199 research outputs found
Angular intricacies in hot gauge field theories
It is argued that in hot gauge field theories, "Hard Thermal Loops" leading
order calculations call for a definite sequence of angular averages and
discontinuity (or Imaginary part prescription) operations, and run otherwise
into incorrect results. The ten years old collinear singularity problem of hot
, provides a striking illustration of that fate.Comment: 14 pages, 1 figur
De Sitter Waves and the Zero Curvature Limit
We show that a particular set of global modes for the massive de Sitter
scalar field (the de Sitter waves) allows to manage the group representations
and the Fourier transform in the flat (Minkowskian) limit. This is in
opposition to the usual acceptance based on a previous result, suggesting the
appearance of negative energy in the limit process. This method also confirms
that the Euclidean vacuum, in de Sitter spacetime, has to be preferred as far
as one wishes to recover ordinary QFT in the flat limit.Comment: 9 pages, latex no figure, to appear in Phys. Rev.
Approximate spectral functions in thermal field theory
Causality requires, that the (anti-) commutator of two interacting field
operators vanishes for space-like coordinate differences. This implies, that
the Fourier transform of the spectral function of this quantum field should
vanish in the space-like domain. We find, that this requirement imposes some
constraints on the use of resummed propagators in high temperature gauge
theory.Comment: 15 pages, latex, 3 figures included. Major rewriting of paper.
Available also at ftp://tpri6c.gsi.de/pub/phenning/h95comm/ or at
http://www.gsi.de/~phenning/pahtft.htm
Universality of low-energy scattering in (2+1) dimensions
We prove that, in (2+1) dimensions, the S-wave phase shift, , k
being the c.m. momentum, vanishes as either as . The constant is universal and .
This result is established first in the framework of the Schr\"odinger equation
for a large class of potentials, second for a massive field theory from proved
analyticity and unitarity, and, finally, we look at perturbation theory in
and study its relation to our non-perturbative result. The
remarkable fact here is that in n-th order the perturbative amplitude diverges
like as , while the full amplitude vanishes as . We show how these two facts can be reconciled.Comment: 23 pages, Late
Conformally invariant wave-equations and massless fields in de Sitter spacetime
Conformally invariant wave equations in de Sitter space, for scalar and
vector fields, are introduced in the present paper. Solutions of their wave
equations and the related two-point functions, in the ambient space notation,
have been calculated. The ``Hilbert'' space structure and the field operator,
in terms of coordinate independent de Sitter plane waves, have been defined.
The construction of the paper is based on the analyticity in the complexified
pseudo-Riemanian manifold, presented first by Bros et al.. Minkowskian limits
of these functions are analyzed. The relation between the ambient space
notation and the intrinsic coordinates is then studied in the final stage.Comment: 21 pages, LaTeX, some details adde
Mutant glycyl-tRNA synthetase (Gars) ameliorates SOD1G93A motor neuron degeneration phenotype but has little affect on Loa dynein heavy chain mutant mice
Background:
In humans, mutations in the enzyme glycyl-tRNA synthetase (GARS) cause motor and sensory axon loss in the peripheral nervous system, and clinical phenotypes ranging from Charcot-Marie-Tooth neuropathy to a severe infantile form of spinal muscular atrophy. GARS is ubiquitously expressed and may have functions in addition to its canonical role in protein synthesis through catalyzing the addition of glycine to cognate tRNAs.
Methodology/Principal findings:
We have recently described a new mouse model with a point mutation in the Gars gene resulting in a cysteine to arginine change at residue 201. Heterozygous Gars^{C201R/+} mice have locomotor and sensory deficits. In an investigation of genetic mutations that lead to death of motor and sensory neurons, we have crossed the Gars^{C201R/+} mice to two other mutants: the TgSOD1^{G93A} model of human amyotrophic lateral sclerosis and the Legs at odd angles mouse (Dync1h1^{Loa}) which has a defect in the heavy chain of the dynein complex. We found the Dync1h1^{Loa/+}; Gars^{C201R/+} double heterozygous mice are more impaired than either parent, and this is may be an additive effect of both mutations. Surprisingly, the Gars^{C201R} mutation significantly delayed disease onset in the SOD1^{G93A}; Gars^{C201R/+} double heterozygous mutant mice and increased lifespan by 29% on the genetic background investigated.
Conclusions/Significance:
These findings raise intriguing possibilities for the study of pathogenetic mechanisms in all three mouse mutant strains
de Sitter symmetry of Neveu-Schwarz spinors
We study the relations between Dirac fields living on the 2-dimensional
Lorentzian cylinder and the ones living on the double-covering of the
2-dimensional de Sitter manifold, here identified as a certain coset space of
the group . We show that there is an extended notion of de Sitter
covariance only for Dirac fields having the Neveu-Schwarz anti-periodicity and
construct the relevant cocycle. Finally, we show that the de Sitter symmetry is
naturally inherited by the Neveu-Schwarz massless Dirac field on the cylinder.Comment: 24 page
A Goldstone Theorem in Thermal Relativistic Quantum Field Theory
We prove a Goldstone Theorem in thermal relativistic quantum field theory,
which relates spontaneous symmetry breaking to the rate of space-like decay of
the two-point function. The critical rate of fall-off coincides with that of
the massless free scalar field theory. Related results and open problems are
briefly discussed
Fluctuation Operators and Spontaneous Symmetry Breaking
We develop an alternative approach to this field, which was to a large extent
developed by Verbeure et al. It is meant to complement their approach, which is
largely based on a non-commutative central limit theorem and coordinate space
estimates. In contrast to that we deal directly with the limits of -point
truncated correlation functions and show that they typically vanish for provided that the respective scaling exponents of the fluctuation
observables are appropriately chosen. This direct approach is greatly
simplified by the introduction of a smooth version of spatial averaging, which
has a much nicer scaling behavior and the systematic developement of Fourier
space and energy-momentum spectral methods. We both analyze the regime of
normal fluctuations, the various regimes of poor clustering and the case of
spontaneous symmetry breaking or Goldstone phenomenon.Comment: 30 pages, Latex, a more detailed discussion in section 7 as to
possible scaling behavior of l-point function
Large Nonreciprocal Propagation of Surface Acoustic Waves in Epitaxial Ferromagnetic/Semiconductor Hybrid Structures
Nonreciprocal propagation of sound, that is, the different transmission of acoustic waves traveling in opposite directions, is a challenging requirement for the realization of devices such as acoustic isolators and circulators. Here, we demonstrate efficient nonreciprocal transmission of surface acoustic waves (SAWs) propagating in opposite directions in a GaAs substrate coated with an epitaxial Fe3Si film. The nonreciprocity arises from the acoustic attenuation induced by the magnetoelastic (ME) interaction between the SAW strain field and spin waves in the ferromagnetic film, which depends on the SAW propagation direction and can be controlled via the amplitude and orientation of an external magnetic field. The acoustic-transmission nonreciprocity, defined as the difference between the transmitted acoustic powers for forward and backward propagation at the ME resonance, reaches values of up to 20%, which are, to our knowledge, the largest values of nonreciprocity reported for SAWs traveling in a semiconducting piezoelectric substrate covered by a ferromagnetic film. The experimental results are well accounted for by a model for the ME interaction, which also shows that the nonreciprocity can be further enhanced by optimization of the sample design. These results make Fe3Si/GaAs a promising platform for the realization of efficient nonreciprocal SAW devices
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