272 research outputs found
Classical ultrarelativistic bremsstrahlung in extra dimensions
The emitted energy and the cross-section of classical scalar bremsstrahlung
in massive particle collisions in D=4+d dimensional Minkowski space M_D as well
as in the brane world M_4 \times T^d is computed to leading ultra-relativistic
order. The particles are taken to interact in the first case via the exchange
of a bulk massless scalar field \Phi and in the second with an additional
massless scalar \phi confined together with the particles on the brane. Energy
is emitted as \Phi radiation in the bulk and/or \phi radiation on the brane. In
contrast to the quantum Born approximation, the classical result is unambiguous
and valid in a kinematical region which is also specified. For D=4 the results
are in agreement with corresponding expressions in classical electrodynamics.Comment: Preprint number adde
Gravitational collapse in braneworld models with curvature corrections
We study the collapse of a homogeneous braneworld dust cloud in the context
of the various curvature correction scenarios, namely, the induced-gravity, the
Gauss-Bonnet, and the combined induced-gravity and Gauss-Bonnet. In accordance
to the Randall-Sundrum model, and contrary to four-dimensional general
relativity, we show in all cases that the exterior spacetime on the brane is
non-static.Comment: References adde
Cosmological Evolution of a Purely Conical Codimension-2 Brane World
We study the cosmological evolution of isotropic matter on an infinitely thin
conical codimension-two brane-world. Our analysis is based on the boundary
dynamics of a six-dimensional model in the presence of an induced gravity term
on the brane and a Gauss-Bonnet term in the bulk. With the assumption that the
bulk contains only a cosmological constant Lambda_B, we find that the isotropic
evolution of the brane-universe imposes a tuned relation between the energy
density and the brane equation of state. The evolution of the system has fixed
points (attractors), which correspond to a final state of radiation for
Lambda_B=0 and to de Sitter state for Lambda_B>0. Furthermore, considering
anisotropic matter on the brane, the tuning of the parameters is lifted, and
new regions of the parametric space are available for the cosmological
evolution of the brane-universe. The analysis of the dynamics of the system
shows that, the isotropic fixed points remain attractors of the system, and for
values of Lambda_B which give acceptable cosmological evolution of the equation
of state, the line of isotropic tuning is a very weak attractor. The initial
conditions, in this case, need to be fine tuned to have an evolution with
acceptably small anisotropy.Comment: 20 pages, 4 figures, typo correcte
Asymmetric embedding in brane cosmology
We derive a system of cosmological equations for a braneworld with induced
curvature which is a junction between several bulk spaces. The permutation
symmetry of the bulk spaces is not imposed, and the values of the fundamental
constants, and even the signatures of the extra dimension, may be different on
different sides of the brane. We then consider the usual partial case of two
asymmetric bulk spaces and derive an exact closed system of scalar equations on
the brane. We apply this result to the cosmological evolution on such a brane
and describe its various partial cases.Comment: 10 page
QCD uncertainties at the LHC and the implications of HERA
Strong interaction physics will be ubiquitous at the Large Hadron Collider
since the colliding beams consist of confined quarks and gluons. Although the
main purpose of the LHC is to study the mechanism of electroweak symmetry
breaking and to search for physics beyond the Standard Model, to maximise the
precision and sensitivity of such anaylses it is necessary to understand in
detail various perturbative, semi-perturbative and non-perturbative QCD
effects. Many of these effects have been extensively studied at HERA and will
be studied further at HERA II. We discuss the impact of the knowledge thus
gained on physics at the LHC.Comment: Contributed to the Proceedings of DIS04, Strbske Pleso, Slovaki
Where the wild things are: Seasonal variation in caribou distribution in relation to climate change
In this study, we develop a method to analyse the relationships between seasonal caribou distribution and climate, to estimate how climatic conditions affect interactions between humans and caribou, and ultimately to predict patterns of distribution relative to climate change. Satellite locations for the Porcupine (Rangifer tarandus granti) and Bathurst (R. t. groenlandicus) caribou herds were analysed for eight ecologically-defined seasons. For each season, two levels of a key environmental factor influencing caribou distribution were identified, as well as the best climate data available to indicate the factor's annual state. Satellite locations were grouped according to the relevant combination of season and environmental factor. Caribou distributions were compared for opposing environmental factors; this comparison was undertaken relative to hunting access for the Porcupine Herd and relative to exposure to mining activity for the Bathurst Herd. Expected climate trends suggest an overall increase in access to Porcupine caribou for Aklavik (NWT) hunters during the winter and rut seasons, for Venetie (Alaska) hunters during midsummer and fall migration and for Arctic Village (Alaska) during midsummer. Arctic Village may experience reduced availability with early snowfalls in the fall, but we expect there to be little directional shift in the spring migration patterns. For the Bathurst Herd, we expect that fewer caribou would be exposed to the mines during the winter, while more caribou would be exposed to the combined Ekati and Diavik mining zone in the early summer and to the Lupin-Jericho mining zone during the fall migration. If changes in climate cause an increased presence of caribou in the mining sites, monitoring and mitigation measures may need to be intensified
Gravitating defects of codimension-two
Thin gravitating defects with conical singularities in higher codimensions
and with generalized Israel matching conditions are known to be inconsistent
for generic energy-momentum. A way to remove this inconsistency is proposed and
is realized for an axially symmetric gravitating codimension-two defect in six
dimensional Einstein gravity. By varying with respect to the brane embedding
fields, alternative matching conditions are derived, which are generalizations
of the Nambu-Goto equations of motion of the defect, consistent with bulk
gravity. For a maximally symmetric defect the standard picture is recovered.
The four-dimensional perfect fluid cosmology coincides with conventional FRW in
the case of radiation, but for dust it has rho^{4/3} instead of rho. A
four-dimensional black hole solution is presented having the Schwarzschild form
with a short-distance correction r^{-2}.Comment: Minor changes, to appear in Classical and Quantum Gravit
Brane-bulk matter relation for a purely conical codimension-2 brane world
We study gravity on an infinitely thin codimension-2 brane world, with purely
conical singularities and in the presence of an induced gravity term on the
brane. We show that in this approximation, the energy momentum tensor of the
bulk is strongly related to the energy momentum tensor of the brane and thus
the gravity dynamics on the brane are induced by the bulk content. This is in
contrast with the gravity dynamics on a codimension-1 brane. We show how this
strong result is relaxed after including a Gauss-Bonnet term in the bulk.Comment: 12 pages, mistake corrected, references adde
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