2,913 research outputs found
Constraints on WIMP Dark Matter from the High Energy PAMELA data
A new calculation of the ratio in cosmic rays is compared to the
recent PAMELA data. The good match up to 100 GeV allows to set constraints on
exotic contributions from thermal WIMP dark matter candidates. We derive
stringent limits on possible enhancements of the WIMP \pbar flux: a =100 GeV (1 TeV) signal cannot be increased by more than a factor 6 (40)
without overrunning PAMELA data. Annihilation through the channel is
also inspected and cross-checked with data. This scenario is
strongly disfavored as it fails to simultaneously reproduce positron and
antiproton measurements.Comment: 5 pages, 5 figures, the bibliography has been updated, minor
modifications have been made in the tex
Galactic secondary positron flux at the Earth
Secondary positrons are produced by spallation of cosmic rays within the
interstellar gas. Measurements have been typically expressed in terms of the
positron fraction, which exhibits an increase above 10 GeV. Many scenarios have
been proposed to explain this feature, among them some additional primary
positrons originating from dark matter annihilation in the Galaxy. The PAMELA
satellite has provided high quality data that has enabled high accuracy
statistical analyses to be made, showing that the increase in the positron
fraction extends up to about 100 GeV. It is therefore of paramount importance
to constrain theoretically the expected secondary positron flux to interpret
the observations in an accurate way. We find the secondary positron flux to be
reproduced well by the available observations, and to have theoretical
uncertainties that we quantify to be as large as about one order of magnitude.
We also discuss the positron fraction issue and find that our predictions may
be consistent with the data taken before PAMELA. For PAMELA data, we find that
an excess is probably present after considering uncertainties in the positron
flux, although its amplitude depends strongly on the assumptions made in
relation to the electron flux. By fitting the current electron data, we show
that when considering a soft electron spectrum, the amplitude of the excess
might be far lower than usually claimed. We provide fresh insights that may
help to explain the positron data with or without new physical model
ingredients. PAMELA observations and the forthcoming AMS-02 mission will allow
stronger constraints to be aplaced on the cosmic--ray transport parameters, and
are likely to reduce drastically the theoretical uncertainties.Comment: 15 pages, 12 figures. The recent PAMELA data on the positron fraction
(arXiv:0810.4995) have been included and the ensuing discussion has been
extended. Accepted version in A&
Infinite barbarians
This paper discusses an infinite regress that looms behind a certain kind of historical explanation. The movement of one barbarian group is often explained by the movement of others, but those movements in turn call for an explanation. While their explanation can again be the movement of yet another group of barbarians, if this sort of explanation does not stop somewhere we are left with an infinite regress of barbarians. While that regress would be vicious, it cannot be accommodated by several general views about what viciousness in infinite regresses amounts to. This example is additional evidence that we should prefer a pluralist approach to infinite regresses
TeV cosmic-ray proton and helium spectra in the myriad model
Recent measurements of cosmic ray proton and helium spectra show a hardening
above a few hundreds of GeV. This excess is hard to understand in the framework
of the conventional models of Galactic cosmic ray production and propagation.
We propose here to explain this anomaly by the presence of local sources
(myriad model). Cosmic ray propagation is described as a diffusion process
taking place inside a two-zone magnetic halo. We calculate the proton and
helium fluxes at the Earth between 50 GeV and 100 TeV. Improving over a similar
analysis, we consistently derive these fluxes by taking into account both local
and remote sources for which a unique injection rate is assumed. We find cosmic
ray propagation parameters compatible with B/C measurements and for which the
proton and helium spectra remarkably agree with the PAMELA and CREAM
measurements over four decades in energy.Comment: 5 pages, 3 figure
On the existence of effective potentials in time-dependent density functional theory
We investigate the existence and properties of effective potentials in
time-dependent density functional theory. We outline conditions for a general
solution of the corresponding Sturm-Liouville boundary value problems. We
define the set of potentials and v-representable densities, give a proof of
existence of the effective potentials under certain restrictions, and show the
set of v-representable densities to be independent of the interaction.Comment: 13 page
Diversification into novel habitats in the Africa clade of Dioscorea (Dioscoreaceae): erect habit and elephant's foot tubers
Background: Dioscorea is a widely distributed and highly diversified genus in tropical regions where it is represented by ten main clades, one of which diversified exclusively in Africa. In southern Africa it is characterised by a distinct group of species with a pachycaul or "elephant''s foot" structure that is partially to fully exposed above the substrate. In contrast to African representatives of the genus from other clades, occurring mainly in forest or woodland, the pachycaul taxa and their southern African relatives occur in diverse habitats ranging from woodland to open vegetation. Here we investigate patterns of diversification in the African clade, time of transition from forest to more open habitat, and morphological traits associated with each habitat and evaluate if such transitions have led to modification of reproductive organs and mode of dispersal. Results: The Africa clade originated in the Oligocene and comprises four subclades. The Dioscorea buchananii subclade (southeastern tropical Africa and South Africa) is sister to the East African subclade, which is respectively sister to the recently evolved sister South African (e. g., Cape and Pachycaul) subclades. The Cape and Pachycaul subclades diversified in the east of the Cape Peninsula in the mid Miocene, in an area with complex geomorphology and climate, where the fynbos, thicket, succulent karoo and forest biomes meet. Conclusions: Diversification out of forest is associated with major shifts in morphology of the perennial tuber (specifically an increase in size and orientation which presumably led them to become pachycaul) and rotation of stem (from twining to non-twining). The iconic elephant''s foot morphology, observed in grasslands and thicket biomes, where its corky bark may offer protection against fire and herbivory, evolved since mid Miocene. A shift in pollination trait is observed within the forest, but entry into open habitat does not show association with reproductive morphology, except in the seed wing, which has switched from winged all round the seed margin to just at the base or at the apex of it, or has been even replaced by an elaiosome
Effect of nano-carbon particle doping on the flux pinning properties of MgB2 superconductor
Polycrystalline MgB2-xCx samples with x=0.05, 0.1, 0.2, 0.3, 0.4
nano-particle carbon powder were prepared using an in-situ reaction method
under well controlled conditions to limit the extent of C substitution. The
phases, lattice parameters, microstructures, superconductivity and flux pinning
were characterized by XRD, TEM, and magnetic measurements. It was found that
both the a-axis lattice parameter and the Tc decreased monotonically with
increasing doping level. For the sample doped with the highest nominal
composition of x=0.4 the Tc dropped only 2.7K. The nano-C-doped samples showed
an improved field dependence of the Jc compared with the undoped sample over a
wide temperature range. The enhancement by C-doping is similar to that of
Si-doping but not as strong as for nano-SiC doped MgB2. X-ray diffraction
results indicate that C reacted with Mg to form nano-size Mg2C3 and MgB2C2
particles. Nano-particle inclusions and substitution, both observed by
transmission electron microscopy, are proposed to be responsible for the
enhancement of flux pinning in high fields.Comment: 9 pages, 12 figure
Comparison of absolute gain photometric calibration between Planck/HFI and Herschel/SPIRE at 545 and 857 GHz
We compare the absolute gain photometric calibration of the Planck/HFI and
Herschel/SPIRE instruments on diffuse emission. The absolute calibration of HFI
and SPIRE each relies on planet flux measurements and comparison with
theoretical far-infrared emission models of planetary atmospheres. We measure
the photometric cross calibration between the instruments at two overlapping
bands, 545 GHz / 500 m and 857 GHz / 350 m. The SPIRE maps used have
been processed in the Herschel Interactive Processing Environment (Version 12)
and the HFI data are from the 2015 Public Data Release 2. For our study we used
15 large fields observed with SPIRE, which cover a total of about 120 deg^2. We
have selected these fields carefully to provide high signal-to-noise ratio,
avoid residual systematics in the SPIRE maps, and span a wide range of surface
brightness. The HFI maps are bandpass-corrected to match the emission observed
by the SPIRE bandpasses. The SPIRE maps are convolved to match the HFI beam and
put on a common pixel grid. We measure the cross-calibration relative gain
between the instruments using two methods in each field, pixel-to-pixel
correlation and angular power spectrum measurements. The SPIRE / HFI relative
gains are 1.047 ( 0.0069) and 1.003 ( 0.0080) at 545 and 857 GHz,
respectively, indicating very good agreement between the instruments. These
relative gains deviate from unity by much less than the uncertainty of the
absolute extended emission calibration, which is about 6.4% and 9.5% for HFI
and SPIRE, respectively, but the deviations are comparable to the values 1.4%
and 5.5% for HFI and SPIRE if the uncertainty from models of the common
calibrator can be discounted. Of the 5.5% uncertainty for SPIRE, 4% arises from
the uncertainty of the effective beam solid angle, which impacts the adopted
SPIRE point source to extended source unit conversion factor (Abridged)Comment: 13 pages, 10 figures; Incorporates revisions in response to referee
comments; cross calibration factors unchange
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