15,142 research outputs found
Macro Dark Matter
Dark matter is a vital component of the current best model of our universe,
CDM. There are leading candidates for what the dark matter could be
(e.g. weakly-interacting massive particles, or axions), but no compelling
observational or experimental evidence exists to support these particular
candidates, nor any beyond-the-Standard-Model physics that might produce such
candidates. This suggests that other dark matter candidates, including ones
that might arise in the Standard Model, should receive increased attention.
Here we consider a general class of dark matter candidates with characteristic
masses and interaction cross-sections characterized in units of grams and
cm, respectively -- we therefore dub these macroscopic objects as Macros.
Such dark matter candidates could potentially be assembled out of Standard
Model particles (quarks and leptons) in the early universe. A combination of
Earth-based, astrophysical, and cosmological observations constrain a portion
of the Macro parameter space. A large region of parameter space remains, most
notably for nuclear-dense objects with masses in the range g and
g, although the lower mass window is closed
for Macros that destabilize ordinary matter.Comment: 13 pages, 1 table, 4 figures. Submitted to MNRAS. v3: corrected small
errors and a few points were made more clear, v4: included CMB bounds on dark
matter-photon coupling from Wilkinson et al. (2014) and references added.
Final revision matches published versio
An Analysis of Applications Development Systems for Remotely Sensed, Multispectral Data for the Earth Observations Division of the NASA Lyndon B. Johnson Space Center
An application development system (ADS) is examined for remotely sensed, multispectral data at the Earth Observations Division (EOD) at Johnson Space Center. Design goals are detailed, along with design objectives that an ideal system should contain. The design objectives were arranged according to the priorities of EOD's program objectives. Four systems available to EOD were then measured against the ideal ADS as defined by the design objectives and their associated priorities. This was accomplished by rating each of the systems on each of the design objectives. Utilizing the established priorities, it was determined how each system stood up as an ADS. Recommendations were made as to possible courses of action for EOD to pursue to obtain a more efficient ADS
Neutron diffraction in a model itinerant metal near a quantum critical point
Neutron diffraction measurements on single crystals of Cr1-xVx (x=0, 0.02,
0.037) show that the ordering moment and the Neel temperature are continuously
suppressed as x approaches 0.037, a proposed Quantum Critical Point (QCP). The
wave vector Q of the spin density wave (SDW) becomes more incommensurate as x
increases in accordance with the two band model. At xc=0.037 we have found
temperature dependent, resolution limited elastic scattering at 4
incommensurate wave vectors Q=(1+/-delta_1,2, 0, 0)*2pi/a, which correspond to
2 SDWs with Neel temperatures of 19 K and 300 K. Our neutron diffraction
measurements indicate that the electronic structure of Cr is robust, and that
tuning Cr to its QCP results not in the suppression of antiferromagnetism, but
instead enables new spin ordering due to novel nesting of the Fermi surface of
Cr.Comment: Submitted as a part of proceedings of LT25 (Amsterdam 2008
The use of the Winograd matrix multiplication algorithm in digital multispectral processing
The Winograd procedure for matrix multiplication provides a method whereby general matrix products may be computed more efficiently than the normal method. The algorithm and the time savings that can be effected are described. A FORTRAN program is provided which performs a general matrix multiply according to this algorithm. A variation of this procedure that may be used to calculate Gaussian probability density functions is also described. It is shown how a time savings can be effected in this calculation. The extension of this method to other similar calculations should yield similar savings
Magnetohydrodynamic Simple Waves
The simple wave solutions, which in ordinary gas dynamics
correspond lo expansion flows or Prandtl-Meyer flows are generalized here to ideal magnetohydrodynamic flows. The one-dimensional unsteady (x, t) case is considered. Due to magnetic effects more than one component
of field and velocity must be considered, To carry out the simple wave formalism the equations of motion (continuity, momentum, induction) are written in terms of flow velocities (u_1, u_2), Alfvén velocities
(b_1, b_2) and sound speed (a), These velocities are then functions only of the phase ξ = x_1 - U(ξ)t; each phase line can be thought of as an infinitesimal wave propagating with a speed c = U - u_1 related to the
flow. By elimination of (u_1, u_2) the system of five first-order ordinary differential equations can be reduced to three (homogeneous) equations. The vanishing of the determinant of coefficients provides a famous
relation for wave speed c and reduces the problem to integration of two first-order equations, The further introduction of dimensionless variables, ratios of wave speeds, reduces the problem to integration
of a single first-order equation, By studying the trajectories of this differential equation an overall view of all possible solutions is obtained;
numerical integration is also carried out in the case of slow waves. As applications of this theory various physical problems are studied, the receding piston and waves produced by a current sheet
Spin dynamics of strongly-doped La_{1-x}Sr_xMnO_3
Cold neutron triple-axis measurements have been used to investigate the
nature of the long-wavelength spin dynamics in strongly-doped
LaSrMnO single crystals with =0.2 and 0.3. Both systems
behave like isotropic ferromagnets at low T, with a gapless ( meV)
quadratic dispersion relation . The values of the spin-wave
stiffness constant are large ( = 166.77 meV for =0.2
and D = 175.87 meV for =0.3), which directly shows that the
electron transfer energy for the band is large. exhibits a power law
behavior as a function of temperature, and appears to collapse as T -> T_C.
Nevertheless, an anomalously strong quasielastic central component develops and
dominates the fluctuation spectrum as T -> T_C. Bragg scattering indicates that
the magnetization near exhibits power law behavior, with for both systems, as expected for a three-dimensional ferromagnet.Comment: 4 pages (RevTex), 3 figures (encapsulated postscript
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