1,933 research outputs found
Pionic Content of Rho-N-N and Rho-N-Delta Vertex Functions
The dynamical content of rho-N-N and rho-N-Delta vertex functions is studied
with a mesonic model. A set of coupled integral equations satisfied by these
vertex functions were solved self-consistently. These soulutions indicate that
the dominant mesonic content arises from di-pion dynamics. With the
experimentally determined pion-baryon-baryon coupling constants and ranges as
input, the model predicts a g_{\rho NN} that agrees with the
meson-exchange-potential results. On the other hand, it predicts a smaller
f_{\rho N\Delta} and much softer form factors. Implications of the findings on
the use of phenomenological coupling constants in nuclear reaction studies are
discussed.Comment: 9 pages, 2 figures will be furnished upon request; LA-UR-94-126
Extended state floating up in a lattice model: Bona fide levitation fingerprints, irrespective of the correlation length
The evolution of extended states with magnetic field and disorder intensities
is investigated for 2D lattice models. The floating-up picture is revealed when
the shift of the extended state, relative to the density of states, is properly
taken into account, either for white-noise or correlated disorder.Comment: 4 pages, 4 figures. EP2DS-14 Pragu
CollapsABEL: An R library for detecting compound heterozygote alleles in genome-wide association studies
Background: Compound Heterozygosity (CH) in classical genetics is the presence of two different recessive mutations at a particular gene locus. A relaxed form of CH alleles may account for an essential proportion of the missing heritability, i.e. heritability of phenotypes so far not accounted for by single genetic variants. Methods to detect CH-like effects in genome-wide association studies (GWAS) may facilitate explaining the missing heritability, but to our knowledge no viable software tools for this purpose are currently available. Results: In this work we present the Generalized Compound Double Heterozygosity (GCDH) test and its implementation in the R package CollapsABEL. Time-consuming procedures are optimized for computational efficiency using Java or C++. Intermediate results are stored either in an SQL database or in a so-called big.matrix file to achieve reasonable memory footprint. Our large scale simulation studies show that GCDH is capable of discovering genetic associations due to CH-like interactions with much higher power than a conventional single-SNP approach under various settings, whether the causal genetic variations are available or not. CollapsABEL provides a user-friendly pipeline for genotype collapsing, statistical testing, power estimation, type I error control and graphics generation in the R language. Conclusions: CollapsABEL provides a computationally efficient solution for screening general forms of CH alleles in densely imputed microarray or whole genome sequencing datasets. The GCDH test provides an improved power over single-SNP based methods in detecting the prevalence of CH in human complex phenotypes, offering an opportunity for tackling the missing heritability problem. Binary and source packages of CollapsABEL are available on CRAN (https://cran.r-project.org/web/packages/CollapsABEL) and the website of the GenABEL project (http://www.genabel.org/packages)
Pion dispersion relation at finite density and temperature
We study the behavior of the pion dispersion relation in a pion medium at
finite density and temperature. We introduce a pion chemical potential to
describe the finite pion number density and argue that such description is
valid during the hadronic phase of a relativistic heavy-ion collision between
chemical and thermal freeze-out. We make use of an effective Lagrangian that
explicitly respects chiral symmetry through the enforcement of the chiral Ward
identities. The pion dispersion relation is computed through the computation of
the pion self-energy in a non-perturbative fashion by giving an approximate
solution to the Schwinger-Dyson equation for this self-energy. The dispersion
relation is described in terms of a density and temperature dependent mass and
an index of refraction which is also temperature, density as well as momentum
dependent. The index of refraction is larger than unity for all values of the
momentum for finite and . We conclude by exploring some of the
possible consequences for the propagation of pions through the boundary between
the medium and vacuum.Comment: 7 pages, 5 figures, 3 new references, published versio
Asymptotics for turbulent flame speeds of the viscous G-equation enhanced by cellular and shear flows
G-equations are well-known front propagation models in turbulent combustion
and describe the front motion law in the form of local normal velocity equal to
a constant (laminar speed) plus the normal projection of fluid velocity. In
level set formulation, G-equations are Hamilton-Jacobi equations with convex
( type) but non-coercive Hamiltonians. Viscous G-equations arise from
either numerical approximations or regularizations by small diffusion. The
nonlinear eigenvalue from the cell problem of the viscous G-equation
can be viewed as an approximation of the inviscid turbulent flame speed .
An important problem in turbulent combustion theory is to study properties of
, in particular how depends on the flow amplitude . In this
paper, we will study the behavior of as at
any fixed diffusion constant . For the cellular flow, we show that
Compared with the inviscid G-equation (), the diffusion dramatically slows
down the front propagation. For the shear flow, the limit
\nit where
is strictly decreasing in , and has zero derivative at .
The linear growth law is also valid for of the curvature dependent
G-equation in shear flows.Comment: 27 pages. We improve the upper bound from no power growth to square
root of log growt
An Attempt to Study T=2 States in 16-N
This research was sponsored by the National Science Foundation Grant NSF PHY 87-1440
Pauli blocking and final-state interaction in electron-nucleus quasielastic scattering
The nucleon final-state interaction in inclusive electron-nucleus
quasielastic scattering is studied. Based on the unitarity equation satisfied
by the scattering-wave operators, a doorway model is developed to take into
account the final-state interaction including the Pauli blocking of nucleon
knockout. The model uses only experimental form factors as the input and can be
readily applied to light- and medium-mass nuclei. Pauli blocking effects in
these latter nuclei are illustrated with the case of the Coulomb interaction.
Significant effects are noted for beam energies below ~ 350 MeV and for low
momentum transfers.Comment: 16 pages, 6 figure
Superradiance of low density Frenkel excitons in a crystal slab of three-level atoms: Quantum interference effect
We systematically study the fluorescence of low density Frenkel excitons in a
crystal slab containing V-type three-level atoms. Based on symmetric
quasi-spin realization of SU(3) in large limit, the two-mode exciton
operators are invoked to depict various collective excitations of the
collection of these V-type atoms starting from their ground state. By making
use of the rotating wave approximation, the light intensity of radiation for
the single lattice layer is investigated in detail. As a quantum coherence
effect, the quantum beat phenomenon is discussed in detail for different
initial excitonic states. We also test the above results analytically without
the consideration of the rotating wave approximation and the self-interaction
of radiance field is also included.Comment: 18pages, 17 figures. Resubmit to Phys. Rev.
eta N S-wave scattering length in a three coupled channel, multiresonance, unitary model
The S-wave scattering length for eta-N elastic scattering is extracted from
the S-wave T-matrix in a three coupled channel, multiresonance unitary model.
Results are compared with values already reported in literature which are
obtained applying multichannel, but single resonance -- no background models. A
dispersion among the previously published values of the real part of the S-wave
scattering length is observed. We demonstrate that the reported spread
originates from the strong sensitivity of the scattering length upon the small
variation of the used input resonance parameters. In addition, we show that
eta-N scattering length value obtained in single resonance -- no background
models significantly increases if background term is added in a unitary way. We
question the reliability of previously reported values based only on the single
resonance -- no background models, and demonstrate that the value of the eta-N
S-wave scattering length obtained in this publication is much more realistic
because of the multiresonance and unitary approach.Comment: revtex, 20 pages + 3 figures (PostScript: gzip + uuencode) included,
submitted to Phys. Rev. C, brief Reports
Quasi-Periodic Releases of Streamer Blobs and Velocity Variability of the Slow Solar Wind near the Sun
We search for persistent and quasi-periodic release events of streamer blobs
during 2007 with the Large Angle Spectrometric Coronagraph on the \textit{Solar
and Heliospheric Observatory} and assess the velocity of the slow solar wind
along the plasma sheet above the corresponding streamer by measuring the
dynamic parameters of blobs. We find 10 quasi-periodic release events of
streamer blobs lasting for three to four days. In each day of these events, we
observe three-five blobs. The results are in line with previous studies using
data observed near the last solar minimum. Using the measured blob velocity as
a proxy for that of the mean flow, we suggest that the velocity of the
background slow solar wind near the Sun can vary significantly within a few
hours. This provides an observational manifestation of the large velocity
variability of the slow solar wind near the Sun.Comment: 14 pages, 5 figures, accepted by Soalr Physic
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