1,309 research outputs found
Gravitino Dark Matter with Weak-Scale Right-Handed Sneutrino
We consider cosmological implications of supersymmetric models with
right-handed (s)neutrinos where the neutrino masses are purely Dirac-type. We
pay particular attention to the case where gravitino is the lightest
superparticle while one of the right-handed sneutrinos is next-to-the-lightest
superparticle. We study constraints from big-bang nuleosynthesis and show that
the constraints could be relaxed compared to the case without right-handed
sneutrinos. As a result, the gravitino-dark-matter scenario becomes viable with
relatively large value of the gravitino mass. We also discuss constraints from
the structure formation; in our model, the free-streaming length of the
gravitino dark matter may be as long as O(1 Mpc), which is comparable to the
present observational upper bound on the scale of free-streaming.Comment: 18 pages, 6 figure
Synchrotron Radiation from the Galactic Center in Decaying Dark Matter Scenario
We discuss the synchrotron radiation flux from the Galactic center in
unstable dark matter scenario. Motivated by the anomalous excess of the
positron fraction recently reported by the PAMELA collaboration, we consider
the case that the dark matter particle is unstable (and long-lived), and that
energetic electron and positron are produced by the decay of dark matter. Then,
the emitted electron and positron becomes the source of the synchrotron
radiation. We calculate the synchrotron radiation flux for models of decaying
dark matter, which can explain the PAMELA positron excess. Taking the lifetime
of the dark matter of O(10^26 sec), which is the suggested value to explain the
PAMELA anomaly, the synchrotron radiation flux is found to be O(1 kJy/str) or
smaller, depending on the particle-physics and cosmological parameters.Comment: 20 pages, 6 figure
Higgs Properties and Fourth Generation Leptons
It is possible that there are additional vector-like generations where the
quarks have mass terms that do not originate from weak symmetry breaking, but
the leptons only get mass through weak symmetry breaking. We discuss the impact
that the new leptons have on Higgs boson decay branching ratios and on the
range of allowed Higgs masses in such a model (with a single new vector-like
generation). We find that if the fourth generation leptons are too heavy to be
produced in Higgs decay, then the new leptons reduce the branching ratio for h
-> gamma gamma to about 30% of its standard-model value. The dependence of this
branching ratio on the new charged lepton masses is weak. Furthermore the
expected Higgs production rate at the LHC is very near its standard-model value
if the new quarks are much heavier than the weak scale. If the new quarks have
masses near the cutoff for the theory then for cutoffs greater than 10^15 GeV,
the new lepton masses cannot be much heavier than about 100 GeV and the Higgs
mass must have a value around 175 GeV.Comment: 8 pages, 8 figures, published versio
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