2,749 research outputs found
The Cosmological Energy Density of Neutrinos from Oscillation Measurements
The emerging structure of the neutrino mass matrix, when combined with the
primordial element abundances, places the most stringent constraint on the
flavor asymmetries in the cosmological neutrino background and therefore its
energy density. I review the mechanism of synchronized neutrino oscillations in
the an early universe with degenerate (asymmetric) neutrino and antineutrino
densities and the implications of refined measurements of neutrino parameters.Comment: 4 pages, Proceedings of NuFact 03, 5th International Workshop on
Neutrino Factories & Superbeams, 5-11 June 2003, Columbia University, New
Yor
Resonantly-Produced 7 keV Sterile Neutrino Dark Matter Models and the Properties of Milky Way Satellites
Sterile neutrinos produced through a resonant Shi-Fuller mechanism are
arguably the simplest model for a dark matter interpretation origin of the
recent unidentified X-ray line seen toward a number of objects harboring dark
matter. Here, I calculate the exact parameters required in this mechanism to
produce the signal. The suppression of small scale structure predicted by these
models is consistent with Local Group and high- galaxy count constraints.
Very significantly, the parameters necessary in these models to produce the
full dark matter density fulfill previously determined requirements to
successfully match the Milky Way Galaxy's total satellite abundance, the
satellites' radial distribution and their mass density profile, or "too big to
fail problem." I also discuss how further precision determinations of the
detailed properties of the candidate sterile neutrino dark matter can probe the
nature of the quark-hadron transition, which takes place during the dark matter
production.Comment: 5 pages, 3 figures. v3: discussion added, matches version accepted to
Phys. Rev. Let
Bulk QCD Thermodynamics and Sterile Neutrino Dark Matter
We point out that the relic densities of singlet (sterile) neutrinos of
interest in viable warm and cold dark matter scenarios, depend on the
characteristics of the QCD transition in the early universe. In the most
promising of these dark matter scenarios the production of the singlets occurs
at or near the QCD transition. Since production of the singlets, their
dilution, and the disappearance of weak scatterers occur simultaneously, we
calculate these processes contemporaneously to obtain accurate predictions of
relic sterile neutrino mass density. Therefore, a determination of the mass and
superweak mixing of the singlet neutrino through, for example, its radiative
decay, along with a determination of its contribution to the critical density,
can provide insight into the finite-temperature QCD transition.Comment: 7 pages, 4 figures, to match version in Phys. Rev.
The orientation of the nuclear obscurer of the AGNs
We examine the distribution of axis ratios of a large sample of disk galaxies
hosting type 2 AGNs selected from the Sloan Digital Sky Survey and compare it
with a well-defined control sample of non-active galaxies. We find them
significantly different, where the type 2 AGNs show both an excess of edge-on
objects and deficit of round objects. This systematical bias can not be
explained by a nuclear obscurer oriented randomly with respect to the stellar
disk. However, a nuclear obscurer coplanar with the stellar disk also does not
fit the data very well. By assuming that the nuclear obscurer having an opening
angle of ~60 degree, we find the observed axis ratio distribution can be nicely
reproduced by a mean tilt angle of ~30 degree between the nuclear obscurer and
the stellar disk.Comment: 5 pages, 3 figures, to appear in ApJ
Opening a new window for warm dark matter
We explore the range of parameters for dark-matter sterile neutrinos in an
extention of the Minimal Standard Model by three singlet fermions with masses
below the electroweak scale (the MSM). This simple model can explain a
wide range of phenomena, including neutrino oscillations, baryogenesis, the
pulsar velocities, and the early reionization. The presence of two heavier
sterile neutrinos and the possibility of entropy production in their decays
broadens the allowed range of parameters for the dark-matter sterile neutrinos
(or other types of dark matter, for example, the gravitino). In addition, the
primordial production of dark matter sterile neutrinos allows to escape most of
the constraints.Comment: 7 pages, version to appear in Phys. Lett. B. A discussion of new
constraints on properties of sterile neutrino, coming from X-ray observations
and from Lyman-alpha forest data is adde
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