7,541 research outputs found
Quasi-Local Formulation of Non-Abelian Finite-Element Gauge Theory
Recently it was shown how to formulate the finite-element equations of motion
of a non-Abelian gauge theory, by gauging the free lattice difference
equations, and simultaneously determining the form of the gauge
transformations. In particular, the gauge-covariant field strength was
explicitly constructed, locally, in terms of a path ordered product of
exponentials (link operators). On the other hand, the Dirac and Yang-Mills
equations were nonlocal, involving sums over the entire prior lattice. Earlier,
Matsuyama had proposed a local Dirac equation constructed from just the
above-mentioned link operators. Here, we show how his scheme, which is closely
related to our earlier one, can be implemented for a non-Abelian gauge theory.
Although both Dirac and Yang-Mills equations are now local, the field strength
is not. The technique is illustrated with a direct calculation of the current
anomalies in two and four space-time dimensions. Unfortunately, unlike the
original finite-element proposal, this scheme is in general nonunitary.Comment: 19 pages, REVTeX, no figure
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Facilitation of transmitter release at squid synapses
Facilitation is shown to decay as a compound exponential with two
time constants (T1, T2) at both giant and non-giant synapses in squid steilate ganglia
bathed in solutions having low extracellular calcium concentrations ([Ca++]o).
Maximum values of facilitation (F~) were significantly larger, and T1 was significantly
smaller in giant than non-giant synapses. Decreases in [Ca++]o or increases in
[Mn++]o had variable effects on T1 and F1, whereas decreases in temperature
increased T~ but had insignificant effects on/'1. The growth of facilitation during
short trains of equal interval stimuli was adequately predicted by the linear
summation model developed by Mallart and Martin (1967.J. Physiol. (Lond.). 193:
676-694) for frog neuromuscular junctions. This result suggests that the underlying
mechanisms of facilitation are similar in squid and other synapses which release
many transmitter quanta.This work was supported by National Science Foundation research grant GB-36949, National
Research Council (Canada) and Grass Fellowships to Dr. Charlton, and a National Institutes of
Health career award (NS-00070) to Dr. Bittner.Neuroscienc
Relativistic Coulomb Resummation in QCD
A relativistic Coulomb-like resummation factor in QCD is suggested, based on
the solution of the quasipotential equation.Comment: 4 pages, 2 eps figures, REVTe
Casimir Energies and Pressures for -function Potentials
The Casimir energies and pressures for a massless scalar field associated
with -function potentials in 1+1 and 3+1 dimensions are calculated. For
parallel plane surfaces, the results are finite, coincide with the pressures
associated with Dirichlet planes in the limit of strong coupling, and for weak
coupling do not possess a power-series expansion in 1+1 dimension. The relation
between Casimir energies and Casimir pressures is clarified,and the former are
shown to involve surface terms. The Casimir energy for a -function
spherical shell in 3+1 dimensions has an expression that reduces to the
familiar result for a Dirichlet shell in the strong-coupling limit. However,
the Casimir energy for finite coupling possesses a logarithmic divergence first
appearing in third order in the weak-coupling expansion, which seems
unremovable. The corresponding energies and pressures for a derivative of a
-function potential for the same spherical geometry generalizes the TM
contributions of electrodynamics. Cancellation of divergences can occur between
the TE (-function) and TM (derivative of -function) Casimir
energies. These results clarify recent discussions in the literature.Comment: 16 pages, 1 eps figure, uses REVTeX
Energy conditions outside a dielectric ball
We show analytically that the vacuum electromagnetic stress-energy tensor
outside a ball with constant dielectric constant and permeability always obeys
the weak, null, dominant, and strong energy conditions. There are still no
known examples in quantum field theory in which the averaged null energy
condition in flat spacetime is violated.Comment: 12 pages, RevTex
Casimir energy, dispersion, and the Lifshitz formula
Despite suggestions to the contrary, we show in this paper that the usual
dispersive form of the electromagnetic energy must be used to derive the
Lifshitz force between parallel dielectric media. This conclusion follows from
the general form of the quantum vacuum energy, which is the basis of the
multiple-scattering formalism. As an illustration, we explicitly derive the
Lifshitz formula for the interaction between parallel dielectric semispaces,
including dispersion, starting from the expression for the total energy of the
system. The issues of constancy of the energy between parallel plates and of
the observability of electrostrictive forces are briefly addressed.Comment: 11 pages, no figure
Probing the helium-graphite interaction
Two separate lines of investigation have recently converged to produce a highly detailed picture of the
behavior of helium atoms physisorbed on graphite basal plane surfaces. Atomic beam scattering experiments
on single crystals have yielded accurate values for the binding energies of several· states for both (^4)He and (^3)He, as well as matrix elements of the largest Fourier component of the periodic part of the interaction potential.
From these data, a complete three-dimensional description of the potential has been constructed, and the
energy band structure of a helium atom moving in this potential calculated. At the same time, accurate
thermodynamic measurements were made on submonolayer helium films adsorbed on Grafoil. The binding
energy and low-coverage specific heat deduced from these measurements are in excellent agreement with
those calculated from the band structures
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