1,791 research outputs found
Fields Annihilation and Particles Creation in DBI inflation
We consider a model of DBI inflation where two stacks of mobile branes are
moving ultra relativistically in a warped throat. The stack closer to the tip
of the throat is annihilated with the background anti-branes while inflation
proceeds by the second stack. The effects of branes annihilation and particles
creation during DBI inflation on the curvature perturbations power spectrum and
the scalar spectral index are studied. We show that for super-horizon scales,
modes which are outside the sound horizon at the time of branes collision, the
spectral index has a shift to blue spectrum compared to the standard DBI
inflation. For small scales the power spectrum approaches to its background DBI
inflation value with the decaying superimposed oscillatory modulations.Comment: First revision: minor changes, the background spectral index is
corrected, new references are added. Second revision: minor changes, new
references are added, accepted for publication in JCA
A self-tuning mechanism in (3+p)d gravity-scalar theory
We present a new type of self-tuning mechanism for ()d brane world
models in the framework of gravity-scalar theory. This new type of self-tuning
mechanism exhibits a remarkable feature. In the limit , being
the string coupling, the geometry of bulk spacetime remains virtually unchanged
by an introduction of the Standard Model(SM)-brane, and consequently it is
virtually unaffected by quantum fluctuations of SM fields with support on the
SM-brane. Such a feature can be obtained by introducing Neveu-Schwarz(NS)-brane
as a background brane on which our SM-brane is to be set. Indeed, field
equations naturally suggest the existence of the background NS-brane. Among the
given such models, of the most interest is the case with , where
represents the bulk cosmological constant. This model contains a pair
of coincident branes (of the SM- and the NS-branes), one of which is a
codimension-2 brane placed at the origin of 2d transverse space (), another a codimension-1 brane placed at the edge of .
These two branes are (anti) T-duals of each other, and one of them may be
identified as our SM-brane plus the background NS-brane. In the presence of the
background NS-brane (and in the absence of ), the 2d transverse space
becomes an orbifold with an appropriate deficit angle.
But this is only possible if the ()d Planck scale and the string
scale () are of the same order, which
accords with the hierarchy assumption \cite{1,2,3} that the electroweak scale
is the only short distance scale existing in nature
Warped Spectroscopy: Localization of Frozen Bulk Modes
We study the 10D equation of motion of dilaton-axion fluctuations in type IIB
string compactifications with three-form flux, taking warping into account.
Using simplified models with physics comparable to actual compactifications, we
argue that the lightest mode localizes in long warped throats and takes a mass
of order the warped string scale. Also, Gukov-Vafa-Witten superpotential is
valid for the lightest mass mode; however, the mass is similar to the
Kaluza-Klein scale, so the dilaton-axion should be integrated out of the
effective theory in this long throat regime (leaving a constant
superpotential). On the other hand, there is a large hierarchy between
flux-induced and KK mass scales for moderate or weak warping. This hierarchy
agrees with arguments given for trivial warping. Along the way, we also
estimate the effect of the other 10D supergravity equations of motion on the
dilaton-axion fluctuation, since these equations act as constraints. We argue
that they give negligible corrections to the simplest approximation.Comment: 24pp + appendices, 6 figs, JHEP3 class; v2. corrected reference; v3.
added clarifications; v4. corrected typo
The General Warped Solution with Conical Branes in Six-dimensional Supergravity
We present the general regular warped solution with 4D Minkowski spacetime in
six-dimensional gauged supergravity. In this framework, we can easily embed
multiple conical branes into the warped geometry by choosing an undetermined
holomorphic function. As an example, for the holomorphic function with many
zeroes, we find warped solutions with multi-branes and discuss the generalized
flux quantization in this case.Comment: 1+19 pages, no figure, JHEP style, version to appear in JHE
General Axisymmetric Solutions and Self-Tuning in 6D Chiral Gauged Supergravity
We re-examine the properties of the axially-symmetric solutions to chiral
gauged 6D supergravity, recently found in refs. hep-th/0307238 and
hep-th/0308064. Ref. hep-th/0307238 finds the most general solutions having two
singularities which are maximally-symmetric in the large 4 dimensions and which
are axially-symmetric in the internal dimensions. We show that not all of these
solutions have purely conical singularities at the brane positions, and that
not all singularities can be interpreted as being the bulk geometry sourced by
neutral 3-branes. The subset of solutions for which the metric singularities
are conical precisely agree with the solutions of ref. hep-th/0308064.
Establishing this connection between the solutions of these two references
resolves a minor conflict concerning whether or not the tensions of the
resulting branes must be negative. The tensions can be both negative and
positive depending on the choice of parameters. We discuss the physical
interpretation of the non-conical solutions, including their significance for
the proposal for using 6-dimensional self-tuning to understand the small size
of the observed vacuum energy. In passing we briefly comment on a recent paper
by Garriga and Porrati which criticizes the realization of self-tuning in 6D
supergravity.Comment: 27 pages, 1 figure; JHEP3 style; Some references added, and
discussion of tension constraints and unwarped solutions made more explici
Supersymmetric codimension-two branes and U(1)_R mediation in 6D gauged supergravity
We construct a consistent supersymmetric action for brane chiral and vector
multiplets in a six-dimensional chiral gauged supergravity. A nonzero brane
tension can be accommodated by allowing for a brane-localized Fayet-Iliopoulos
term proportional to the brane tension. When the brane chiral multiplet is
charged under the bulk U(1)_R, we obtain a nontrivial coupling to the extra
component of the U(1)_R gauge field strength as well as a singular scalar
self-interaction term. Dimensionally reducing to 4D on a football
supersymmetric solution, we discuss the implication of such interactions for
obtaining the U(1)_R D-term in the 4D effective supergravity. By assuming the
bulk gaugino condensates and nonzero brane F- and/or D-term for the uplifting
potential, we have all the moduli stabilized with a vanishing cosmological
constant. The brane scalar with nonzero R charge then gets a soft mass of order
the gravitino mass. The overall sign of the soft mass squared depends on the
sign of the R charge as well as whether the brane F- or D-term dominates.Comment: 28 pages, no figures, version to appear in JHE
Nontrival Cosmological Constant in Brane Worlds with Unorthodox Lagrangians
In self-tuning brane-world models with extra dimensions, large contributions
to the cosmological constant are absorbed into the curvature of extra
dimensions and consistent with flat 4d geometry. In models with conventional
Lagrangians fine-tuning is needed nevertheless to ensure a finite effective
Planck mass. Here, we consider a class of models with non conventional
Lagrangian in which known problems can be avoided. Unfortunately these models
are found to suffer from tachyonic instabilities. An attempt to cure these
instabilities leads to the prediction of a positive cosmological constant,
which in turn needs a fine-tuning to be consistent with observations.Comment: 17 pages, 1 figur
Regular S-Brane Backgrounds
We construct time-dependent S-brane solutions to the supergravity field
equations in various dimensions which (unlike most such geometries) do not
contain curvature singularities. The configurations we consider are less
symmetric than are earlier solutions, with our simplest solution being obtained
by a simple analytical continuation of the Kerr geometry. We discuss in detail
the global structure and properties of this background. We then generalize it
to higher dimensions and to include more complicated field configurations -
like non vanishing scalars and antisymmetric tensor gauge potentials - by the
usual artifice of applying duality symmetries.Comment: 22 pages, 3 figures. Typos in eq.(2.6) correcte
The Coupled Cluster Method in Hamiltonian Lattice Field Theory
The coupled cluster or exp S form of the eigenvalue problem for lattice
Hamiltonian QCD (without quarks) is investigated. A new construction
prescription is given for the calculation of the relevant coupled cluster
matrix elements with respect to an orthogonal and independent loop space basis.
The method avoids the explicit introduction of gauge group coupling
coefficients by mapping the eigenvalue problem onto a suitable set of character
functions, which allows a simplified procedure. Using appropriate group
theoretical methods, we show that it is possible to set up the eigenvalue
problem for eigenstates having arbitrary lattice momentum and lattice angular
momentum.Comment: LaTeX, no figur
Inter- and Intragranular Effects in Superconducting Compacted Platinum Powders
Compacted platinum powders exhibit a sharp onset of diamagnetic screening at
mK in zero magnetic field in all samples investigated. This
sharp onset is interpreted in terms of the intragranular transition into the
superconducting state. At lower temperatures, the magnetic ac susceptibility
strongly depends on the ac field amplitude and reflects the small intergranular
critical current density . This critical current density shows a strong
dependence on the packing fraction f of the granular samples. Surprisingly,
increases significantly with decreasing f ( A/cm for f = 0.67 and A/cm for f
= 0.50). The temperature dependence of shows strong positive curvature
over a wide temperature range for both samples. The phase diagrams of inter-
and intragranular superconductivity for different samples indicate that the
granular structure might play the key role for an understanding of the origin
of superconductivity in the platinum compacts.Comment: 11 pages including 9 figures. To appear in Phys. Rev. B in Nov. 0
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