19,582 research outputs found
The UV behavior of Gravity at Large N
A first step in the analysis of the renormalizability of gravity at Large N
is carried on. Suitable resummations of planar diagrams give rise to a theory
in which there is only a finite number of primitive superficially divergent
Feynman diagrams. The mechanism is similar to the the one which makes
renormalizable the 3D Gross-Neveu model at large N. Some potential problems in
fulfilling the Slavnov-Taylor and the Zinn-Justin equations are also pointed
out.Comment: 17 pages, 9 figures. To appear on Phys. Rev. D. Two more references,
further technical details and the discussion of the KLT relations at large N
have been include
Comparison of liquid-metal magnetohydrodynamic power conversion cycles
Comparison of liquid metal magnetohydrodynamic power conversion cycle
Convergence of density-matrix expansions for nuclear interactions
We extend density-matrix expansions in nuclei to higher orders in derivatives
of densities and test their convergence properties. The expansions allow for
converting the interaction energies characteristic to finite- and short-range
nuclear effective forces into quasi-local density functionals. We also propose
a new type of expansion that has excellent convergence properties when
benchmarked against the binding energies obtained for the Gogny interaction.Comment: 4 pages, 3 figure
Hadron Structure in the Non-Perturbative Regime of QCD: Isospin Symmetry and its Violation
I discuss recent progress made in calculating electromagnetic corrections in
the framework of the effective field theory of QCD. In the case of elastic
pion-pion scattering, strong interaction predictions have been worked out to
two loop accuracy. I present first results for the electromagnetic corrections
in the case of neutral pions. Here, the only sizeable effect comes from the
charged to neutral pion mass difference. In the presence of nucleons, isospin
violation can be measured in threshold pion photoproduction. I review the
present status of the theoretical predictions and the experimental data. I
argue that a deeper understanding of isospin violation based on a more precise
study of such reactions can be achieved.Comment: 10 pp, LaTeX file, 3 figures, uses espcrc1.sty and epsf, plenary
talk, QULEN '97, Osaka, May 1997, to be published in the proceeding
Development of improved amorphous materials for laser systems
Crystallization calculations were performed in order to determine the possibility of forming a particular type of laser glass with the avoidance of devitrification in an outer space laboratory. It was demonstrated that under the homogenuous nucleating conditions obtainable in a zero gravity laboratory this laser glass may be easily quenched to a virtually crystal-free product. Experimental evidence is provided that use of this material as a host in a neodymium glass laser would result in more than a 10 percent increase in efficiency when compared to laser glass rods of a similar composition currently commercially available. Differential thermal analysis, thermal gradient oven, X-ray diffraction, and liquidus determination experiments were carried out to determine the basics of the crystallization behavior of the glass, and small-angle X-ray scattering and splat-cooling experiments were performed in order to provide additional evidence for the feasibility of producing this laser glass material, crystal free, in an outer space environment
Cosmological Density Perturbations with a Scale-Dependent Newton's G
We explore possible cosmological consequences of a running Newton's constant
, as suggested by the non-trivial ultraviolet fixed point
scenario in the quantum field-theoretic treatment of Einstein gravity with a
cosmological constant term. In particular we focus here on what possible
effects the scale-dependent coupling might have on large scale cosmological
density perturbations. Starting from a set of manifestly covariant effective
field equations derived earlier, we systematically develop the linear theory of
density perturbations for a non-relativistic, pressure-less fluid. The result
is a modified equation for the matter density contrast, which can be solved and
thus provides an estimate for the growth index parameter in the
presence of a running . We complete our analysis by comparing the fully
relativistic treatment with the corresponding results for the non-relativistic
(Newtonian) case, the latter also with a weakly scale dependent .Comment: 54 pages, 4 figure
Aspects of Nucleon Chiral Perturbation Theory
I review recent progress made in the calculation of nucleon properties in the
framework of heavy baryon CHPT. Topics include: Compton scattering,
scattering, the anatomy of a low-energy constant and the induced pseudoscalar
form factor.Comment: plain TeX (macro included), 12pp, lecture delivered at the workshop
on "Chiral Dynamics: Theory and Experiments", MIT, July 25-29, 199
Effective Nonlocal Euclidean Gravity
A nonlocal form of the effective gravitational action could cure the
unboundedness of euclidean gravity with Einstein action. On sub-horizon length
scales the modified gravitational field equations seem compatible with all
present tests of general relativity and post-Newtonian gravity. They induce a
difference in the effective Newton's constant between regions of space with
vanishing or nonvanishing curvature scalar (or Ricci tensor). In cosmology they
may lead to a value for the critical density after inflation. The
simplest model considered here appears to be in conflict with nucleosynthesis,
but generalizations consistent with all cosmological observations seem
conceivable.Comment: 12 pages, LaTe
Growth of covariant perturbations in the contracting phase of a bouncing universe
In this paper we examine the validity of the linear perturbation theory near
a bounce in the covariant analysis. Some linearity parameters are defined to
set up conditions for a linear theory. Linear evolution of density perturbation
and gravitational waves have been computed previously. We have calculated the
vector and scalar induced parts of the shear tensor. For radiationlike and
dustlike single fluid dominated collapsing Friedmann-Lemaitre-Robertson-Walker
background it is shown that the linearity conditions are not satisfied near a
bounce.Comment: 9 pages, final versio
Higgs-Flavor Groups, Naturalness, and Dark Matter
In the absence of low-energy supersymmetry, a multiplicity of weak-scale
Higgs doublets would require additional fine-tunings unless they formed an
irreducible multiplet of a non-abelian symmetry. Remnants of such symmetry
typically render some Higgs fields stable, giving several dark matter particles
of various masses. The non-abelian symmetry also typically gives simple,
testable mass relations.Comment: Some comments added after Eqs. (2) and (12
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