792 research outputs found
Production and Evolution of Perturbations of Sterile Neutrino Dark Matter
Sterile neutrinos, fermions with no standard model couplings [SU(2)
singlets], are predicted by most extensions of the standard model, and may be
the dark matter. I describe the nonthermal production and linear perturbation
evolution in the early universe of this dark matter candidate. I calculate
production of sterile neutrino dark matter including effects of Friedmann
dynamics dictated by the quark-hadron transition and particle population, the
alteration of finite temperature effective mass of active neutrinos due to the
presence of thermal leptons, and heating of the coupled species due to the
disappearance of degrees of freedom in the plasma. These effects leave the
sterile neutrinos with a non-trivial momentum distribution. I also calculate
the evolution of sterile neutrino density perturbations in the early universe
through the linear regime and provide a fitting function form for the transfer
function describing the suppression of small scale fluctuations for this warm
dark matter candidate. The results presented here differ quantitatively from
previous work due to the inclusion here of the relevant physical effects during
the production epoch.Comment: v4: matches version in Phys. Rev.
Constraints on the parameters of radiatively decaying dark matter from the dark matter halo of the Milky Way and Ursa Minor
We improve the earlier restrictions on parameters of the dark matter (DM) in
the form of a sterile neutrino. The results were obtained from non-observing
the DM decay line in the X-ray spectrum of the Milky Way (using the recent
XMM-Newton PN blank sky data). We also present a similar constraint coming from
the recent XMM-Newton observation of Ursa Minor -- dark, X-ray quiet dwarf
spheroidal galaxy. The new Milky way data improve on (by as much as the order
of magnitude at masses ~3.5 keV) existing constraints. Although the observation
of Ursa Minor has relatively poor statistics, the constraints are comparable to
those recently obtained using observations of the Large Magellanic Cloud or
M31. This confirms a recent proposal that dwarf satellites of the MW are very
interesting candidates for the DM search and dedicated studies should be made
to this purpose.Comment: 8 pp. v.2 - Final version to appear in A&
Origins of Hidden Sector Dark Matter I: Cosmology
We present a systematic cosmological study of a universe in which the visible
sector is coupled, albeit very weakly, to a hidden sector comprised of its own
set of particles and interactions. Assuming that dark matter (DM) resides in
the hidden sector and is charged under a stabilizing symmetry shared by both
sectors, we determine all possible origins of weak-scale DM allowed within this
broad framework. We show that DM can arise only through a handful of
mechanisms, lending particular focus to Freeze-Out and Decay and Freeze-In, as
well as their variations involving late time re-annihilations of DM and DM
particle anti-particle asymmetries. Much like standard Freeze-Out, where the
abundance of DM depends only on the annihilation cross-section of the DM
particle, these mechanisms depend only on a very small subset of physical
parameters, many of which may be measured directly at the LHC. In particular,
we show that each DM production mechanism is associated with a distinctive
window in lifetimes and cross-sections for particles which may be produced in
the near future. We evaluate prospects for employing the LHC to definitively
reconstruct the origin of DM in a companion paper.Comment: 32 pages, 19 figures; v2: references added, published versio
Hadronic Axion Model in Gauge-Mediated Supersymmetry Breaking and Cosmology of Saxion
Recently we have proposed a simple hadronic axion model within gauge-mediated
supersymmetry breaking. In this paper we discuss various cosmological
consequences of the model in great detail. A particular attention is paid to a
saxion, a scalar partner of an axion, which is produced as a coherent
oscillation in the early universe. We show that our model is cosmologically
viable, if the reheating temperature of inflation is sufficiently low. We also
discuss the late decay of the saxion which gives a preferable power spectrum of
the density fluctuation in the standard cold dark matter model when compared
with the observation.Comment: 24 pages, 3 figure
Roles of two successive phase transitions in new spin-Peierls system TiOBr
In this sturdy, we determine the roles of two successive phase transitions in
the new spin-Peierls system TiOBr by electron and synchrotron X-ray diffraction
analyses. Results show an incommensurate superstructure along the h- and
k-directions between Tc1=27K and Tc2=47K, and a twofold superstructure which is
related to a spin-Peierls lattice distortion below Tc1. The diffuse scattering
observed above Tc2 indicates that a structural correlation develops at a high
temperature. We conclude that Tc2 is a second-order lock-in temperature, which
is related to the spin-Peierls lattice distortion with the incommensurate
structure, and that Tc1 is from incommensurate to commensurate phase transition
temperature accompanying the first-order spin-Peierls lattice distortion.Comment: 4 pages, 5 figure
Cosmological Moduli Problem and Thermal Inflation Models
In superstring theories, there exist various dilaton and modulus fields which
masses are expected to be of the order of the gravitino mass . These
fields lead to serious cosmological difficulties, so called ``cosmological
moduli problem'', because a large number of moduli particles are produced as
the coherent oscillations after the primordial inflation. We make a
comprehensive study whether the thermal inflation can solve the cosmological
moduli problem in the whole modulus mass region
predicted by both hidden sector supersymmetry (SUSY) breaking and
gauge-mediated SUSY breaking models. In particular, we take into account the
primordial inflation model whose reheating temperature is so low that its
reheating process finishes after the thermal inflation ends. We find that the
above mass region survives from
various cosmological constraints in the presence of the thermal inflation.Comment: 49 pages, 17 figure
Search for the light dark matter with an X-ray spectrometer
Sterile neutrinos with the mass in the keV range are interesting warm dark
matter (WDM) candidates. The restrictions on their parameters (mass and mixing
angle) obtained by current X-ray missions (XMM-Newton or Chandra) can only be
improved by less than an order of magnitude in the near future. Therefore the
new strategy of search is needed. We compare the sensitivities of existing and
planned X-ray missions for the detection of WDM particles with the mass ~1-20
keV. We show that existing technology allows an improvement in sensitivity by a
factor of 100. Namely, two different designs can achieve such an improvement:
[A] a spectrometer with the high spectral resolving power of 0.1%, wide
(steradian) field of view, with small effective area of about cm^2 (which can
be achieved without focusing optics) or [B] the same type of spectrometer with
a smaller (degree) field of view but with a much larger effective area of 10^3
cm^2 (achieved with the help of focusing optics). To illustrate the use of the
"type A" design we present the bounds on parameters of the sterile neutrino
obtained from analysis of the data taken by an X-ray microcalorimeter. In spite
of the very short exposure time (100 sec) the derived bound is comparable to
the one found from long XMM-Newton observation.Comment: 9pp, revtex
On initial conditions for the Hot Big Bang
We analyse the process of reheating the Universe in the electroweak theory
where the Higgs field plays a role of the inflaton. We estimate the maximal
temperature of the Universe and fix the initial conditions for
radiation-dominated phase of the Universe expansion in the framework of the
Standard Model (SM) and of the nuMSM -- the minimal extension of the SM by
three right-handed singlet fermions. We show that the inflationary epoch is
followed by a matter dominated stage related to the Higgs field oscillations.
We investigate the energy transfer from Higgs-inflaton to the SM particles and
show that the radiation dominated phase of the Universe expansion starts at
temperature T_r~(3-15)*10^{13} GeV, where the upper bound depends on the Higgs
boson mass. We estimate the production rate of singlet fermions at preheating
and find that their concentrations at T_r are negligibly small. This suggests
that the sterile neutrino Dark Matter (DM) production and baryogenesis in the
nuMSM with Higgs-driven inflation are low energy phenomena, having nothing to
do with inflation. We study then a modification of the nuMSM, adding to its
Lagrangian higher dimensional operators suppressed by the Planck scale. The
role of these operators in Higgs-driven inflation is clarified. We find that
these operators do not contribute to the production of Warm Dark Matter (WDM)
and to baryogenesis. We also demonstrate that the sterile neutrino with mass
exceeding 100 keV (a Cold Dark Matter (CDM) candidate) can be created during
the reheating stage of the Universe in necessary amounts. We argue that the
mass of DM sterile neutrino should not exceed few MeV in order not to overclose
the Universe.Comment: 41 pages, 5 figures. Journal version accepted in JCA
On the hadronic contribution to sterile neutrino production
Sterile neutrinos with masses in the keV range are considered to be a viable
candidate for warm dark matter. The rate of their production through
active-sterile neutrino transitions peaks, however, at temperatures of the
order of the QCD scale, which makes it difficult to estimate their relic
abundance quantitatively, even if the mass of the sterile neutrino and its
mixing angle were known. We derive here a relation, valid to all orders in the
strong coupling constant, which expresses the production rate in terms of the
spectral function associated with active neutrinos. The latter can in turn be
expressed as a certain convolution of the spectral functions related to various
mesonic current-current correlation functions, which are being actively studied
in other physics contexts. In the naive weak coupling limit, the appropriate
Boltzmann equations can be derived from our general formulae.Comment: 28 pages. v2: small clarifications added, published versio
Topological Inflation in Supergravity
We investigate a topological inflation model in supergravity. By means of
numerical simulations, it is confirmed that topological inflation can take
place in supergravity. We also show that the condition for successful inflation
depends not only on the vacuum-expectation value (VEV) of inflaton field but
also on the form of its K\"ahler potential. In fact, it is found that the
required VEV of the inflaton can be as small as , where is the gravitational scale.Comment: 17 pages, 7 figures. To appear in Phys. Rev.
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