13,415 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
Bi-partite entanglement entropy in massive two-dimensional quantum field theory
Recently, Cardy, Castro Alvaredo and the author obtained the first
exponential correction to saturation of the bi-partite entanglement entropy at
large region length, in massive two-dimensional integrable quantum field
theory. It only depends on the particle content of the model, and not on the
way particles scatter. Based on general analyticity arguments for form factors,
we propose that this result is universal, and holds for any massive
two-dimensional model (also out of integrability). We suggest a link of this
result with counting pair creations far in the past.Comment: 5 pages, 2 figures. v2: improved presentation of result and
calculations, section "discussion" adjusted, references adjuste
Dressing the electromagnetic nucleon current
A field-theory-based approach to pion photoproduction off the nucleon is used
to derive a microscopically consistent formulation of the fully dressed
electromagnetic nucleon current in an effective Lagrangian formalism. It is
shown how the rigorous implementation of local gauge invariance at all levels
of the reaction dynamics provides equations that lend themselves to practically
manageable truncations of the underlying nonlinearities of the problem. The
requirement of consistency also suggests a novel way of treating the pion
photoproduction problem. Guided by a phenomenological implementation of gauge
invariance for the truncated equations that has proved successful for pion
photoproduction, an expression for the fully dressed nucleon current is given
that satisfies the Ward-Takahashi identity for a fully dressed nucleon
propagator as a matter of course. Possible applications include meson photo-
and electroproduction processes, bremsstrahlung, Compton scattering, and
processes off nucleons.Comment: 10 pages, 9 figure
A Test of the Adhesion Approximation for Gravitational Clustering
We quantitatively compare a particle implementation of the adhesion
approximation to fully non--linear, numerical nbody simulations. Our primary
tool, cross--correlation of nbody simulations with the adhesion approximation,
indicates good agreement, better than that found by the same test performed
with the Zel'dovich approximation (hereafter ZA). However, the
cross--correlation is not as good as that of the truncated Zel'dovich
approximation (TZA), obtained by applying the Zel'dovich approximation after
smoothing the initial density field with a Gaussian filter. We confirm that the
adhesion approximation produces an excessively filamentary distribution.
Relative to the nbody results, we also find that: (a) the power spectrum
obtained from the adhesion approximation is more accurate than that from ZA or
TZA, (b) the error in the phase angle of Fourier components is worse than that
from TZA, and (c) the mass distribution function is more accurate than that
from ZA or TZA. It appears that adhesion performs well statistically, but that
TZA is more accurate dynamically, in the sense of moving mass to the right
place.
Subject Heading: Galaxies, formation, clustering--large--scale structure of
the UniverseComment: TeX, 7 pages excluding figures (contact
[email protected]). submitted to Ap
The resultant parameters of effective theory
This is the 4-th paper in the series devoted to a systematic study of the
problem of mathematically correct formulation of the rules needed to manage an
effective field theory. Here we consider the problem of constructing the full
set of essential parameters in the case of the most general effective
scattering theory containing no massless particles with spin J > 1/2. We
perform the detailed classification of combinations of the Hamiltonian coupling
constants and select those which appear in the expressions for renormalized
S-matrix elements at a given loop order.Comment: 21 pages, 4 LaTeX figures, submitted to Phys. Rev.
Fading Gravity and Self-Inflation
We study the cosmology of a toy modified theory of gravity in which gravity
shuts off at short distances, as in the fat graviton scenario of Sundrum. In
the weak-field limit, the theory is perturbatively local, ghost-free and
unitary, although likely suffers from non-perturbative instabilities. We derive
novel self-inflationary solutions from the vacuum equations of the theory,
without invoking scalar fields or other forms of stress energy. The modified
perturbation equation expressed in terms of the Newtonian potential closely
resembles its counterpart for inflaton fluctuations. The resulting scalar
spectrum is therefore slightly red, akin to the simplest scalar-driven
inflationary models. A key difference, however, is that the gravitational wave
spectrum is generically not scale invariant. In particular the tensor spectrum
can have a blue tilt, a distinguishing feature from standard inflation.Comment: 35 pages, 4 figures. v3: version to appear in Phys. Rev.
On the power counting of loop diagrams in general relativity
A class of loop diagrams in general relativity appears to have a behavior
which would upset the utility of the energy expansion for quantum effects. We
show through the study of specific diagrams that cancellations occur which
restore the expected behaviour of the energy expansion. By considering the
power counting in a physical gauge we show that the apparent bad behavior is a
gauge artifact, and that the quantum loops enter with a well behaved energy
expansion.Comment: 29 pages, uses axodraw and epsfig.tex, one small .eps file is
included. The full PostScript version is also available as
http://het.phast.umass.edu/students/kakukk/powercount_hepth.p
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
Signatures of gravitational fixed points at the LHC
We study quantum-gravitational signatures at the CERN Large Hadron Collider (LHC) in the context of theories with extra spatial dimensions and a low fundamental Planck scale in the TeV range. Implications of a gravitational fixed point at high energies are worked out using Wilson¿s renormalization group. We find that relevant cross sections involving virtual gravitons become finite. Based on gravitational lepton pair production we conclude that the LHC is sensitive to a fundamental Planck scale of up to 6 TeV
Comment on "Does Gluons Carry Half of the Nucleon Momentum?" by X. S. Chen et. al. (PRL103, 062001 (2009))
The authors claim to have found a "proper", "gauge-invariant" definition of a
charged-particle's momentum in gauge theory, which is more "superior" than the
textbook version. I show that their result arises from a misunderstanding of
gauge symmetry by generalizing the Coulomb gauge result indiscriminately and is
not physical
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