435 research outputs found
Axinos as Dark Matter
Supersymmetric extensions of the Standard Model that incorporate the axion
solution to the strong CP problem necessarily contain also the axino, the
fermionic partner of the axion. In contrast to the neutralino and the
gravitino, the axino mass is generically not of the order of the
supersymmetry-breaking scale and can be much smaller. The axino is therefore an
intriguing candidate for a stable superpartner. In a previous Letter [1] it was
shown that axinos are a natural candidate for cold dark matter in the Universe
when they are generated non-thermally through out-of-equilibrium neutralino
decays. Here, we extend the study of non-thermal production and include a
competing thermal production mechanism through scatterings and decays of
particles in the plasma. We identify axino masses in the range of tens of MeV
to several GeV (depending on the scenario) as corresponding to cold axino
relics if the reheating temperature \treh is less than about
5\times10^4\gev. At higher \treh and lower mass, axinos could constitute
warm dark matter. In the scenario with axinos as relics, the gravitino problem
finds a natural solution. The lightest superpartner of the Standard Model
spectrum remains effectively stable in high-energy detectors but may be either
neutral or charged. The usual highly restrictive constraint \abundchi\lsim1
on the relic abundance of the lightest neutralino becomes void.Comment: 38 pages, a version to be published at JHE
Hybrid inflation with running inflaton mass
We realize and study a model of hybrid inflation in the context of softly
broken supersymmetry. The inflaton is taken to be a flat direction in the
superfield space and, due to unsuppressed couplings, its soft supersymmetry
breaking mass runs with scale. Both gauge and Yukawa couplings are taken into
account and different inflationary scenarios are investigated depending on the
relative strenght of the couplings and the mass spectrum.Comment: latex file, 14 pages with 3 eps figures, uses epsf.st
Running-mass models of inflation, and their observational constraints
If the inflaton sector is described by softly broken supersymmetry, and the
inflaton has unsuppressed couplings, the inflaton mass will run strongly with
scale. Four types of model are possible. The prediction for the spectral index
involves two parameters, while the COBE normalization involves a third, all of
them calculable functions of the relevant masses and couplings. A crude
estimate is made of the region of parameter space allowed by present
observation.Comment: Latex file, 20 pages, 11 figures, uses epsf.sty. Comment on the
observation of the spectral index scale dependence added; Fig. 3-6 improve
Observational constraints on the spectral index of the cosmological curvature perturbation
We evaluate the observational constraints on the spectral index , in the
context of the CDM hypothesis which represents the simplest viable
cosmology. We first take to be practically scale-independent. Ignoring
reionization, we find at a nominal 2- level . If
we make the more realisitic assumption that reionization occurs when a fraction
to 1 of the matter has collapsed, the 2- lower bound is
unchanged while the 1- bound rises slightly. These constraints are
compared with the prediction of various inflation models. Then we investigate
the two-parameter scale-dependent spectral index, predicted by running-mass
inflation models, and find that present data allow significant scale-dependence
of , which occurs in a physically reasonable regime of parameter space.Comment: ReVTeX, 15 pages, 5 figures and 3 tables, uses epsf.sty Improved
treatment of reionization and small bug fixed in the constant n case; more
convenient parameterization and better treatment of the n dependence in the
CMB anisotropy for the running mass case; conclusions basically unchanged;
references adde
Axino dark matter in brane world cosmology
We discuss dark matter in the brane world scenario. We work in the
Randall-Sundrum type II brane world and assume that the lightest supersymmetric
particle is the axino. We find that the axinos can play the role of cold dark
matter in the universe, provided that the five-dimensional Planck mass is
bounded both from below and from above. This is possible for higher reheating
temperatures compared to the conventional four-dimensional cosmology due to a
novel expansion law for the universe.Comment: 1+11 pages, version submitted to JCA
Thermal production of axino Dark Matter
We reconsider thermal production of axinos in the early universe, adding: a)
missed terms in the axino interaction; b) production via gluon decays
kinematically allowed by thermal masses; c) a precise modeling of reheating. We
find an axino abunance a few times larger than previous computations.Comment: 6 pages, 2 figures. Final version, to appear on JHE
New Q-ball Solutions in Gauge-Mediation, Affleck-Dine Baryogenesis and Gravitino Dark Matter
Affleck-Dine (AD) baryogenesis along a d=6 flat direction in gauge-mediated
supersymmetry-breaking (GMSB) models can produce unstable Q-balls which
naturally have field strength similar to the messenger scale. In this case a
new kind of Q-ball is formed, intermediate between gravity-mediated and
gauge-mediated type. We study in detail these new Q-ball solutions, showing how
their properties interpolate between standard gravity-mediated and
gauge-mediated Q-balls as the AD field becomes larger than the messenger scale.
It is shown that E/Q for the Q-balls can be greater than the nucleon mass but
less than the MSSM-LSP mass, leading to Q-ball decay directly to Standard Model
fermions with no MSSM-LSP production. More significantly, if E/Q is greater
than the MSSM-LSP mass, decaying Q-balls can provide a natural source of
non-thermal MSSM-LSPs, which can subsequently decay to gravitino dark matter
without violating nucleosynthesis constraints. The model therefore provides a
minimal scenario for baryogenesis and gravitino dark matter in the
gauge-mediated MSSM, requiring no new fields.Comment: 13 pages, 9 figures. Some corrections and additional discussion.
Version published in JCA
Possible Evidence For Axino Dark Matter In The Galactic Bulge
Recently, the SPI spectrometer on the INTEGRAL satellite observed strong 511
keV line emission from the galactic bulge. Although the angular distribution
(spherically symmetric with width of \sim 9 degree) of this emission is
difficult to account for with traditional astrophysical scenarios, light dark
matter particles could account for the observation. In this letter, we consider
the possibility that decaying axinos in an R-parity violating model of
supersymmetry may be the source of this emission. We find that \sim 1-300 MeV
axinos with R-parity violating couplings can naturally produce the observed
emission.Comment: 4 pages, 1 figure. Version accepted by Physical Review
Leptogenesis with "Fuzzy Mass Shell" for Majorana Neutrinos
We study the mixing of elementary and composite particles. In quantum field
theory the mixing of composite particles originates in the couplings of the
constituent quarks and for neutrinos in self-energy diagrams. In the event that
the incoming and outgoing neutrinos have different masses, the self-energy
diagrams vanish because energy is not conserved but the finite decaying widths
make the mixing possible. We can consider the neutrinos to be "fuzzy" states on
their mass shell and the mixing is understood as the overlap of two
wavefunctions. These considerations restrict the mass difference to be
approximately equal to or smaller than the largest of the two widths: abs(M_i -
M_j) lessorequal max(Gamma_i, Gamma_j).Comment: 11 pages, 1 figur
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