1,187 research outputs found
The origin of the positron excess in cosmic rays
We show that the positron excess measured by the PAMELA experiment in the
region between 10 and 100 GeV may well be a natural consequence of the standard
scenario for the origin of Galactic cosmic rays. The 'excess' arises because of
positrons created as secondary products of hadronic interactions inside the
sources, but the crucial physical ingredient which leads to a natural
explanation of the positron flux is the fact that the secondary production
takes place in the same region where cosmic rays are being accelerated.
Therefore secondary positrons (and electrons) participate in the acceleration
process and turn out to have a very flat spectrum, which is responsible, after
propagation in the Galaxy, for the observed positron 'excess'. This effect
cannot be avoided though its strength depends on the values of the
environmental parameters during the late stages of evolution of supernova
remnants.Comment: 4 Pages, 2 figures. Some references and discussion adde
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
Looking Good With Flickr Faves: Gaussian Processes for Finding Difference Makers in Personality Impressions
Flickr allows its users to generate galleries of "faves", i.e., pictures that they have tagged as favourite. According to recent studies, the faves are predictive of the personality traits that people attribute to Flickr users. This article investigates the phenomenon and shows that faves allow one to predict whether a Flickr user is perceived to be above median or not with respect to each of the Big-Five Traits (accuracy up to 79\% depending on the trait). The classifier - based on Gaussian Processes with a new kernel designed for this work - allows one to identify the visual characteristics of faves that better account for the prediction outcome
The Sensitivity of the IceCube Neutrino Detector to Dark Matter Annihilating in Dwarf Galaxies
In this paper, we compare the relative sensitivities of gamma-ray and
neutrino observations to the dark matter annihilation cross section in
leptophilic models such as have been designed to explain PAMELA data. We
investigate whether the high energy neutrino telescope IceCube will be
competitive with current and upcoming searches by gamma-ray telescopes, such as
the Atmospheric Cerenkov Telescopes (ACTs) (HESS, VERITAS and MAGIC), or the
Fermi Gamma Ray Space Telescope, in detecting or constraining dark matter
particles annihilating in dwarf spheroidal galaxies. We find that after ten
years of observation of the most promising nearby dwarfs, IceCube will have
sensitivity comparable to the current sensitivity of gamma-ray telescopes only
for very heavy (m_X > 7 TeV) or relatively light (m_X < 200 GeV) dark matter
particles which annihilate primarily to mu+mu-. If dark matter particles
annihilate primarily to tau+tau-, IceCube will have superior sensitivity only
for dark matter particle masses below the 200 GeV threshold of current ACTs. If
dark matter annihilations proceed directly to neutrino-antineutrino pairs a
substantial fraction of the time, IceCube will be competitive with gamma-ray
telescopes for a much wider range of dark matter masses.Comment: 7 pages, 3 figures. v2: references added and minor revisions. v3: as
published in PRD
Quality Control of Cement Deep Soil Mixing Work for the Port of Oakland Projects
The Cement Deep Soil Mixing (CDSM) method is an in situ soil treatment technology that introduces and mixes cementitious materials with native soils using hollow-stem rotating shafts equipped with a cutting tool at the tip and mixing paddles above the tip. The successful use of the soil-cement produced by CDSM relies on the selection of acceptance criteria and construction quality control during the in situ soil mixing process. Two CDSM projects for the Port of Oakland are used as case examples to present the acceptance criteria set and the execution of the quality control program for the soil mixing work. This quality control program ensures that the geometric and material design parameters of the CDSM structure have been obtained. The data acquired from these two projects are presented and compared with strength data from two other projects to illustrate the influence of acceptance criteria over the CDSM products
Discriminating different scenarios to account for the cosmic excess by synchrotron and inverse Compton radiation
The excesses of the cosmic positron fraction recently measured by PAMELA and
the electron spectra by ATIC, PPB-BETS, Fermi and H.E.S.S. indicate the
existence of primary electron and positron sources. The possible explanations
include dark matter annihilation, decay, and astrophysical origin, like
pulsars. In this work we show that these three scenarios can all explain the
experimental results of the cosmic excess. However, it may be difficult
to discriminate these different scenarios by the local measurements of
electrons and positrons. We propose possible discriminations among these
scenarios through the synchrotron and inverse Compton radiation of the primary
electrons/positrons from the region close to the Galactic center. Taking
typical configurations, we find the three scenarios predict quite different
spectra and skymaps of the synchrotron and inverse Compton radiation, though
there are relatively large uncertainties. The most prominent differences come
from the energy band MHz for synchrotron emission and GeV for inverse Compton emission. It might be able to discriminate at least
the annihilating dark matter scenario from the other two given the high
precision synchrotron and diffuse -ray skymaps in the future.Comment: published in Pr
Measurement and Modeling of Infrared Nonlinear Absorption Coefficients and Laser-induced Damage Thresholds in Ge and GaSb
Using a simultaneous fitting technique to extract nonlinear absorption coefficients from data at two pulse widths, we measure two-photon and free-carrier absorption coefficients for Ge and GaSb at 2.05 and 2.5 μm for the first time, to our knowledge. Results agreed well with published theory. Single-shot damage thresholds were also measured at 2.5 μm and agreed well with modeled thresholds using experimentally determined parameters including nonlinear absorption coefficients and temperature dependent linear absorption. The damage threshold for a single-layer Al2O3 anti-reflective coating on Ge was 55% or 35% lower than the uncoated threshold for picosecond or nanosecond pulses, respectively
Reliability Testing of AlGaN/GaN HEMTs Under Multiple Stressors
We performed an experiment on AlGaN/GaN HEMTs with high voltage and high power as stressors. We found that devices tested under high power generally degraded more than those tested under high voltage. In particular, the high-voltage-tested devices did not degrade significantly as suggested by some papers in the literature. The same papers in the literature also suggest that high voltages cause cracks and pits. However, the high-voltage-tested devices in this study do not exhibit cracks or pits in TEM images, while the high-power-tested devices exhibit pits
Searching for Dark Matter with Future Cosmic Positron Experiments
Dark matter particles annihilating in the Galactic halo can provide a flux of
positrons potentially observable in upcoming experiments, such as PAMELA and
AMS-02. We discuss the spectral features which may be associated with dark
matter annihilation in the positron spectrum and assess the prospects for
observing such features in future experiments. Although we focus on some
specific dark matter candidates, neutralinos and Kaluza-Klein states, we carry
out our study in a model independent fashion. We also revisit the positron
spectrum observed by HEAT.Comment: 19 pages, 33 figure
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