3,885 research outputs found
Neutrino properties from Yukawa structure
We discuss the implications for lepton mixing and CP violation of structure
in the lepton mass matrices, for the case that neutrino masses are generated by
the see-saw mechanism with an hierarchical structure for the Majorana masses.
For a particularly interesting case with enhanced symmetry in which the lepton
Dirac mass matrices are related to those in the quark sector, the CHOOZ angle
is near the present limit and the CP violating phase relevant to thermal
leptogenesis and to decay is near maximal.Comment: 13 pages, 7 figures. References added and typos corrected. Mistake in
the discussion of leptogenesis correcte
PAMELA's cosmic positron from decaying LSP in SO(10) SUSY GUT
We propose two viable scenarios explaining the recent observations on cosmic
positron excess. In both scenarios, the present relic density in the Universe
is assumed to be still supported by thermally produced WIMP or LSP (\chi). One
of the scenarios is based on two dark matter (DM) components (\chi,X) scenario,
and the other is on SO(10) SUSY GUT. In the two DM components scenario,
extremely small amount of non-thermally produced meta-stable DM component
[O(10^{-10}) < n_X /n_\chi] explains the cosmic positron excess. In the SO(10)
model, extremely small R-parity violation for LSP decay to e^\pm is naturally
achieved with a non-zero VEV of the superpartner of one right-handed neutrino
(\tilde{\nu}^c) and a global symmetry.Comment: 6 pages, Talks presented in PASCOS, SUSY, and COSMO/CosPA in 201
Probing the stability of superheavy dark matter particles with high-energy neutrinos
Two of the most fundamental properties of the dark matter particle, the mass
and the lifetime, are only weakly constrained by the astronomical and
cosmological evidence of dark matter. We derive in this paper lower limits on
the lifetime of dark matter particles with masses in the range 10 TeV-10^15 TeV
from the non-observation of ultrahigh energy neutrinos in the AMANDA, IceCube,
Auger and ANITA experiments. For dark matter particles which produce neutrinos
in a two body or a three body decay, we find that the dark matter lifetime must
be longer than O(10^26-10^28) s for masses between 10 TeV and the Grand
Unification scale. Finally, we also calculate, for concrete particle physics
scenarios, the limits on the strength of the interactions that induce the dark
matter decay.Comment: 17 pages, 6 figures; v2: references added, discussion improved,
matches the version published at JCA
Supergraph Techniques and Two-Loop Beta-Functions for Renormalizable and Non-Renormalizable Operators
We present a construction kit for calculating two-loop beta functions in N=1
supersymmetric theories for the operators of the superpotential using
supergraph techniques. In particular, it allows to compute the beta functions
for every desired, even higher dimensional, operator of the superpotential from
the wavefunction renormalization constants of the theory. We apply this method
to calculate the two-loop beta functions for the lowest-dimensional effective
neutrino mass operator in the Minimal Supersymmetric Standard Model (MSSM) and
for the Yukawa couplings in the MSSM extended by singlet superfields and the
mass matrix for the latter. Our method can be applied to any N=1 supersymmetric
theory.Comment: 15 pages, 13 figures; error in two-loop trace terms correcte
Gamma-rays from Heavy Minimal Dark Matter
We consider the annihilation into gamma rays of Minimal Dark Matter
candidates in the fermionic 5-plet and scalar 7-plet representations of
, taking into account both the Sommerfeld effect and the internal
bremsstrahlung. Assuming the Einasto profile, we show that present measurements
of the Galactic Center by the H.E.S.S. instrument exclude the 5-plet and 7-plet
as the dominant form of dark matter for masses between 1 TeV and 20 TeV, in
particular, the 5-plet mass leading to the observed dark matter density via
thermal freeze-out. We also discuss prospects for the upcoming Cherenkov
Telescope Array, which will be able to probe even heavier dark matter masses,
including the scenario where the scalar 7-plet is thermally produced.Comment: 27 pages, 8 figures. Matches the version accepted for publication by
JCA
Validity of the N\'{e}el-Arrhenius model for highly anisotropic Co_xFe_{3-x}O_4 nanoparticles
We report a systematic study on the structural and magnetic properties of
Co_{x}Fe_{3-x}O_{4} magnetic nanoparticles with sizes between to nm,
prepared by thermal decomposition of Fe(acac)_{3} and Co(acac)_{2}. The large
magneto-crystalline anisotropy of the synthesized particles resulted in high
blocking temperatures ( K \leqq K for d nm ) and large coercive fields ( kA/m for K).
The smallest particles ( nm) revealed the existence of a magnetically
hard, spin-disordered surface. The thermal dependence of static and dynamic
magnetic properties of the whole series of samples could be explained within
the N\'{e}el-Arrhenius relaxation framework without the need of ad-hoc
corrections, by including the thermal dependence of the magnetocrystalline
anisotropy constant through the empirical Br\"{u}khatov-Kirensky
relation. This approach provided values very similar to the bulk
material from either static or dynamic magnetic measurements, as well as
realistic values for the response times ( s).
Deviations from the bulk anisotropy values found for the smallest particles
could be qualitatively explained based on Zener\'{}s relation between
and M(T)
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