1,781 research outputs found
Lepton Flavor Violation without Supersymmetry
We study the lepton flavor violating (LFV) processes mu -> e gamma, mu -> 3e,
and mu -> e conversion in nuclei in the left-right symmetric model without
supersymmetry and perform the first complete computation of the LFV branching
ratios B(mu -> f) to leading non-trivial order in the ratio of left- and
right-handed symmetry breaking scales. To this order, B(mu -> e gamma) and B(mu
-> e) are governed by the same combination of LFV violating couplings, and
their ratio is naturally of order unity. We also find B(mu -> 3 e)/B(mu -> e)
\sim 100 under slightly stronger assumptions. Existing limits on the branching
ratios already substantially constrain mass splittings and/or mixings in the
heavy neutrino sector. When combined with future collider studies and precision
electroweak measurements, improved limits on LFV processes will test the
viability of low-scale, non-supersymmetric LFV scenarios.Comment: 24 pages, 7 figures, 2 table
Cosmological Hysteresis and the Cyclic Universe
A Universe filled with a homogeneous scalar field exhibits `Cosmological
hysteresis'. Cosmological hysteresis is caused by the asymmetry in the equation
of state during expansion and contraction. This asymmetry results in the
formation of a hysteresis loop: , whose value can be non-vanishing
during each oscillatory cycle. For flat potentials, a negative value of the
hysteresis loop leads to the increase in amplitude of consecutive cycles and to
a universe with older and larger successive cycles. Such a universe appears to
possess an arrow of time even though entropy production is absent and all of
the equations respect time-reversal symmetry ! Cosmological hysteresis appears
to be widespread and exists for a large class of scalar field potentials and
mechanisms for making the universe bounce. For steep potentials, the value of
the hysteresis loop can be positive as well as negative. The expansion factor
in this case displays quasi-periodic behaviour in which successive cycles can
be both larger as well as smaller than previous ones. This quasi-regular
pattern resembles the phenomenon of BEATS displayed by acoustic systems.
Remarkably, the expression relating the increase/decrease in oscillatory cycles
to the quantum of hysteresis appears to be model independent. The cyclic
scenario is extended to spatially anisotropic models and it is shown that the
anisotropy density decreases during successive cycles if the hysteresis loop is
negative.Comment: 31 pages, 8 figures. Matches version published in Phys Rev D85,
123542 (2012
Physical constants and the Gurzadyan-Xue formula for the dark energy
We consider cosmological implications of the formula for the dark energy
density derived by Gurzadyan and Xue which predicts a value fitting the
observational one. Cosmological models with varying by time physical constants,
namely, speed of light and gravitational constant and/or their combinations,
are considered. In one of the models, for example, vacuum energy density
induces effective negative curvature, while another one has an unusual
asymptotic. This analysis also explicitely rises the issue of the meaning and
content of physical units and constants in cosmological context.Comment: version corrected to match the one to appear in Modern Physics
Letters
Turbulence without pressure
We develop exact field theoretic methods to treat turbulence when the effect
of pressure is negligible. We find explicit forms of certain probability
distributions, demonstrate that the breakdown of Galilean invariance is
responsible for intermittency and establish the operator product expansion. We
also indicate how the effects of pressure can be turned on perturbatively.Comment: 12 page
HyRec: A fast and highly accurate primordial hydrogen and helium recombination code
We present a state-of-the-art primordial recombination code, HyRec, including
all the physical effects that have been shown to significantly affect
recombination. The computation of helium recombination includes simple analytic
treatments of hydrogen continuum opacity in the He I 2 1P - 1 1S line, the He
I] 2 3P - 1 1S line, and treats feedback between these lines within the
on-the-spot approximation. Hydrogen recombination is computed using the
effective multilevel atom method, virtually accounting for an infinite number
of excited states. We account for two-photon transitions from 2s and higher
levels as well as frequency diffusion in Lyman-alpha with a full radiative
transfer calculation. We present a new method to evolve the radiation field
simultaneously with the level populations and the free electron fraction. These
computations are sped up by taking advantage of the particular sparseness
pattern of the equations describing the radiative transfer. The computation
time for a full recombination history is ~2 seconds. This makes our code well
suited for inclusion in Monte Carlo Markov chains for cosmological parameter
estimation from upcoming high-precision cosmic microwave background anisotropy
measurements.Comment: Version accepted by PRD. Numerical integration switches adapted to be
well behaved for a wide range of cosmologies (Sec. V E). HyRec is available
at http://www.tapir.caltech.edu/~yacine/hyrec/hyrec.htm
Reflection coefficient for superresonant scattering
We investigate superresonant scattering of acoustic disturbances from a
rotating acoustic black hole in the low frequency range. We derive an
expression for the reflection coefficient, exhibiting its frequency dependence
in this regime.Comment: 7 page
On domain shapes and processes in supersymmetric theories
A supersymmetric theory with several scalar superfields generically has
several domain wall type classical configurations which interpolate between
various supersymmetric vacua of the scalar fields. Depending on the couplings,
some of these configurations develop instability and decay into multiple domain
walls, others can form intersections in space. These phenomena are considered
here in a simplest, yet non-trivial, model with two scalar superfields.Comment: 10 pages, LaTeX, 5 figures in LaTe
On the small-scale stability of thermonuclear flames in Type Ia supernovae
We present a numerical model which allows us to investigate thermonuclear
flames in Type Ia supernova explosions. The model is based on a finite-volume
explicit hydrodynamics solver employing PPM. Using the level-set technique
combined with in-cell reconstruction and flux-splitting schemes we are able to
describe the flame in the discontinuity approximation. We apply our
implementation to flame propagation in Chandrasekhar-mass Type Ia supernova
models. In particular we concentrate on intermediate scales between the flame
width and the Gibson-scale, where the burning front is subject to the
Landau-Darrieus instability. We are able to reproduce the theoretical
prediction on the growth rates of perturbations in the linear regime and
observe the stabilization of the flame in a cellular shape. The increase of the
mean burning velocity due to the enlarged flame surface is measured. Results of
our simulation are in agreement with semianalytical studies.Comment: 9 pages, 7 figures, Uses AASTEX, emulateapj5.sty, onecolfloat.sty.
Replaced with accepted version (ApJ), Figures 1 and 3 are ne
An Early Universe Model with Stiff Matter and a Cosmological Constant
In the present work, we study the quantum cosmology description of a
Friedmann-Robertson-Walker model in the presence of a stiff matter perfect
fluid and a negative cosmological constant. We work in the Schutz's variational
formalism and the spatial sections have constant negative curvature. We
quantize the model and obtain the appropriate Wheeler-DeWitt equation. In this
model the states are bounded therefore we compute the discrete energy spectrum
and the corresponding eigenfunctions. In the present work, we consider only the
negative eigenvalues and their corresponding eigenfunctions. This choice
implies that the energy density of the perfect fluid is negative. A stiff
matter perfect fluid with this property produces a model with a bouncing
solution, at the classical level, free from an initial singularity. After that,
we use the eigenfunctions in order to construct wave packets and evaluate the
time-dependent expectation value of the scale factor. We find that it
oscillates between maximum and minimum values. Since the expectation value of
the scale factor never vanishes, we confirm that this model is free from an
initial singularity, also, at the quantum level.Comment: 12 Pages, 4 Figures. Final version. Accepted for publication in the
Proceedings of the 8th Friedmann Seminar, Rio de Janeiro, 2011. We restricted
our attention to treat the case where the stiff matter has negative energy
eigenvalues, following the referee's suggestio
Constraints On The Delayed Transition to Detonation in Type Ia Supernovae
We investigate the possibility of a delayed detonation in a type Ia supernova
under the assumption that the transition to detonation is triggered by
turbulence only. Our discussion is based on the Zeldovich mechanism and
suggests that typical turbulent velocities present during the explosion are not
strong enough to allow this transition to occur. Although we are able to show
that in carbon-rich matter (e.g., C) the possibility of a
deflagration to detonation transition (DDT) is enhanced, even in this case the
turbulent velocities needed are larger than the expected value of on a length-scale of cm. Thus we
conclude that a DDT may not be a common event during a thermonuclear explosion
of a Chandrasekhar-mass white dwarf.Comment: 18 pages, 5 figures, accepted for publication in the Ap
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