16,522 research outputs found
Relic density and PAMELA events in a heavy wino dark matter model with Sommerfeld effect
In a wino LSP scenario the annihilation cross section of winos
gravitationally bound in galaxies can be boosted by a Sommerfeld enhancement
factor which arises due to the ladder of exchanged W bosons between the initial
states. The boost factor obtained can be in the range S ~ 10^4 if the mass is
close to the resonance value of M ~ 4 TeV. In this paper we show that if one
takes into account the Sommerfeld enhancement in the relic abundance
calculation then the correct relic density is obtained for 4 TeV wino mass due
to the enhanced annihilation after their kinetic decoupling. At the same time
the Sommerfeld enhancement in the \chi \chi --> W^+ W^- annihilation channel is
sufficient to explain the positron flux seen in PAMELA data without
significantly exceeding the observed antiproton signal. We also show that (e^-
+ e^+) and gamma ray signals are broadly compatible with the Fermi-LAT
observations. In conclusion we show that a 4 TeV wino DM can explain the
positron and antiproton fluxes observed by PAMELA and at the same time give a
thermal relic abundance of CDM consistent with WMAP observations.Comment: 24 pages, 12 figures, 1 table; title corrected in arxiv metadat
Measuring Fundamental Parameters of Substellar Objects. II: Masses and Radii
We present mass and radius derivations for a sample of very young, mid- to
late M, low-mass stellar and substellar objects in Upper Sco and Taurus. In a
previous paper, we determined effective temperatures and surface gravities for
these targets, from an analysis of their high-resolution optical spectra and
comparisons to the latest synthetic spectra. We now derive extinctions, radii,
masses and luminosities by combining our previous results with observed
photometry, surface fluxes from the synthetic spectra and the known cluster
distances. These are the first mass and radius estimates for young, very low
mass bodies that are independent of theoretical evolutionary models (though our
estimates do depend on spectral modeling). We find that for most of our sample,
our derived mass-radius and mass-luminosity relationships are in very good
agreement with the theoretical predictions. However, our results diverge from
the evolutionary model values for the coolest, lowest-mass targets: our
inferred radii and luminosities are significantly larger than predicted for
these objects at the likely cluster ages, causing them to appear much younger
than expected. We suggest that uncertainties in the evolutionary models - e.g.,
in the choice of initial conditions and/or treatment of interior convection -
may be responsible for this discrepancy. Finally, two of our late-M objects
(USco 128 and 130) appear to have masses close to the deuterium-fusion boundary
(9--14 Jupiters, within a factor of 2). This conclusion is primarily a
consequence of their considerable faintness compared to other targets with
similar extinction, spectral type and temperature (difference of 1 mag). Our
result suggests that the faintest young late-M or cooler objects may be
significantly lower in mass than the current theoretical tracks indicate.Comment: 54 pages, incl. 5 figs, accepted Ap
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Secure Anonymous Routing for MANETs Using Distributed Dynamic Random Path Selection
Most of the MANET security research has so far focused on providing routing security and confidentiality to the data packets, but less has been done to ensure privacy and anonymity of the communicating entities. In this paper, we propose a routing protocol which ensures anonymity, privacy of the user. This is achieved by randomly selecting next hop at each intermediate. This protocol also provides data security using public key ciphers. The protocol is simulated using in-house simulator written in C with OpenSSL crypto APIs. The robustness of our protocol is evaluated against known security attacks
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