172 research outputs found
Brane Universes with Gauss-Bonnet-Induced-Gravity
The DGP brane world model allows us to get the observed late time
acceleration via modified gravity, without the need for a ``dark energy''
field. This can then be generalised by the inclusion of high energy terms, in
the form of a Gauss-Bonnet bulk. This is the basis of the
Gauss-Bonnet-Induced-Gravity (GBIG) model explored here with both early and
late time modifications to the cosmological evolution. Recently the simplest
GBIG models (Minkowski bulk and no brane tension) have been analysed. Two of
the three possible branches in these models start with a finite density
``Big-Bang'' and with late time acceleration. Here we present a comprehensive
analysis of more general models where we include a bulk cosmological constant
and brane tension. We show that by including these factors it is possible to
have late time phantom behaviour.Comment: 12 pages, 19 figures. Minor modifications to text, comments on
phantom behaviour added. References added. As submitted to JCA
Brane Decay of a (4+n)-Dimensional Rotating Black Hole. III: spin-1/2 particles
In this work, we have continued the study of the Hawking radiation on the
brane from a higher-dimensional rotating black hole by investigating the
emission of fermionic modes. A comprehensive analysis is performed that leads
to the particle, power and angular momentum emission rates, and sheds light on
their dependence on fundamental parameters of the theory, such as the spacetime
dimension and angular momentum of the black hole. In addition, the angular
distribution of the emitted modes, in terms of the number of particles and
energy, is thoroughly studied. Our results are valid for arbitrary values of
the energy of the emitted particles, dimension of spacetime and angular
momentum of the black hole, and complement previous results on the emission of
brane-localised scalars and gauge bosons.Comment: Latex file, JHEP style, 34 pages, 16 figures Energy range in plots
increased, minor changes, version published in JHE
Generating socially appropriate tutorial dialog
Analysis of student-tutor coaching dialogs suggest that good human tutors attend to and attempt to influence the motivational state of learners. Moreover, they are sensitive to the social face of the learner, and seek to mitigate the potential face threat of their comments. This paper describes a dialog generator for pedagogical agents that takes motivation and face threat factors into account. This enables the agent to interact with learners in a socially appropriate fashion, and foster intrinsic motivation on the part of the learner, which in turn may lead to more positive learner affective states
Split Fermions in Extra Dimensions and Exponentially Small Cross-Sections at Future Colliders
We point out a dramatic new experimental signature for a class of theories
with extra dimensions, where quarks and leptons are localized at slightly
separated parallel ``walls'' whereas gauge and Higgs fields live in the bulk of
the extra dimensions. The separation forbids direct local couplings between
quarks and leptons, allowing for an elegant solution to the proton decay
problem. We show that scattering cross sections for collisions of fermions
which are separated in the extra dimensions vanish exponentially at energies
high enough to probe the separation distance. This is because the separation
puts a lower bound on the attainable impact parameter in the collision. We
present cross sections for two body high energy scattering and estimate the
power with which future colliders can probe this scenario, finding sensitivity
to inverse fermion separations of order 10-70 TeV.Comment: 18 pages, 3 figure
Constraining the Littlest Higgs
Little Higgs models offer a new way to address the hierarchy problem, and
give rise to a weakly-coupled Higgs sector. These theories predict the
existence of new states which are necessary to cancel the quadratic divergences
of the Standard Model. The simplest version of these models, the Littlest
Higgs, is based on an non-linear sigma model and predicts that
four new gauge bosons, a weak isosinglet quark, , with , as well as
an isotriplet scalar field exist at the TeV scale. We consider the
contributions of these new states to precision electroweak observables, and
examine their production at the Tevatron. We thoroughly explore the parameter
space of this model and find that small regions are allowed by the precision
data where the model parameters take on their natural values. These regions
are, however, excluded by the Tevatron data. Combined, the direct and indirect
effects of these new states constrain the `decay constant' f\gsim 3.5 TeV and
m_{t'}\gsim 7 TeV. These bounds imply that significant fine-tuning be
present in order for this model to resolve the hierarchy problem.Comment: 31 pgs, 26 figures; bound on t' mass fixed to mt'>2f, conclusions
unchange
Brane decay of a (4+n)-dimensional rotating black hole: spin-0 particles
In this work, we study the `scalar channel' of the emission of Hawking
radiation from a (4+n)-dimensional, rotating black hole on the brane. We
numerically solve both the radial and angular part of the equation of motion
for the scalar field, and determine the exact values of the absorption
probability and of the spheroidal harmonics, respectively. With these, we
calculate the particle, energy and angular momentum emission rates, as well as
the angular variation in the flux and power spectra -- a distinctive feature of
emission during the spin-down phase of the life of the produced black hole. Our
analysis is free from any approximations, with our results being valid for
arbitrarily large values of the energy of the emitted particle, angular
momentum of the black hole and dimensionality of spacetime. We finally compute
the total emissivities for the number of particles, energy and angular momentum
and compare their relative behaviour for different values of the parameters of
the theory.Comment: 24 pages, 13 figure
Birefringence of interferential mirrors at normal incidence Experimental and computational study
In this paper we present a review of the existing data on interferential
mirror birefringence. We also report new measurements of two sets of mirrors
that confirm that mirror phase retardation per reflection decreases when mirror
reflectivity increases. We finally developed a computational code to calculate
the expected phase retardation per reflection as a function of the total number
of layers constituting the mirror. Different cases have been studied and we
have compared computational results with the trend of the experimental data.
Our study indicates that the origin of the mirror intrinsic birefringence can
be ascribed to the reflecting layers close to the substrate.Comment: To be published in Applied Physics
Brane Decay of a (4+n)-Dimensional Rotating Black Hole. II: spin-1 particles
The present works complements and expands a previous one, focused on the
emission of scalar fields by a (4+n)-dimensional rotating black hole on the
brane, by studying the emission of gauge fields on the brane from a similar
black hole. A comprehensive analysis of the particle, energy and angular
momentum emission rates is undertaken, for arbitrary angular momentum of the
black hole and dimensionality of spacetime. Our analysis reveals the existence
of a number of distinct features associated with the emission of spin-1 fields
from a rotating black hole on the brane, such as the behaviour and magnitude of
the different emission rates, the angular distribution of particles and energy,
the relative enhancement compared to the scalar fields, and the magnitude of
the superradiance effect. Apart from their theoretical interest, these features
can comprise clear signatures of the emission of Hawking radiation from a
brane-world black hole during its spin-down phase upon successful detection of
this effect during an experiment.Comment: 35 pages, 19 figures, Latex fil
Region graph partition function expansion and approximate free energy landscapes: Theory and some numerical results
Graphical models for finite-dimensional spin glasses and real-world
combinatorial optimization and satisfaction problems usually have an abundant
number of short loops. The cluster variation method and its extension, the
region graph method, are theoretical approaches for treating the complicated
short-loop-induced local correlations. For graphical models represented by
non-redundant or redundant region graphs, approximate free energy landscapes
are constructed in this paper through the mathematical framework of region
graph partition function expansion. Several free energy functionals are
obtained, each of which use a set of probability distribution functions or
functionals as order parameters. These probability distribution
function/functionals are required to satisfy the region graph
belief-propagation equation or the region graph survey-propagation equation to
ensure vanishing correction contributions of region subgraphs with dangling
edges. As a simple application of the general theory, we perform region graph
belief-propagation simulations on the square-lattice ferromagnetic Ising model
and the Edwards-Anderson model. Considerable improvements over the conventional
Bethe-Peierls approximation are achieved. Collective domains of different sizes
in the disordered and frustrated square lattice are identified by the
message-passing procedure. Such collective domains and the frustrations among
them are responsible for the low-temperature glass-like dynamical behaviors of
the system.Comment: 30 pages, 11 figures. More discussion on redundant region graphs. To
be published by Journal of Statistical Physic
Experimental Probes of Localized Gravity: On and Off the Wall
The phenomenology of the Randall-Sundrum model of localized gravity is
analyzed in detail for the two scenarios where the Standard Model (SM) gauge
and matter fields are either confined to a TeV scale 3-brane or may propagate
in a slice of five dimensional anti-deSitter space. In the latter instance, we
derive the interactions of the graviton, gauge, and fermion Kaluza-Klein (KK)
states. The resulting phenomenological signatures are shown to be highly
dependent on the value of the 5-dimensional fermion mass and differ
substantially from the case where the SM fields lie on the TeV-brane. In both
scenarios, we examine the collider signatures for direct production of the
graviton and gauge KK towers as well as their induced contributions to
precision electroweak observables. These direct and indirect signatures are
found to play a complementary role in the exploration of the model parameter
space. In the case where the SM field content resides on the TeV-brane, we show
that the LHC can probe the full parameter space and hence will either discover
or exclude this model if the scale of electroweak physics on the 3-brane is
less than 10 TeV. We also show that spontaneous electroweak symmetry breaking
of the SM must take place on the TeV-brane.Comment: 62 pages, Latex, 22 figure
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