3,538 research outputs found
Astrophysical Uncertainties in the Cosmic Ray Electron and Positron Spectrum From Annihilating Dark Matter
In recent years, a number of experiments have been conducted with the goal of
studying cosmic rays at GeV to TeV energies. This is a particularly interesting
regime from the perspective of indirect dark matter detection. To draw reliable
conclusions regarding dark matter from cosmic ray measurements, however, it is
important to first understand the propagation of cosmic rays through the
magnetic and radiation fields of the Milky Way. In this paper, we constrain the
characteristics of the cosmic ray propagation model through comparison with
observational inputs, including recent data from the CREAM experiment, and use
these constraints to estimate the corresponding uncertainties in the spectrum
of cosmic ray electrons and positrons from dark matter particles annihilating
in the halo of the Milky Way.Comment: 21 pages, 9 figure
Making Tunnel Barriers (Including Metals) Transparent
Ian R. Hooper, T. W. Preist, and J. Roy Sambles, Physical Review Letters, Vol. 97, article 053902 (2006). "Copyright © 2006 by the American Physical Society."The classical "brick wall," which may, according to quantum mechanics, leak via tunneling, is here shown to be completely transparent when appropriate impedance matching media are placed both in front of and behind the "wall." Optical experiments involving beyond-critical-angle-tunnel barriers in the frustrated total internal reflection scheme which mimic quantum mechanical systems provide convincing proof of this remarkable effect. The same mechanism also allows vastly enhanced transmission through unstructured thin metal films without the need for surface wave excitation
Broad-band polarization conversion from a finite periodic structure in the microwave regime
Copyright © 2004 American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics. The following article appeared in Applied Physics Letters 84 (2004) and may be found at http://link.aip.org/link/?APPLAB/84/849/1A reflection grating demonstrating extraordinarily broad-band polarization conversion in a nondiffractive regime has been studied at microwave frequencies. This single-element structure has been fabricated by electrolessly plating a metallic layer onto a stereo-lithographically produced resin profile. Angle-dependent microwave reflectivity data collected from the grating indicates polarization conversion of greater than 80% over a spectral bandwidth equivalent to the entire visible regime (factor of 2 in frequency). This supports an earlier publication in which it was predicted that a broad-band polarization converter could be created from a suitably profiled diffraction grating
Nambu-Goldstone Dark Matter and Cosmic Ray Electron and Positron Excess
We propose a model of dark matter identified with a pseudo-Nambu-Goldstone
boson in the dynamical supersymmetry breaking sector in a gauge mediation
scenario. The dark matter particles annihilate via a below-threshold narrow
resonance into a pair of R-axions each of which subsequently decays into a pair
of light leptons. The Breit-Wigner enhancement explains the excess electron and
positron fluxes reported in the recent cosmic ray experiments PAMELA, ATIC and
PPB-BETS without postulating an overdensity in halo, and the limit on
anti-proton flux from PAMELA is naturally evaded.Comment: 3 figure
Surface plasmon polaritons on narrow-ridged short-pitch metal gratings
Ian R. Hooper and J. Roy Sambles, Physical Review B, Vol. 66, article 205408 (2002). "Copyright © 2002 by the American Physical Society."The reflectivity of short pitch metal gratings consisting of a series of narrow Gaussian ridges in the classical mount has been modeled as a function of frequency and in-plane wave vector (the plane of incidence containing the grating vector) for various ridge heights. Surface plasmon polaritons (SPP’s) are found to be excited even in the zero-order region of the spectrum. These may result in strong absorption of radiation polarized with its electric field in the plane of incidence (transverse magnetic). For zero in-plane wave vector the SPP modes consist of a symmetric charge distribution on either side of the grating ridges, a family of these modes existing with different numbers of field maxima per grating period. Because of the charge symmetry these modes may only be coupled to at angles away from normal incidence where strong resonant absorption may then occur. The dispersion of these SPP modes as a function of the in-plane wave vector is found to be complex arising from the formation of very large band gaps due to the harmonic content of the grating profile, the creation of a pseudo high-energy mode, and through strong interactions between different SPP bands
ASCA and contemporaneous ground-based observations of the BL Lacertae objects 1749+096 and 2200+420 (BL Lac)
We present ASCA observations of the radio-selected BL Lacertae objects
1749+096 (z=0.32) and 2200+420 (BL Lac, z=0.069) performed in 1995 Sept and
Nov, respectively. The ASCA spectra of both sources can be described as a first
approximation by a power law with photon index Gamma ~ 2. This is flatter than
for most X-ray-selected BL Lacs observed with ASCA, in agreement with the
predictions of current blazar unification models. While 1749+096 exhibits
tentative evidence for spectral flattening at low energies, a concave continuum
is detected for 2200+420: the steep low-energy component is consistent the
high-energy tail of the synchrotron emission responsible for the longer
wavelengths, while the harder tail at higher energies is the onset of the
Compton component. The spectral energy distributions from radio to gamma-rays
are consistent with synchrotron-self Compton emission from a single homogeneous
region shortward of the IR/optical wavelengths, with a second component in the
radio domain related to a more extended emission region. For 2200+420,
comparing the 1995 Nov state with the optical/GeV flare of 1997 July, we find
that models requiring inverse Compton scattering of external photons provide a
viable mechanism for the production of the highest (GeV) energies during the
flare. An increase of the external radiation density and of the power injected
in the jet can reproduce the flat gamma-ray continuum observed in 1997 July. A
directly testable prediction of this model is that the line luminosity in
2200+420 should vary shortly after (~1 month) a non-thermal synchrotron flare.Comment: 28 pages,6 figures, 5 tables; LaTeX document. accepted for
publication in the Astrophysical Journa
Results from PAMELA, ATIC and FERMI : Pulsars or Dark Matter ?
It is well known that the dark matter dominates the dynamics of galaxies and
clusters of galaxies. Its constituents remain a mystery despite an assiduous
search for them over the past three decades. Recent results from the
satellite-based PAMELA experiment detect an excess in the positron fraction at
energies between 10-100 GeV in the secondary cosmic ray spectrum. Other
experiments namely ATIC, HESS and FERMI show an excess in the total electron
(\ps + \el) spectrum for energies greater 100 GeV. These excesses in the
positron fraction as well as the electron spectrum could arise in local
astrophysical processes like pulsars, or can be attributed to the annihilation
of the dark matter particles. The second possibility gives clues to the
possible candidates for the dark matter in galaxies and other astrophysical
systems. In this article, we give a report of these exciting developments.Comment: 27 Pages, extensively revised and significantly extended, to appear
in Pramana as topical revie
IceCube-Plus: An Ultra-High Energy Neutrino Telescope
While the first kilometer-scale neutrino telescope, IceCube, is under
construction, alternative plans exist to build even larger detectors that will,
however, b e limited by a much higher neutrino energy threshold of 10 PeV or
higher rather than 10 to 100 GeV. These future projects detect radio and
acoustic pulses as w ell as air showers initiated by ultra-high energy
neutrinos. As an alternative, we here propose an expansion of IceCube, using
the same strings, placed on a gri d with a spacing of order 500 m. Unlike other
proposals, the expanded detector uses methods that are understood and
calibrated on atmospheric neutrinos. Atmosp heric neutrinos represent the only
background at the energies under consideratio n and is totally negligible.
Also, the cost of such a detector is understood. We conclude that supplementing
the 81 IceCube strings with a modest number of addi tional strings spaced at
large distances can almost double the effective volume of the detector.
Doubling the number of strings on a 800 m grid can deliver a d etector that
this a factor of 5 larger for horizontal muons at modest cost.Comment: Version to be published in JCA
Two photon annihilation of Kaluza-Klein dark matter
We investigate the fermionic one-loop cross section for the two photon
annihilation of Kaluza-Klein (KK) dark matter particles in a model of universal
extra dimensions (UED). This process gives a nearly mono-energetic gamma-ray
line with energy equal to the KK dark matter particle mass. We find that the
cross section is large enough that if a continuum signature is detected, the
energy distribution of gamma-rays should end at the particle mass with a peak
that is visible for an energy resolution of the detector at the percent level.
This would give an unmistakable signature of a dark matter origin of the
gamma-rays, and a unique determination of the dark matter particle mass, which
in the case studied should be around 800 GeV. Unlike the situation for
supersymmetric models where the two-gamma peak may or may not be visible
depending on parameters, this feature seems to be quite robust in UED models,
and should be similar in other models where annihilation into fermions is not
helicity suppressed. The observability of the signal still depends on largely
unknown astrophysical parameters related to the structure of the dark matter
halo. If the dark matter near the galactic center is adiabatically contracted
by the central star cluster, or if the dark matter halo has substructure
surviving tidal effects, prospects for detection look promising.Comment: 17 pages, 3 figures; slightly revised versio
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