276 research outputs found
Kaluza-Klein Dark Matter and the Positron Excess
The excess of cosmic positrons observed by the HEAT experiment may be the
result of Kaluza-Klein dark matter annihilating in the galactic halo.
Kaluza-Klein dark matter annihilates dominantly into charged leptons that yield
a large number and hard spectrum of positrons per annihilation. Given a
Kaluza-Klein dark matter particle with a mass in the range of 300-400 GeV, no
exceptional substructure or clumping is needed in the local distribution of
dark matter to generate a positron flux that explains the HEAT observations.
This is in contrast to supersymmetric dark matter that requires unnaturally
large amounts of dark substructure to produce the observed positron excess.
Future astrophysical and collider tests are outlined that will confirm or rule
out this explanation of the HEAT data.Comment: 5 pages, 3 figures, REVTeX
Trapping of strangelets in the geomagnetic field
Strangelets coming from the interstellar medium (ISM) are an interesting
target to experiments searching for evidence of this hypothetic state of
hadronic matter. We entertain the possibility of a {\it trapped} strangelet
population, quite analogous to ordinary nuclei and electron belts. For a
population of strangelets to be trapped by the geomagnetic field, these
incoming particles would have to fulfill certain conditions, namely having
magnetic rigidities above the geomagnetic cutoff and below a certain threshold
for adiabatic motion to hold. We show in this work that, for fully ionized
strangelets, there is a narrow window for stable trapping. An estimate of the
stationary population is presented and the dominant loss mechanisms discussed.
It is shown that the population would be substantially enhanced with respect to
the ISM flux (up to two orders of magnitude) due to quasi-stable trapping.Comment: 10 pp., 5 figure
Positron Propagation and Fluxes from Neutralino Annihilation in the Halo
Supersymmetric neutralinos are one of the most promising candidates for the
dark matter in the Universe. If they exist, they should make up some fraction
of the Milky Way halo. We investigate the fluxes of positrons expected at the
Earth from neutralino annihilation in the halo. Positron propagation is treated
in a diffusion model including energy loss. The positron source function
includes contributions from both continuum and monochromatic positrons. We find
that, for a "canonical" halo model and propagation parameters, the fluxes are
generally too low to be visible. Given the large uncertainties in both
propagation and halo structure, it is however possible to obtain observable
fluxes. We also investigate the shapes of the positron spectra, including fits
to a feature indicated by the results of the HEAT experiment.Comment: 16 pages, 19 figures, uses revte
Evidence for Remnant Flare Suprathermals In The Source Population of Solar Energetic Particles In The 2000 Bastille Day Event
The energy spectra of Fe in the very large solar energetic particle (SEP) event of 2000 July 14 are strikingly
different from those of lighter species. We show that this difference can be explained by shock acceleration from
a two-component source population, comprising solar wind suprathermals and a small (∼5%) admixture of remnant
flare particles, as previously proposed to explain enhanced ^3He/^4He in some gradual SEP events. Flare remnants
can also account for several previously unexplained features of high-energy solar heavy ions as well as important aspects of SEP event-to-event variability. These results offer a new perspective on the enduring controversy over the relative roles of flares and coronal mass ejections (CMEs) in producing SEPs. Flare activity clearly makes a unique and critical contribution to the source population. But the predominate accelerator in large gradual SEP events is the CME-driven shock, and many spectral, compositional, and charge state characteristics of highenergy heavy ions can be understood without invoking other acceleration mechanisms
Positrons from particle dark-matter annihilation in the Galactic halo: propagation Green's functions
We have made a calculation of the propagation of positrons from dark-matter
particle annihilation in the Galactic halo in different models of the dark
matter halo distribution using our 3D code, and present fits to our numerical
propagation Green's functions. We show that the Green's functions are not very
sensitive to the dark matter distribution for the same local dark matter energy
density. We compare our predictions with computed cosmic ray positron spectra
(``background'') for the ``conventional'' CR nucleon spectrum which matches the
local measurements, and a modified spectrum which respects the limits imposed
by measurements of diffuse Galactic gamma-rays, antiprotons, and positrons. We
conclude that significant detection of a dark matter signal requires favourable
conditions and precise measurements unless the dark matter is clumpy which
would produce a stronger signal. Although our conclusion qualitatively agrees
with that of previous authors, it is based on a more realistic model of
particle propagation and thus reduces the scope for future speculations.
Reliable background evaluation requires new accurate positron measurements and
further developments in modelling production and propagation of cosmic ray
species in the Galaxy.Comment: 8 pages, 6 ps-figures, 3 tables, uses revtex. Accepted for
publication in Physical Review D. More details can be found at
http://www.gamma.mpe-garching.mpg.de/~aws/aws.htm
Non-Thermal Production of WIMPs, Cosmic Excesses and -rays from the Galactic Center
In this paper we propose a dark matter model and study aspects of its
phenomenology. Our model is based on a new dark matter sector with a U(1)'
gauge symmetry plus a discrete symmetry added to the Standard Model of particle
physics. The new fields of the dark matter sector have no hadronic charges and
couple only to leptons. Our model can not only give rise to the observed
neutrino mass hierarchy, but can also generate the baryon number asymmetry via
non-thermal leptogenesis. The breaking of the new U(1)' symmetry produces
cosmic strings. The dark matter particles are produced non-thermally from
cosmic string loop decay which allows one to obtain sufficiently large
annihilation cross sections to explain the observed cosmic ray positron and
electron fluxes recently measured by the PAMELA, ATIC, PPB-BETS, Fermi-LAT, and
HESS experiments while maintaining the required overall dark matter energy
density. The high velocity of the dark matter particles from cosmic string loop
decay leads to a low phase space density and thus to a dark matter profile with
a constant density core in contrast to what happens in a scenario with
thermally produced cold dark matter where the density keeps rising towards the
center. As a result, the flux of gamma rays radiated from the final leptonic
states of dark matter annihilation from the Galactic center is suppressed and
satisfies the constraints from the HESS gamma-ray observations.Comment: 23 pages, 2 figure
On the Estimation of Solar Energetic Particle Injection Timing from Onset Times near Earth
We examine the accuracy of a common technique for estimating the start time
of solar energetic particle injection based on a linear fit to the observed
onset time versus 1/(particle velocity). This is based on a concept that the
first arriving particles move directly along the magnetic field with no
scattering. We check this by performing numerical simulations of the transport
of solar protons between 2 and 2000 MeV from the Sun to the Earth, for several
assumptions regarding interplanetary scattering and the duration of particle
injection, and analyzing the results using the inverse velocity fit. We find
that in most cases, the onset times align close to a straight line as a
function of inverse velocity. Despite this, the estimated injection time can be
in error by several minutes. Also, the estimated path length can deviate
greatly from the actual path length along the interplanetary magnetic field.
The major difference between the estimated and actual path lengths implies that
the first arriving particles cannot be viewed as moving directly along the
interplanetary magnetic field.Comment: 19 pages, 3 Postscript figures. Astrophys. J., in pres
Temporal evolution of solar energetic particle spectra
During solar flares and coronal mass ejections, solar energetic par- ticles (SEPs) may be released into the interplanetary medium and near-Earth locations. The energy spectra of SEP events at 1 AU are typically averaged over the entire event or studied in a few snapshots. In this paper we analyze the time evolution of the energy spectra of four large selected SEP events using a large number of snapshots. We use a multi-spacecraft and multi-instrument approach for the observations, obtained over a wide SEP energy range. We find large differences in the spectra at the beginning of the events as measured by different instruments. We show that over time, a wave-like structure is observed traveling through the spectra from the highest energies to the lowest energies, creating an “arch” shape which then straightens into a power law later in the event, after times of the order of 10 hours. We discuss the processes that determine SEP intensities and their role in shaping the spectral time evolution
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