15,628 research outputs found
Six-dimensional Methods for Four-dimensional Conformal Field Theories
The calculation of both spinor and tensor Green's functions in
four-dimensional conformally invariant field theories can be greatly simplified
by six-dimensional methods. For this purpose, four-dimensional fields are
constructed as projections of fields on the hypercone in six-dimensional
projective space, satisfying certain transversality conditions. In this way
some Green's functions in conformal field theories are shown to have structures
more general than those commonly found by use of the inversion operator. These
methods fit in well with the assumption of AdS/CFT duality. In particular, it
is transparent that if fields on AdS approach finite limits on the boundary
of AdS, then in the conformal field theory on this boundary these limits
transform with conformal dimensionality zero if they are tensors (of any rank),
but with conformal dimension 1/2 if they are spinors or spinor-tensors.Comment: Version accepted for publication in Physical Review D. References to
earlier work added in footnote 2. Minor errors corrected. 24 page
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
On Local Dilatation Invariance
The relationship between local Weyl scaling invariant models and local
dilatation invariant actions is critically scrutinized. While actions invariant
under local Weyl scalings can be constructed in a straightforward manner,
actions invariant under local dilatation transformations can only be achieved
in a very restrictive case. The invariant couplings of matter fields to an
Abelian vector field carrying a non-trivial scaling weight can be easily built,
but an invariant Abelian vector kinetic term can only be realized when the
local scale symmetry is spontaneously broken.Comment: 3 page
On the relation between mass of pion, fundamental physical constants and cosmological parameters
In this article we reconsider the old mysterious relation, advocated by Dirac
and Weinberg, between the mass of the pion, the fundamental physical constants,
and the Hubble parameter. By introducing the cosmological density parameters,
we show how the corresponding equation may be written in a form that is
invariant with respect to the expansion of the Universe and without invoking a
varying gravitational "constant", as was originaly proposed by Dirac. It is
suggest that, through this relation, Nature gives a hint that virtual pions
dominante the "content" of the quantum vacuum
Cosmic Acceleration from Causal Backreaction with Recursive Nonlinearities
We revisit the causal backreaction paradigm, in which the need for Dark
Energy is eliminated via the generation of an apparent cosmic acceleration from
the causal flow of inhomogeneity information coming in towards each observer
from distant structure-forming regions. This second-generation formalism
incorporates "recursive nonlinearities": the process by which
already-established metric perturbations will then act to slow down all future
flows of inhomogeneity information. Here, the long-range effects of causal
backreaction are now damped, weakening its impact for models that were
previously best-fit cosmologies. Nevertheless, we find that causal backreaction
can be recovered as a replacement for Dark Energy via the adoption of larger
values for the dimensionless `strength' of the clustering evolution functions
being modeled -- a change justified by the hierarchical nature of clustering
and virialization in the universe, occurring on multiple cosmic length scales
simultaneously. With this, and with one new model parameter representing the
slowdown of clustering due to astrophysical feedback processes, an alternative
cosmic concordance can once again be achieved for a matter-only universe in
which the apparent acceleration is generated entirely by causal backreaction
effects. One drawback is a new degeneracy which broadens our predicted range
for the observed jerk parameter , thus removing what had
appeared to be a clear signature for distinguishing causal backreaction from
Cosmological Constant CDM. As for the long-term fate of the universe,
incorporating recursive nonlinearities appears to make the possibility of an
`eternal' acceleration due to causal backreaction far less likely; though this
does not take into account gravitational nonlinearities or the large-scale
breakdown of cosmological isotropy, effects not easily modeled within this
formalism.Comment: 53 pages, 7 figures, 3 tables. This paper is an advancement of
previous research on Causal Backreaction; the earlier work is available at
arXiv:1109.4686 and arXiv:1109.515
A Simple UV-Completion of QED in 5D
We construct a Lifshitz-like version of five-dimensional (5D) QED which is UV
- completed and reduces at low energies to ordinary 5D QED. The UV quantum
behaviour of this theory is very smooth. In particular, the gauge coupling
constant is finite at all energy scales and at all orders in perturbation
theory. We study the IR properties of this theory, when compactified on a
circle, and compare the one-loop energy dependence of the coupling in the
Lifshitz theory with that coming from the standard 5D QED effective field
theory. The range of validity of the 5D effective field theory is found to
agree with the more conservative version of Naive Dimensional Analysis.Comment: 24 pages, 7 figures; v2: Minor improvements, matches journal versio
Gravitational radiation from rotating monopole-string systems
We study the gravitational radiation from a rotating monopole-antimonopole
pair connected by a string. While at not too high frequencies the emitted
gravitational spectrum is described asymptotically by , the
spectrum is exponentially suppressed in the high-frequency limit, . Below , the emitted spectrum of gravitational
waves is very similar to the case of an oscillating monopole pair connected by
a string, and we argue therefore that the spectrum found holds approximately
for any moving monopole-string system. As application, we discuss the
stochastic gravitational wave background generated by monopole-antimonopole
pairs connected by strings in the early Universe and gravitational wave bursts
emitted at present by monopole-string networks. We confirm that advanced
gravitational wave detectors have the potential to detect a signal for string
tensions as small as .Comment: 8 pages, 2 figures, revtex4; v2: minor corrections, matches published
versio
The spin-statistics connection in classical field theory
The spin-statistics connection is obtained for a simple formulation of a
classical field theory containing even and odd Grassmann variables. To that
end, the construction of irreducible canonical realizations of the rotation
group corresponding to general causal fields is reviewed. The connection is
obtained by imposing local commutativity on the fields and exploiting the
parity operation to exchange spatial coordinates in the scalar product of
classical field evaluated at one spatial location with the same field evaluated
at a distinct location. The spin-statistics connection for irreducible
canonical realizations of the Poincar\'{e} group of spin is obtained in the
form: Classical fields and their conjugate momenta satisfy fundamental
field-theoretic Poisson bracket relations for 2 even, and fundamental
Poisson antibracket relations for 2 oddComment: 27 pages. Typos and sign error corrected; minor revisions to tex
Effective Field Theory for the Quantum Electrodynamics of a Graphene Wire
We study the low-energy quantum electrodynamics of electrons and holes, in a
thin graphene wire. We develop an effective field theory (EFT) based on an
expansion in p/p_T, where p_T is the typical momentum of electrons and holes in
the transverse direction, while p are the momenta in the longitudinal
direction. We show that, to the lowest-order in (p/p_T), our EFT theory is
formally equivalent to the exactly solvable Schwinger model. By exploiting such
an analogy, we find that the ground state of the quantum wire contains a
condensate of electron-hole pairs. The excitation spectrum is saturated by
electron-hole collective bound-states, and we calculate the dispersion law of
such modes. We also compute the DC conductivity per unit length at zero
chemical potential and find g_s =e^2/h, where g_s=4 is the degeneracy factor.Comment: 7 pages, 2 figures. Definitive version, accepted for publication on
Phys. Rev.
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