9,321 research outputs found
Gravitino dark matter from increased thermal relic particles
We investigate the so-called superWIMP scenario with gravitino as the
lightest supersymmetric particle (LSP) in the context of non-standard
cosmology, in particular, brane world cosmology. As a candidate of the
next-to-LSP (NLSP), we examine slepton and sneutrino. Brane world cosmological
effects dramatically enhance the relic density of the slepton or sneutrino
NLSP, so that the NLSP with mass of order 100 GeV can provide the correct
abundance of gravitino dark matter through its decay. We find that with an
appropriate five dimensional Planck mass, this scenario can be realized
consistently with the constraints from Big Bang Nucleosynthesis (BBN) for both
NLSP candidates of slepton and sneutrino. The BBN constraints for slepton NLSP
are more stringent than that for sneutrino, as the result, the gravitino must
be rather warm in the slepton NLSP case. The energy density of gravitino
produced by thermal scattering is highly suppressed and negligible due to the
brane world cosmological effects.Comment: 15 pages, discussion and references added, the final versio
Neutralino dark matter in brane world cosmology
The thermal relic density of the neutralino dark matter in the brane world
cosmology is studied. Since the expansion law at a high energy regime in the
brane world cosmology is modified from the one in the standard cosmology, the
resultant relic density can be altered. It has been found that, if the five
dimensional Planck mass is lower than TeV, the brane world
cosmological effect is significant at the decoupling time and the resultant
relic density is enhanced. We calculate the neutralino relic density in the
Constrained Minimal Supersymmetric Standard Model (CMSSM) and show that the
allowed region is dramatically modified from the one in the standard cosmology
and eventually disappears as is decreasing. We also find a new lower
bound on TeV based on the neutralino dark matter hypothesis,
namely the lower bound in order for the allowed region of the neutralino dark
matter to exist.Comment: 12 pages, 8 figure
Gauss-Bonnet braneworld cosmological effect on relic density of dark matter
In Gauss-Bonnet braneworld cosmology, the Friedmann equation of our
four-dimensional universe on 3-brane is modified in a high energy regime
(Gauss-Bonnet regime), while the standard expansion law is reproduced in low
energies (standard regime). We investigate the Gauss-Bonnet braneworld
cosmological effect on the thermal relic density of cold dark matter when the
freeze-out of the dark matter occurs in the Gaugss-Bonnet regime. We find that
the resultant relic density is considerably reduced when the transition
temperature, which connects the Gauss-Bonnet regime with the standard regime,
is low enough. This result is in sharp contrast with the result previously
obtained in the Randall-Sundrum braneworld cosmology, where the relic density
is enhanced.Comment: 11 pages, 2 figures, new references were added, some typos were
correcte
Statistical mechanics of lossy compression using multilayer perceptrons
Statistical mechanics is applied to lossy compression using multilayer
perceptrons for unbiased Boolean messages. We utilize a tree-like committee
machine (committee tree) and tree-like parity machine (parity tree) whose
transfer functions are monotonic. For compression using committee tree, a lower
bound of achievable distortion becomes small as the number of hidden units K
increases. However, it cannot reach the Shannon bound even where K -> infty.
For a compression using a parity tree with K >= 2 hidden units, the rate
distortion function, which is known as the theoretical limit for compression,
is derived where the code length becomes infinity.Comment: 12 pages, 5 figure
The X-ray binary population in M33: II. X-ray spectra and variability
In this paper we investigate the X-ray spectra and X-ray spectral variability
of compact X-ray sources for 3 Chandra observations of the Local Group galaxy
M33. The observations are centered on the nucleus and the star forming region
NGC 604. In the observations 261 sources have been detected. For a total of 43
sources the number of net counts is above 100, sufficient for a more detailed
spectral fitting. Of these sources, 25 have been observed in more than one
observation, allowing the study of spectral variability on ~months timescales.
A quarter of the sources are found to be variable between observations.
However, except for two foreground sources, no source is variable within any
observation above the 99% confidence level. Only six sources show significant
spectral variability between observations. A comparison of N_H values with HI
observations shows that X-ray absorption values are consistent with Galactic
X-ray binaries and most sources in M33 are intrinsically absorbed. The pattern
of variability and the spectral parameters of these sources are consistent with
the M33 X-ray source population being dominated by X-ray binaries: Two thirds
of the 43 bright sources have spectral and timing properties consistent with
X-ray binaries; we also find two candidates for super-soft sources and two
candidates for quasi-soft sources.Comment: 25 pages, ApJ accepte
Thermal leptogenesis in brane world cosmology
The thermal leptogenesis in brane world cosmology is studied. In brane world
cosmology, the expansion law is modified from the four-dimensional standard
cosmological one at high temperature regime in the early universe. As a result,
the well-known upper bound on the lightest light neutrino mass induced by the
condition for the out-of-equilibrium decay of the lightest heavy neutrino,
eV, can be moderated to be in the case of with the
lightest heavy neutrino mass () and the ``transition temperature''
(), at which the modified expansion law in brane world cosmology is
smoothly connecting with the standard one. This implies that the degenerate
mass spectrum of the light neutrinos can be consistent with the thermal
leptogenesis scenario. Furthermore, as recently pointed out, the gravitino
problem in supersymmetric case can be solved if the transition temperature is
low enough GeV. Therefore, even in the supersymmetric
case, thermal leptogenesis scenario can be successfully realized in brane world
cosmology.Comment: 9 pages, final versio
Resonant Leptogenesis in the Minimal B-L Extended Standard Model at TeV
We investigate the resonant leptogenesis scenario in the minimal B-L extended
standard model(SM) with the B-L symmetry breaking at the TeV scale. Through
detailed analysis of the Boltzmann equations, we show how much the resultant
baryon asymmetry via leptogenesis is enhanced or suppressed, depending on the
model parameters, in particular, the neutrino Dirac Yukawa couplings and the
TeV-scale Majorana masses of heavy degenerate neutrinos. In order to consider a
realistic case, we impose a simple ansatz for the model parameters and analyze
the neutrino oscillation parameters and the baryon asymmetry via leptogenesis
as a function of only a single CP-phase. We find that for a fixed CP-phase all
neutrino oscillation data and the observed baryon asymmetry of the present
universe can be simultaneously reproduced.Comment: 25 pages, 15 figures, version to be published in Phys. Rev.
Electroweak phase transition in a nonminimal supersymmetric model
The Higgs potential of the minimal nonminimal supersymmetric standard model
(MNMSSM) is investigated within the context of electroweak phase transition. We
investigate the allowed parameter space yielding correct electroweak phase
transitoin employing a high temperature approximation. We devote to
phenomenological consequences for the Higgs sector of the MNMSSM for
electron-positron colliders. It is observed that a future linear
collider with GeV will be able to test the model with regard
to electroweak baryogenesis.Comment: 28 pages, 5 tables, 12 figure
Leptogenesis in models with multi-Higgs bosons
We study the leptogenesis scenario in models with multi-Higgs doublets. It is
pointed out that the washing-out process through the effective dimension five
interactions, which has not been taken into account seriously in the
conventional scenario, can be effective, and the resultant baryon asymmetry can
be exponentially suppressed. This fact implies new possible scenario where the
observed baryon asymmetry is the remnant of the washed out lepton asymmetry
which was originally much larger than the one in the conventional scenario. Our
new scenario is applicable to some neutrino mass matrix models which predict
too large CP-violating parameter and makes them viable through the washing-out
process.Comment: Latex 2e, 11 pages, 2 figures. Many parts in the original manuscript
have been revised, but conclusions are unchange
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