10,615 research outputs found
Thermal Abundances of Heavy Particles
Matsumoto and Yoshimura [hep-ph/9910393] have argued that there are loop
corrections to the number density of heavy particles (in thermal equilibrium
with a gas of light particles) that are not Boltzmann suppressed by a factor of
e^(-M/T) at temperatures T well below the mass M of the heavy particle. We
argue, however, that their definition of the number density does not correspond
to a quantity that could be measured in a realistic experiment. We consider a
model where the heavy particles carry a conserved U(1) charge, and the light
particles do not. The fluctuations of the net charge in a given volume then
provide a measure of the total number of heavy particles in that same volume.
We show that these charge fluctuations are Boltzmann suppressed (to all orders
in perturbation theory). Therefore, we argue, the number density of heavy
particles is also Boltzmann suppressed.Comment: 9 pages, 1 figure; minor improvements in revised versio
Temperature Power Law of Equilibrium Heavy Particle Density
A standard calculation of the energy density of heavy stable particles that
may pair-annihilate into light particles making up thermal medium is performed
to second order of coupling, using the technique of thermal field theory. At
very low temperatures a power law of temperature is derived for the energy
density of the heavy particle. This is in sharp contrast to the exponentially
suppressed contribution estimated from the ideal gas distribution function. The
result supports a previous dynamical calculation based on the Hartree
approximation, and implies that the relic abundance of dark matter particles is
enhanced compared to that based on the Boltzmann equation.Comment: 12 pages, LATEX file with 6 PS figure
Topological Origin of Zero-Energy Edge States in Particle-Hole Symmetric Systems
A criterion to determine the existence of zero-energy edge states is
discussed for a class of particle-hole symmetric Hamiltonians. A ``loop'' in a
parameter space is assigned for each one-dimensional bulk Hamiltonian, and its
topological properties, combined with the chiral symmetry, play an essential
role. It provides a unified framework to discuss zero-energy edge modes for
several systems such as fully gapped superconductors, two-dimensional d-wave
superconductors, and graphite ribbons. A variants of the Peierls instability
caused by the presence of edges is also discussed.Comment: Completely rewritten. Discussions on coexistence of is- or
id_{xy}-wave order parameter near edges in d_{x^{2}-y^{2}}-wave
superconductors are added; 4 pages, 3 figure
Boltzmann Suppression of Interacting Heavy Particles
Matsumoto and Yoshimura have recently argued that the number density of heavy
particles in a thermal bath is not necessarily Boltzmann-suppressed for T << M,
as power law corrections may emerge at higher orders in perturbation theory.
This fact might have important implications on the determination of WIMP relic
densities. On the other hand, the definition of number densities in a
interacting theory is not a straightforward procedure. It usually requires
renormalization of composite operators and operator mixing, which obscure the
physical interpretation of the computed thermal average. We propose a new
definition for the thermal average of a composite operator, which does not
require any new renormalization counterterm and is thus free from such
ambiguities. Applying this definition to the model of Matsumoto and Yoshimura
we find that it gives number densities which are Boltzmann-suppressed at any
order in perturbation theory. We discuss also heavy particles which are
unstable already at T=0, showing that power law corrections do in general
emerge in this case.Comment: 7 pages, 5 figures. New section added, with the discussion of the
case of an unstable heavy particle. Version to appear on Phys. Rev.
Energy landscape and phase transitions in the self-gravitating ring model
We apply a recently proposed criterion for the existence of phase
transitions, which is based on the properties of the saddles of the energy
landscape, to a simplified model of a system with gravitational interactions,
referred to as the self-gravitating ring model. We show analytically that the
criterion correctly singles out the phase transition between a homogeneous and
a clustered phase and also suggests the presence of another phase transition,
not previously known. On the basis of the properties of the energy landscape we
conjecture on the nature of the latter transition
Intraoperative Neurophysiological Monitoring for Endoscopic Endonasal Approaches to the Skull Base: A Technical Guide.
Intraoperative neurophysiological monitoring during endoscopic, endonasal approaches to the skull base is both feasible and safe. Numerous reports have recently emerged from the literature evaluating the efficacy of different neuromonitoring tests during endonasal procedures, making them relatively well-studied. The authors report on a comprehensive, multimodality approach to monitoring the functional integrity of at risk nervous system structures, including the cerebral cortex, brainstem, cranial nerves, corticospinal tract, corticobulbar tract, and the thalamocortical somatosensory system during endonasal surgery of the skull base. The modalities employed include electroencephalography, somatosensory evoked potentials, free-running and electrically triggered electromyography, transcranial electric motor evoked potentials, and auditory evoked potentials. Methodological considerations as well as benefits and limitations are discussed. The authors argue that, while individual modalities have their limitations, multimodality neuromonitoring provides a real-time, comprehensive assessment of nervous system function and allows for safer, more aggressive management of skull base tumors via the endonasal route
Cooperative Jahn-Teller Effect and Electron-Phonon Coupling in
A classical model for the lattice distortions of \lax is derived and, in a
mean field approximation, solved. The model is based on previous work by
Kanamori and involves localized Mn d-electrons (which induce tetragonal
distortions of the oxygen octahedra surrounding the Mn) and localized holes
(which induce breathing distortions). Parameters are determined by fitting to
the room temperature structure of . The energy gained by formation of
a local lattice distortion is found to be large, most likely eV
per site, implying a strong electorn-phonon coupling and supporting polaronic
models of transport in the doped materials. The structural transition is shown
to be of the order-disorder type; the rapid x-dependence of the transition
temperature is argued to occur because added holes produce a "random" field
which misaligns the nearby sites.Comment: 24 pages. No figures. One Table. Late
On Witten multiple zeta-functions associated with semisimple Lie algebras IV
In our previous work, we established the theory of multi-variable Witten
zeta-functions, which are called the zeta-functions of root systems. We have
already considered the cases of types , , , and . In
this paper, we consider the case of -type. We define certain analogues of
Bernoulli polynomials of -type and study the generating functions of them
to determine the coefficients of Witten's volume formulas of -type. Next
we consider the meromorphic continuation of the zeta-function of -type and
determine its possible singularities. Finally, by using our previous method, we
give explicit functional relations for them which include Witten's volume
formulas.Comment: 22 pag
Numerical Renormalization Group Study of Kondo Effect in Unconventional Superconductors
Orbital degrees of freedom of a Cooper pair play an important role in the
unconventional superconductivity. To elucidate the orbital effect in the Kondo
problem, we investigated a single magnetic impurity coupled to Cooper pairs
with a () symmetry using the numerical
renormalization group method. It is found that the ground state is always a
spin doublet. The analytical solution for the strong coupling limit explicitly
shows that the orbital dynamics of the Cooper pair generates the spin 1/2 of
the ground state.Comment: 4 pages, 2 figures, JPSJ.sty, to be published in J. Phys. Soc. Jpn.
70 (2001) No. 1
Tomography increases key rates of quantum-key-distribution protocols
We construct a practically implementable classical processing for the BB84
protocol and the six-state protocol that fully utilizes the accurate channel
estimation method, which is also known as the quantum tomography. Our proposed
processing yields at least as high key rate as the standard processing by Shor
and Preskill. We show two examples of quantum channels over which the key rate
of our proposed processing is strictly higher than the standard processing. In
the second example, the BB84 protocol with our proposed processing yields a
positive key rate even though the so-called error rate is higher than the 25%
limit.Comment: 13 pages, 1 figure, REVTeX4. To be published in PRA. Version 2 adds
many references, a closed form key rate formula for unital channels, and a
procedure for the maximum likelihood channel estimatio
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