that an astrophysical solution to UHECR problem is viable. The
pectral features of extragalactic protons interacting with CMB are calculated
in model-independent way. Using the power-law generation spectrum as the only assumption, we analyze four features of the proton
spectrum: the GZK cutoff, dip, bump and the second dip. We found the dip,
induced by electron-positron production on CMB, as the most robust feature,
existing in energy range eV. Its shape is
stable relative to various phenomena included in calculations. The dip is well
confirmed by observations of AGASA, HiRes, Fly's Eye and Yakutsk detectors. The
best fit is reached at , with the allowed range 2.55 - 2.75. The
dip is used for energy calibration of the detectors. After the energy
calibration the fluxes and spectra of all three detectors agree perfectly, with
discrepancy between AGASA and HiRes at eV being not
statistically significant. The agreement of the dip with observations should be
considered as confirmation of UHE proton interaction with CMB. The dip has two
flattenings. The high energy flattening at eV
automatically explains ankle. The low-energy flattening at eV provides the transition to galactic cosmic rays. This transition is
studied quantitatively. The UHECR sources, AGN and GRBs, are studied in a
model-dependent way, and acceleration is discussed. Based on the agreement of
the dip with existing data, we make the robust prediction for the spectrum at
eV to be measured in the nearest future by
Auger detector.Comment: Revised version as published in Phys.Rev. D47 (2006) 043005 with a
small additio
THE EXTERNAL ELECTRON BEAM OF THE CAMBRIDGE ELECTRON ACCELERATOR
The resonant beam extraction system of Cambridge Synchrotron is described, and measurements of the extraction efficiency and extracted beam properties are compared with values predicted by calculations. (auth
De Haas-van Alphen effect and Fermi surface properties of single crystal CrB2
We report the angular dependence of three distinct de Haas-van Alphen (dHvA)
frequencies of the torque magnetization in the itinerant antiferromagnet CrB2
at temperatures down to 0.3K and magnetic fields up to 14T. Comparison with the
calculated Fermi surface of nonmagnetic CrB2 suggests that two of the observed
dHvA oscillations arise from electron-like Fermi surface sheets formed by bands
with strong B-px,y character which should be rather insensitive to exchange
splitting. The measured effective masses of these Fermi surface sheets display
strong enhancements of up to a factor of two over the calculated band masses
which we attribute to electron-phonon coupling and electronic correlations. For
the temperature and field range studied, we do not observe signatures
reminiscent of the heavy d-electron bands expected for antiferromagnetic CrB2.
In view that the B-p bands are at the heart of conventional high-temperature
superconductivity in the isostructural MgB2, we consider possible implications
of our findings for nonmagnetic CrB2 and an interplay of itinerant
antiferromagnetism with superconductivity.Comment: 8 pages, 4 figure
Optimizing SUV Analysis: A Multicenter Study on Preclinical FDG-PET/CT Highlights the Impact of Standardization
PURPOSE: Preclinical imaging, with translational potential, lacks a standardized method for defining volumes of interest (VOIs), impacting data reproducibility. The aim of this study was to determine the interobserver variability of VOI sizes and standard uptake values (SUV mean and SUV max) of different organs using the same [ 18F]FDG-PET and PET/CT datasets analyzed by multiple observers. In addition, the effect of a standardized analysis approach was evaluated. PROCEDURES: In total, 12 observers (4 beginners and 8 experts) analyzed identical preclinical [ 18F]FDG-PET-only and PET/CT datasets according to their local default image analysis protocols for multiple organs. Furthermore, a standardized protocol was defined, including detailed information on the respective VOI size and position for multiple organs, and all observers reanalyzed the PET/CT datasets following this protocol. RESULTS: Without standardization, significant differences in the SUV mean and SUV max were found among the observers. Coregistering CT images with PET images improved the comparability to a limited extent. The introduction of a standardized protocol that details the VOI size and position for multiple organs reduced interobserver variability and enhanced comparability. CONCLUSIONS: The protocol offered clear guidelines and was particularly beneficial for beginners, resulting in improved comparability of SUV mean and SUV max values for various organs. The study suggested that incorporating an additional VOI template could further enhance the comparability of the findings in preclinical imaging analyses. </p
Quark model approach to the meson electroproduction on the proton
A recently developed quark model approach to pseudoscalar meson
photoproduction is extended to electroproduction process for the meson
in the kinematics of momentum transfer 4 (GeV/c) and total
center of mass energy 1.6 GeV. Existing data are well reproduced and
the roles of the and resonances are closely
investigated. In the study of the longitudinal excitation of the
resonance, a reliable constraint on the properties is obtained
by cleanly removing the electromagnetic transition from the amplitude. Thus, the fitted quantities can be
determined with an uncertainty of about 15%. This could be the first direct
constraint on the properties in theory.Comment: 31 pages, 9 figures; Substantially modified version, to appear on
Jour. of Phys.
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