917 research outputs found
Higgs Mass in the Standard Model from Coupling Constant Reduction
Plausible interrelations between parameters of the standard model are
studied. The empirical value of the top quark mass, when used in the
renormalization group equations, suggests that the ratio of the colour SU(3)
gauge coupling , and the top coupling is independent of the
renormalization scale. On the other hand, variety of top-condensate models
suggest that the Higgs self-coupling is proportional to .
Invoking the requirement that the ratio is independent of
the renormalization scale , fixes the Higgs mass. The pole mass of the Higgs
[which differs from the renormalization group mass by a few percent] is found
to be GeV for the one-loop equations and GeV for the
two-loop equations.Comment: 17 pages RevTeX including 7 figure
Metal-nanoparticle single-electron transistors fabricated using electromigration
We have fabricated single-electron transistors from individual metal
nanoparticles using a geometry that provides improved coupling between the
particle and the gate electrode. This is accomplished by incorporating a
nanoparticle into a gap created between two electrodes using electromigration,
all on top of an oxidized aluminum gate. We achieve sufficient gate coupling to
access more than ten charge states of individual gold nanoparticles (5-15 nm in
diameter). The devices are sufficiently stable to permit spectroscopic studies
of the electron-in-a-box level spectra within the nanoparticle as its charge
state is varied.Comment: 3 pages, 3 figures, submitted to AP
Qualitative and Quantitative Detection of Chlamydophila pneumoniae DNA in Cerebrospinal Fluid from Multiple Sclerosis Patients and Controls
A standardized molecular test for the detection of Chlamydophila pneumoniae DNA in cerebrospinal fluid (CSF) would assist the further assessment of the association of C. pneumoniae with multiple sclerosis (MS). We developed and validated a qualitative colorimetric microtiter plate-based PCR assay (PCR-EIA) and a real-time quantitative PCR assay (TaqMan) for detection of C. pneumoniae DNA in CSF specimens from MS patients and controls. Compared to a touchdown nested-PCR assay, the sensitivity, specificity, and concordance of the PCR-EIA assay were 88.5%, 93.2%, and 90.5%, respectively, on a total of 137 CSF specimens. PCR-EIA presented a significantly higher sensitivity in MS patients (p = 0.008) and a higher specificity in other neurological diseases (p = 0.018). Test reproducibility of the PCR-EIA assay was statistically related to the volumes of extract DNA included in the test (p = 0.033); a high volume, which was equivalent to 100 µl of CSF per reaction, yielded a concordance of 96.8% between two medical technologists running the test at different times. The TaqMan quantitative PCR assay detected 26 of 63 (41.3%) of positive CSF specimens that tested positive by both PCR-EIA and nested-PCR qualitative assays. None of the CSF specimens that were negative by the two qualitative PCR methods were detected by the TaqMan quantitative PCR. The PCR-EIA assay detected a minimum of 25 copies/ml C. pneumoniae DNA in plasmid-spiked CSF, which was at least 10 times more sensitive than TaqMan. These data indicated that the PCR-EIA assay possessed a sensitivity that was equal to the nested-PCR procedures for the detection of C. pneumoniae DNA in CSF. The TaqMan system may not be sensitive enough for diagnostic purposes due to the low C. pneumoniae copies existing in the majority of CSF specimens from MS patients
The derivative of the topological susceptibility at zero momentum and an estimate of mass in the chiral limit
The anomaly-anomaly correlator is studied using QCD sum rules. Using the
matrix elements of anomaly between vacuum and pseudoscalars and
, the derivative of correlator is evaluated and found to be
GeV. Assuming that has no
significant dependence on quark masses, the mass of in the chiral limit
is found to be 723 MeV. The same calculation also yields for the
singlet pseudoscalar decay constant in the chiral limit a value of MeV.Comment: LaTeX, 7 pages, 2 figures, uses cernrep.cls (included
Spin splitting and Kondo effect in quantum dots coupled to noncollinear ferromagnetic leads
We study the Kondo effect in a quantum dot coupled to two noncollinear
ferromagnetic leads. First, we study the spin splitting
of an energy level
in the quantum dot by tunnel couplings to the ferromagnetic leads, using the
Poor man's scaling method. The spin splitting takes place in an intermediate
direction between magnetic moments in the two leads. , where is the spin
polarization in the leads, is the angle between the magnetic moments,
and is an asymmetric factor of tunnel barriers (). Hence the spin
splitting is always maximal in the parallel alignment of two ferromagnets
() and minimal in the antiparallel alignment (). Second,
we calculate the Kondo temperature . The scaling calculation
yields an analytical expression of as a function of
and , , when .
is a decreasing function with respect to
. When is
relevant, we evaluate using the
slave-boson mean-field theory. The Kondo resonance is split into two by finite
, which results in the spin accumulation in the quantum dot and
suppression of the Kondo effect.Comment: 11 pages, 8 figures, revised versio
Axial Vector Current Matrix Elements and QCD Sum Rules
The matrix element of the isoscalar axial vector current,
, between nucleon
states is computed using the external field QCD sum rule method. The external
field induced correlator, , is calculated
from the spectrum of the isoscalar axial vector meson states. Since it is
difficult to ascertain, from QCD sum rule for hyperons, the accuracy of
validity of flavour SU(3) symmetry in hyperon decays when strange quark mass is
taken into account, we rely on the empirical validity of Cabbibo theory to
dertermine the matrix element between
nucleon states. Combining with our calculation of and the well known nucleon -decay
constant allows us to determine
occuring in the Bjorken sum rule. The result is in reasonable agreement with
experiment. We also discuss the role of the anomaly in maintaining flavour
symmetry and validity of OZI rule.Comment: 8 pages, 4 figures, revtex
Mechanically-adjustable and electrically-gated single-molecule transistors
We demonstrate a device geometry for single-molecule electronics experiments
that combines both the ability to adjust the spacing between the electrodes
mechanically and the ability to shift the energy levels in the molecule using a
gate electrode. With the independent in-situ variations of molecular properties
provided by these two experimental "knobs", we are able to achieve a much more
detailed characterization of electron transport through the molecule than is
possible with either technique separately. We illustrate the devices'
performance using C60 molecules.Comment: 15 pages, 3 figure
A Fractional Viscoelastic Model Of The Axon In Brain White Matter
Traumatic axonal injury occurs when loads experienced on the tissue-scale are
transferred to the individual axons. Mechanical characterization of axon
deformation especially under dynamic loads however is extremely difficult owing
to their viscoelastic properties. The viscoelastic characterization of axon
properties that are based on interpretation of results from in-vivo brain
Magnetic Resonance Elastography (MRE) are dependent on the specific frequencies
used to generate shear waves with which measurements are made. In this study,
we aim to develop a fractional viscoelastic model to characterize the time
dependent behavior of the properties of the axons in a composite white matter
(WM) model. The viscoelastic powerlaw behavior observed at the tissue level is
assumed to exist across scales, from the continuum macroscopic level to that of
the microstructural realm of the axons. The material parameters of the axons
and glia are fitted to a springpot model. The 3D fractional viscoelastic
springpot model is implemented within a finite element framework. The
constitutive equations defining the fractional model are coded using a
vectorized user defined material (VUMAT) subroutine in ABAQUS finite element
software. Using this material characterization, representative volume elements
(RVE) of axons embedded in glia with periodic boundary conditions are developed
and subjected to a relaxation displacement boundary condition. The homogenized
orthotropic fractional material properties of the axon-matrix system as a
function of the volume fraction of axons in the ECM are extracted by solving
the inverse problem.Comment: Accepted for publication at the 12th International Conference on
Mathematical Modeling in Physical Science
Nonequilibrium Kondo Effect in a Quantum Dot Coupled to Ferromagnetic Leads
We study the Kondo effect in the electron transport through a quantum dot
coupled to ferromagnetic leads, using a real-time diagrammatic technique which
provides a systematic description of the nonequilibrium dynamics of a system
with strong local electron correlations. We evaluate the theory in an extension
of the `resonant tunneling approximation', introduced earlier, by introducing
the self-energy of the off-diagonal component of the reduced propagator in spin
space. In this way we develop a charge and spin conserving approximation that
accounts not only for Kondo correlations but also for the spin splitting and
spin accumulation out of equilibrium. We show that the Kondo resonances, split
by the applied bias voltage, may be spin polarized. A left-right asymmetry in
the coupling strength and/or spin polarization of the electrodes significantly
affects both the spin accumulation and the weight of the split Kondo resonances
out of equilibrium. The effects are observable in the nonlinear differential
conductance. We also discuss the influence of decoherence on the Kondo
resonance in the frame of the real-time formulation.Comment: 13 pages, 13 figure
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