219 research outputs found
Gauge Coupling Variation in Brane Models
We consider the space-time variation of gauge couplings in brane-world models
induced by the coupling to a bulk scalar field. A variation is generated by the
running of the gauge couplings with energy and a conformal anomaly while going
from the Jordan to the Einstein frame. We indicate that the one-loop
corrections cancel implying that one obtains a variation of the fine structure
constant by either directly coupling the gauge fields to the bulk scalar field
or having bulk scalar field dependent Yukawa couplings. Taking into account the
cosmological dynamics of the bulk scalar field, we constrain the strength of
the gauge coupling dependence on the bulk scalar field and relate it to
modifications of gravity at low energy.Comment: 4 pages, 1 figur
The effect of dietary intervention on the metabolic and behavioural impairments generated by short term high fat feeding in the rat
AbstractPrevious studies have shown that rats fed a high calorie diet rich in saturated fat for 12weeks exhibit peripheral insulin resistance and impairments of behavioural flexibility when switched from an operant delayed matching to place (DMTP) schedule to a delayed non-matching to place (DNMTP) schedule. However, the metabolic changes evoked by feeding a high fat (HF) diet can be observed within two weeks of commencing the diet. The current study has confirmed that 4weeks exposure to an HF diet resulted in increased body weight, peripheral insulin resistance and plasma leptin. Studies performed during weeks 3 and 4 on the HF diet revealed suppressed lever pressing rates and impaired behavioural flexibility in the operant DMTP/DNMTP task. When animals fed the HF diet were then returned to a standard chow (SC) diet for 5weeks their weight and blood biochemistry no longer differed from those measured in animals that had never been exposed to the HF diet. The animals restored to the SC diet exhibited a clear ability to acquire the DNMTP schedule of reinforcement although these animals continued to lever press at a lower rate when compared with animals that received the SC diet throughout. The data suggest that exposure to an HF diet diminishes the motivation to respond for a reward and, thus, the capacity to adapt behavioural performance. This deficit was ameliorated, but not totally reversed, by the dietary intervention. If also true for humans, the results suggest that deficits in behavioural flexibility develop after only a short period on a high calorie diet but may be largely reversible through simple dietary intervention, at least in the early stages of deficit development. However, the putative effects of short-term exposure to an HF diet on behavioural motivation may persist for some time after switching to a healthier low fat diet and remain a problem for those seeking to adopt a healthier diet
Robust signatures of solar neutrino oscillation solutions
With the goal of identifying signatures that select specific neutrino
oscillation parameters, we test the robustness of global oscillation solutions
that fit all the available solar and reactor experimental data. We use three
global analysis strategies previously applied by different authors and also
determine the sensitivity of the oscillation solutions to the critical nuclear
fusion cross section, S_{17}(0), for the production of 8B. The favored
solutions are LMA, LOW, and VAC in order of g.o.f. The neutral current to
charged current ratio for SNO is predicted to be 3.5 +- 0.6 (1 sigma), which is
separated from the no-oscillation value of 1.0 by much more than the expected
experimental error. The predicted range of the day-night difference in charged
current rates is (8.2 +- 5.2)% and is strongly correlated with the day-night
effect for neutrino-electron scattering. A measurement by SNO of either a NC to
CC ratio > 3.3 or a day-night difference > 10%, would favor a small region of
the currently allowed LMA neutrino parameter space. The global oscillation
solutions predict a 7Be neutrino-electron scattering rate in BOREXINO and
KamLAND in the range 0.66 +- 0.04 of the BP00 standard solar model rate, a
prediction which can be used to test both the solar model and the neutrino
oscillation theory. Only the LOW solution predicts a large day-night effect(<
42%) in BOREXINO and KamLAND. For the KamLAND reactor experiment, the LMA
solution predicts 0.44 of the standard model rate; we evaluate 1 sigma and 3
sigma uncertainties and the first and second moments of the energy spectrum.Comment: Included predictions for KamLAND reactor experiment and updated to
include 1496 days of Super-Kamiokande observation
Interacting fermions in self-similar potentials
We consider interacting spinless fermions in one dimension embedded in
self-similar quasiperiodic potentials. We examine generalizations of the
Fibonacci potential known as precious mean potentials. Using a bosonization
technique and a renormalization group analysis, we study the low-energy physics
of the system. We show that it undergoes a metal-insulator transition for any
filling factor, with a critical interaction that strongly depends on the
position of the Fermi level in the Fourier spectrum of the potential. For some
positions of the Fermi level the metal-insulator transition occurs at the non
interacting point. The repulsive side is an insulator with a gapped spectrum
whereas in the attractive side the spectrum is gapless and the properties of
the system are described by a Luttinger liquid. We compute the transport
properties and give the characteristic exponents associated to the frequency
and temperature dependence of the conductivity.Comment: 18 pages, 10 EPS figure
Natural Orbitals and BEC in traps, a diffusion Monte Carlo analysis
We investigate the properties of hard core Bosons in harmonic traps over a
wide range of densities. Bose-Einstein condensation is formulated using the
one-body Density Matrix (OBDM) which is equally valid at low and high
densities. The OBDM is calculated using diffusion Monte Carlo methods and it is
diagonalized to obtain the "natural" single particle orbitals and their
occupation, including the condensate fraction. At low Boson density, , where and is the hard core diameter, the condensate is
localized at the center of the trap. As increases, the condensate moves
to the edges of the trap. At high density it is localized at the edges of the
trap. At the Gross-Pitaevskii theory of the condensate
describes the whole system within 1%. At corrections are
3% to the GP energy but 30% to the Bogoliubov prediction of the condensate
depletion. At , mean field theory fails. At , the Bosons behave more like a liquid He droplet than a trapped Boson
gas.Comment: 13 pages, 14 figures, submitted Phys. Rev.
From weak-scale observables to leptogenesis
Thermal leptogenesis is an attractive mechanism for generating the baryon
asymmetry of the Universe. However, in supersymmetric models, the parameter
space is severely restricted by the gravitino bound on the reheat temperature
. For hierarchical light neutrino masses, it is shown that thermal
leptogenesis {\it can} work when GeV. The low-energy
observable consequences of this scenario are . For higher , thermal leptogenesis works in a
larger area of parameter space, whose observable consequences are more
ambiguous. A parametrisation of the seesaw in terms of weak-scale inputs is
used, so the results are independent of the texture chosen for the GUT-scale
Yukawa matrices.Comment: a few references adde
Neutrino Masses with "Zero Sum" Condition:
It is well known that the neutrino mass matrix contains more parameters than
experimentalists can hope to measure in the foreseeable future even if we
impose CP invariance. Thus, various authors have proposed ansatzes to restrict
the form of the neutrino mass matrix further. Here we propose that ; this ``zero sum'' condition can occur in certain
class of models, such as models whose neutrino mass matrix can be expressed as
commutator of two matrices. With this condition, the absolute neutrino mass can
be obtained in terms of the mass-squared differences. When combined with the
accumulated experimental data this condition predicts two types of mass
hierarchies, with one of them characterized by eV, and the other by eV and eV. The mass ranges
predicted is just below the cosmological upper bound of 0.23 eV from recent
WMAP data and can be probed in the near future. We also point out some
implications for direct laboratory measurement of neutrino masses, and the
neutrino mass matrix.Comment: Latex 12 pages. No figures. New references adde
Recent Advances in Understanding Particle Acceleration Processes in Solar Flares
We review basic theoretical concepts in particle acceleration, with
particular emphasis on processes likely to occur in regions of magnetic
reconnection. Several new developments are discussed, including detailed
studies of reconnection in three-dimensional magnetic field configurations
(e.g., current sheets, collapsing traps, separatrix regions) and stochastic
acceleration in a turbulent environment. Fluid, test-particle, and
particle-in-cell approaches are used and results compared. While these studies
show considerable promise in accounting for the various observational
manifestations of solar flares, they are limited by a number of factors, mostly
relating to available computational power. Not the least of these issues is the
need to explicitly incorporate the electrodynamic feedback of the accelerated
particles themselves on the environment in which they are accelerated. A brief
prognosis for future advancement is offered.Comment: This is a chapter in a monograph on the physics of solar flares,
inspired by RHESSI observations. The individual articles are to appear in
Space Science Reviews (2011
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