1,781 research outputs found
Horava Gravity and Gravitons at a Conformal Point
Recently Horava proposed a renormalizable gravity theory with higher
derivatives by abandoning the Lorenz invariance in UV. Here, I study the Horava
model at , where an anisotropic Weyl symmetry exists in the UV
limit, in addition to the foliation-preserving diffeomorphism. By considering
linear perturbations around Minkowski vacuum, I show that the scalar graviton
mode is completely disappeared and only the usual tensor graviton modes remain
in the physical spectrum. The existence of the UV conformal symmetry is unique
to the theory with the detailed balance and it is quite probable that
be the UV fixed point. This situation is analogous to
, which is Lorentz invariant in the IR limit and is believed to be
the IR fixed point.Comment: Added comments and references, Accepted in GER
Poisson Algebra of Diffeomorphism Generators in a Spacetime Containing a Bifurcation
In this article we will analyze the possibility of a nontrivial central
extension of the Poisson algebra of the diffeomorphism generators, which
respect certain boundary conditions on the black hole bifurcation. The origin
of a possible central extension in the algebra is due to the existence of
boundary terms in the in the canonical generators. The existence of such
boundary terms depend on the exact boundary conditions one takes. We will check
two possible boundary conditions i.e. fixed bolt metric and fixed surface
gravity. In the case of fixed metric the the action acquires a boundary term
associated to the bifurcation but this is canceled in the Legendre
transformation and so absent in the canonical generator and so in this case the
possibility of a nontrivial central extension is ruled out. In the case of
fixed surface gravity the boundary term in the action is absent but present in
the Hamiltonian. Also in this case we will see that there is no nontrivial
central extension, also if there exist a boundary term in the generator.Comment: LaTex 20 pages, some misprints corrected, 2 references added.
Accepted for publication on Phys. Rev.
Scattering of electromagnetic waves by many thin cylinders: theory and computational modeling
Electromagnetic (EM) wave scattering by many parallel infinite cylinders is
studied asymptotically as a tends to 0, where a is the radius of the cylinders.
It is assumed that the centres of the cylinders are distributed so that their
numbers is determined by some positive function N(x). The function N(x) >= 0 is
a given continuous function. An equation for the self-consistent (limiting)
field is derived as a tends to 0. The cylinders are assumed perfectly
conducting. Formula for the effective refraction coefficient of the new medium,
obtained by embedding many thin cylinders into a given region, is derived. The
numerical results presented demonstrate the validity of the proposed approach
and its efficiency for solving the many-body scattering problems, as well as
the possibility to create media with negative refraction coefficients.Comment: 21 pages, 13 figure
Numerically Efficient Analysis of Planar Microstrip Configurations Using Closed-Form Green's Functions
An efficient technique for the analysis of a general class of microstrip structures with a substrate and a superstrate is investigated in this paper using newly-derived closed-form spatial domain Green's functions employed in conjunction with the Method of Moments (MoM). The computed current distributions on the microstrip structure are used to determine the scattering parameters of microstrip discontinuities and the input impedances of microstrip patch antennas. It is shown that the use of the closed-form Green's functions in the context of the MoM provides a computational advantage in terms of the CPU time by almost two orders of magnitude over the conventional spectral domain approach employing the transformed version of the Green's functions. © 1995 IEE
Kondo effect of non-magnetic impurities and the co-existing charge order in the cuprate superconductors
We present a theory of Kondo effect caused by an induced magnetic moment near
non-magnetic impurities such as Zn and Li in the cuprate superconductors. Based
on the co-existence of charge order and superconductivity, a natural
description of the induced moment and the resulting Kondo effect is obtained in
the framework of bond-operator theory of microscopic t-J-V Hamiltonian. The
local density of state near impurities is computed in a self-consistent
Bogoliubov-de Gennes theory which shows a low-energy peak in the middle of
superconducting gap. Our theory also suggests that the charge order can be
enhanced near impuries.Comment: 5 pages, 4 figure
A two-step learning approach for solving full and almost full cold start problems in dyadic prediction
Dyadic prediction methods operate on pairs of objects (dyads), aiming to
infer labels for out-of-sample dyads. We consider the full and almost full cold
start problem in dyadic prediction, a setting that occurs when both objects in
an out-of-sample dyad have not been observed during training, or if one of them
has been observed, but very few times. A popular approach for addressing this
problem is to train a model that makes predictions based on a pairwise feature
representation of the dyads, or, in case of kernel methods, based on a tensor
product pairwise kernel. As an alternative to such a kernel approach, we
introduce a novel two-step learning algorithm that borrows ideas from the
fields of pairwise learning and spectral filtering. We show theoretically that
the two-step method is very closely related to the tensor product kernel
approach, and experimentally that it yields a slightly better predictive
performance. Moreover, unlike existing tensor product kernel methods, the
two-step method allows closed-form solutions for training and parameter
selection via cross-validation estimates both in the full and almost full cold
start settings, making the approach much more efficient and straightforward to
implement
Absorption cross section in the topologically massive gravity at the critical point
The absorption cross section for the the warped AdS black hole background
shows that it is larger than the area even if the s-wave limit is considered.
It raises some question whether the deviation from the areal cross section is
due to the warped configuration of the geometry or the rotating coordinate
system, where these two effects are mixed up in the warped AdS black hole.
So, we study the low-frequency scattering dynamics of propagating scalar fields
under the warped AdS background at the critical point which reduces to the
BTZ black hole in the rotating frame without the warped factor, which shows
that the deformation effect at the critical point does not appear.Comment: 12 pages, LaTe
Toy Model for Pion Production II: The role of three-particle singularities
The influence of three-particle breakup singularities on s-wave meson
production in nucleon-nucleon collisions is studied within the distorted wave
Born approximation. This study is based on a simple scalar model for the
two-nucleon interaction and the production mechanism. An algorithm for the
exact numerical treatment of the inherent three-body cuts, together with its
straightforward implementation is presented. It is also shown that two
often-used approximations to avoid the calculation of the three-body breakup
are not justified. The possible impact on pion production observables is
discussed.Comment: 14 pages, 6 figure
Thermodynamics of a black hole based on a generalized uncertainty principle
We study thermodynamic quantities and the stability of a black hole in a
cavity using the Euclidean action formalism by Gibbons and Hawking based on the
generalized uncertainty relation which is extended in a symmetric way with
respect to the space and momentum without loss of generality. Two parameters in
the uncertainty relation affect the thermodynamical quantities such as energy,
entropy, and the heat capacity. In particular, it can be shown that the small
black hole is unstable and it may decay either into a minimal black hole or a
large black hole. We discuss a constraint for a large black hole comparable to
the size of the cavity in connection with the critical mass.Comment: 12 pages, 4 figures; v2. to appear in JHE
Black p-Branes versus black holes in non-asymptotically flat Einstein-Yang-Mills theory
We present a class of non-asymptotically flat (NAF) charged black p-branes
(BpB) with p-compact dimensions in higher dimensional Einstein-Yang-Mills
theory. Asymptotically the NAF structure manifests itself as an anti-de-sitter
spacetime. We determine the total mass / energy enclosed in a thin-shell
located outside the event horizon. By comparing the entropies of BpB with those
of black holes in same dimensions we derive transition criteria between the two
types of black objects. Given certain conditions satisfied our analysis shows
that BpB can be considered excited states of black holes. An event horizon
versus charge square plot \ for the BpB reveals such a
transition where is related to the horizon radius of the black
hole (BH) both with the common charge Comment: 10 pages, 1 figure, updated version. Final version to be published in
EPJ
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