39 research outputs found
New bulk scalar field solutions in brane worlds
We use nonlinear perturbation theory to obtain new solutions for brane world
models that incorporate a massive bulk scalar field. We then consider tensor
perturbations and show that Newtonian gravity is recovered on the brane for
both a light scalar field and for a bulk field with large negative mass. This
latter result points to the viability of higher-derivative theories of gravity
in the context of bulk extra dimensions.Comment: 4+\epsilon pages, no figure
Scaling Solutions to 6D Gauged Chiral Supergravity
We construct explicitly time-dependent exact solutions to the field equations
of 6D gauged chiral supergravity, compactified to 4D in the presence of up to
two 3-branes situated within the extra dimensions. The solutions we find are
scaling solutions, and are plausibly attractors which represent the late-time
evolution of a broad class of initial conditions. By matching their near-brane
boundary conditions to physical brane properties we argue that these solutions
(together with the known maximally-symmetric solutions and a new class of
non-Lorentz-invariant static solutions, which we also present here) describe
the bulk geometry between a pair of 3-branes with non-trivial on-brane
equations of state.Comment: Contribution to the New Journal of Physics focus issue on Dark
Energy; 28 page
Kicking the Rugby Ball: Perturbations of 6D Gauged Chiral Supergravity
We analyze the axially-symmetric scalar perturbations of 6D chiral gauged
supergravity compactified on the general warped geometries in the presence of
two source branes. We find all of the conical geometries are marginally stable
for normalizable perturbations (in disagreement with some recent calculations)
and the nonconical for regular perturbations, even though none of them are
supersymmetric (apart from the trivial Salam-Sezgin solution, for which there
are no source branes). The marginal direction is the one whose presence is
required by the classical scaling property of the field equations, and all
other modes have positive squared mass. In the special case of the conical
solutions, including (but not restricted to) the unwarped `rugby-ball'
solutions, we find closed-form expressions for the mode functions in terms of
Legendre and Hypergeometric functions. In so doing we show how to match the
asymptotic near-brane form for the solution to the physics of the source
branes, and thereby how to physically interpret perturbations which can be
singular at the brane positions.Comment: 21 pages + appendices, references adde
Brane matter, hidden or mirror matter, their various avatars and mixings: many faces of the same physics
Numerous papers deal with the phenomenology related to photon-hidden photon
kinetic mixing and with the effects of a mass mixing on particle-hidden
particle oscillations. In addition, recent papers underline the existence of a
geometrical mixing between branes which would allow a matter swapping between
branes. These approaches and their phenomenologies are reminiscent of each
other but rely on different physical concepts. In the present paper, we suggest
there is no rivalry between these models, which are probably many faces of the
same physics. We discuss some phenomenological consequences of a global
framework.Comment: 9 pages. Typo corrected. Published in European Physical Journal
Spin-2 spectrum of defect theories
We study spin-2 excitations in the background of the recently-discovered
type-IIB solutions of D'Hoker et al. These are holographically-dual to defect
conformal field theories, and they are also of interest in the context of the
Karch-Randall proposal for a string-theory embedding of localized gravity. We
first generalize an argument by Csaki et al to show that for any solution with
four-dimensional anti-de Sitter, Poincare or de Sitter invariance the spin-2
excitations obey the massless scalar wave equation in ten dimensions. For the
interface solutions at hand this reduces to a Laplace-Beltrami equation on a
Riemann surface with disk topology, and in the simplest case of the
supersymmetric Janus solution it further reduces to an ordinary differential
equation known as Heun's equation. We solve this equation numerically, and
exhibit the spectrum as a function of the dilaton-jump parameter .
In the limit of large a nearly-flat linear-dilaton dimension grows
large, and the Janus geometry becomes effectively five-dimensional. We also
discuss the difficulties of localizing four-dimensional gravity in the more
general backgrounds with NS5-brane or D5-brane charge, which will be analyzed
in detail in a companion paper.Comment: 41 pages, 6 figure
The neutron and its role in cosmology and particle physics
Experiments with cold and ultracold neutrons have reached a level of
precision such that problems far beyond the scale of the present Standard Model
of particle physics become accessible to experimental investigation. Due to the
close links between particle physics and cosmology, these studies also permit a
deep look into the very first instances of our universe. First addressed in
this article, both in theory and experiment, is the problem of baryogenesis ...
The question how baryogenesis could have happened is open to experimental
tests, and it turns out that this problem can be curbed by the very stringent
limits on an electric dipole moment of the neutron, a quantity that also has
deep implications for particle physics. Then we discuss the recent spectacular
observation of neutron quantization in the earth's gravitational field and of
resonance transitions between such gravitational energy states. These
measurements, together with new evaluations of neutron scattering data, set new
constraints on deviations from Newton's gravitational law at the picometer
scale. Such deviations are predicted in modern theories with extra-dimensions
that propose unification of the Planck scale with the scale of the Standard
Model ... Another main topic is the weak-interaction parameters in various
fields of physics and astrophysics that must all be derived from measured
neutron decay data. Up to now, about 10 different neutron decay observables
have been measured, much more than needed in the electroweak Standard Model.
This allows various precise tests for new physics beyond the Standard Model,
competing with or surpassing similar tests at high-energy. The review ends with
a discussion of neutron and nuclear data required in the synthesis of the
elements during the "first three minutes" and later on in stellar
nucleosynthesis.Comment: 91 pages, 30 figures, accepted by Reviews of Modern Physic
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Relativistic klystrons for high-gradient accelerators
Experimental work is being performed by collaborators at LLNL, SLAC, and LBL to investigate relativistic klystrons as a possible rf power source for future high-gradient accelerators. We have learned how to overcome or previously reported problem of high power rf pulse shortening and have achieved peak rf power levels of 330 MW using an 11.4-GHz high-gain tube with multiple output structures. In these experiments the rf pulse is of the same duration as the beam current pulse. In addition, experiments have been performed on two short sections of a high-gradient accelerator using the rf power from a relativistic klystron. An average accelerating gradient of 84 MV/m has been achieved with 80-MW of rf power