1,103 research outputs found
Testing Lorentz Invariance with Ultra High Energy Cosmic Ray Spectrum
The GZK cutoff predicted at the Ultra High Energy Cosmic Ray (UHECR) spectrum
as been observed by the HiRes and Auger experiments. The results put severe
constraints on the effect of Lorentz Invariance Violation(LIV) which has been
introduced to explain the absence of GZK cutoff indicated in the AGASA data.
Assuming homogeneous source distribution with a single power law spectrum, we
calculate the spectrum of UHECRs observed on Earth by taking the processes of
photopion production, pair production and adiabatic energy loss into
account. The effect of LIV is also taken into account in the calculation. By
fitting the HiRes monocular spectra and the Auger combined spectra, we show
that the LIV parameter is constrained to
and respectively, which is well consistent
with strict Lorentz Invariance up to the highest energy.Comment: Accepted for publication in Physical Review D 12 pages, 4 figure
Neutrino emission from dark matter annihilation/decay in light of cosmic and data
A self-consistent global fitting method based on the Markov Chain Monte Carlo
technique to study the dark matter (DM) property associated with the cosmic ray
electron/positron excesses was developed in our previous work. In this work we
further improve the previous study to include the hadronic branching ratio of
DM annihilation/decay. The PAMELA data are employed to constrain
the hadronic branching ratio. We find that the 95% () upper limits of
the quark branching ratio allowed by the PAMELA data is for DM annihilation and for DM decay respectively. This
result shows that the DM coupling to pure leptons is indeed favored by the
current data. Based on the global fitting results, we further study the
neutrino emission from DM in the Galactic center. Our predicted neutrino flux
is some smaller than previous works since the constraint from -rays is
involved. However, it is still capable to be detected by the forth-coming
neutrino detector such as IceCube. The improved points of the present study
compared with previous works include: 1) the DM parameters, both the particle
physical ones and astrophysical ones, are derived in a global fitting way, 2)
constraints from various species of data sets, including -rays and
antiprotons are included, and 3) the expectation of neutrino emission is fully
self-consistent.Comment: 13 pages, 2 figures, 1 table; Published in IJMPA 201
Constraints on the Dark Matter Annihilations by Neutrinos with Substructure Effects Included
Dark matter (DM) annihilations in the Galaxy may produce high energy
neutrinos, which can be detected by the neutrino telescopes, for example
IceCube, ANTARES and Super-Kamiokande. The neutrinos can also arise from
hadronic interaction between cosmic ray and atmosphere around the Earth, known
as atmospheric neutrino. Current measurements on neutrino flux is consistent
with theoretical prediction of atmospheric neutrino within the uncertainties.
In this paper, by requiring that the DM annihilation neutrino flux is less than
the current measurements, we obtain an upper bound on the cross section of dark
matter annihilation . Compared with previous investigations,
we improve the bound by including DM substructure contributions. In our paper,
two kinds of substructure effects are scrutinized. One is the substructure
average contribution over all directions. The other is point source effect by
single massive sub-halo. We found that the former can improve the bound by
several times, while the latter can improve the bound by
utilizing the excellent angular resolution of neutrino telescope IceCube. The
exact improvement depends on the DM profile and the sub-halo concentration
model. In some model, IceCube can achieve the sensitivity of .Comment: 19 pages, 5 figures, 1 tabl
The Galactic positron flux and dark matter substructures
In this paper we calculate the Galactic positron flux from dark matter
annihilation in the frame of supersymmetry, taking the enhancement of the flux
by existence of dark matter substructures into account. The propagation of
positrons in the Galactic magnetic field is solved in a realistic numerical
model GALPROP. The secondary positron flux is recalculated in the GLAPROP
model. The total positron flux from secondary products and dark matter
annihilation can fit the HEAT data well when taking a cuspy density profile of
the substructures.Comment: 16 pages, 10 figures, accepted by JCA
A study on the sharp knee and fine structures of cosmic ray spectra
The paper investigates the overall and detailed features of cosmic ray (CR)
spectra in the knee region using the scenario of nuclei-photon interactions
around the acceleration sources. Young supernova remnants can be the physical
realities of such kind of CR acceleration sites. The results show that the
model can well explain the following problems simultaneously with one set of
source parameters: the knee of CR spectra and the sharpness of the knee, the
detailed irregular structures of CR spectra, the so-called "component B" of
Galactic CRs, and the electron/positron excesses reported by recent
observations. The coherent explanation serves as evidence that at least a
portion of CRs might be accelerated at the sources similar to young supernova
remnants, and one set of source parameters indicates that this portion mainly
comes from standard sources or from a single source.Comment: 13 pages, 4 figures, accepted for publication in SCIENCE CHINA
Physics, Mechanics & Astronomy
Linear stability analysis of retrieval state in associative memory neural networks of spiking neurons
We study associative memory neural networks of the Hodgkin-Huxley type of
spiking neurons in which multiple periodic spatio-temporal patterns of spike
timing are memorized as limit-cycle-type attractors. In encoding the
spatio-temporal patterns, we assume the spike-timing-dependent synaptic
plasticity with the asymmetric time window. Analysis for periodic solution of
retrieval state reveals that if the area of the negative part of the time
window is equivalent to the positive part, then crosstalk among encoded
patterns vanishes. Phase transition due to the loss of the stability of
periodic solution is observed when we assume fast alpha-function for direct
interaction among neurons. In order to evaluate the critical point of this
phase transition, we employ Floquet theory in which the stability problem of
the infinite number of spiking neurons interacting with alpha-function is
reduced into the eigenvalue problem with the finite size of matrix. Numerical
integration of the single-body dynamics yields the explicit value of the
matrix, which enables us to determine the critical point of the phase
transition with a high degree of precision.Comment: Accepted for publication in Phys. Rev.
Search for dark matter signals with Fermi-LAT observation of globular clusters NGC 6388 and M 15
The globular clusters are probably good targets for dark matter (DM) searches
in -rays due to the possible adiabatic contraction of DM by baryons. In
this work we analyse the three-year data collected by {\it Fermi} Large Area
Telescope of globular clusters NGC 6388 and M 15 to search for possible DM
signals. For NGC 6388 the detection of -ray emission was reported by
{\it Fermi} collaboration, which is consistent with the emission of a
population of millisecond pulsars. The spectral shape of NGC 6388 is also shown
to be consistent with a DM contribution if assuming the annihilation final
state is . No significant -ray emission from M 15 is
observed. We give the upper limits of DM contribution to -ray emission
in both NGC 6388 and M 15, for annihilation final states , ,
, and monochromatic line. The constraints are
stronger than that derived from observation of dwarf galaxies by {\it Fermi}.Comment: 17 pages, 6 figures, accepted by JCA
Clumpiness enhancement of charged cosmic rays from dark matter annihilation with Sommerfeld effect
Boost factors of dark matter annihilation into antiprotons and
electrons/positrons due to the clumpiness of dark matter distribution are
studied in detail in this work, taking the Sommerfeld effect into account. It
has been thought that the Sommerfeld effect, if exists, will be more remarkable
in substructures because they are colder than the host halo, and may result in
a larger boost factor. We give a full calculation of the boost factors based on
the recent N-body simulations. Three typical cases of Sommerfeld effects, the
non-resonant, moderately resonant and strongly resonant cases are considered.
We find that for the non-resonant and moderately resonant cases the enhancement
effects of substructures due to the Sommerfeld effect are very small () because of the saturation behavior of the Sommerfeld effect.
For the strongly resonant case the boost factor is typically smaller than . However, it is possible in some very extreme cases that DM
distribution is adopted to give the maximal annihilation the boost factor can
reach up to . The variances of the boost factors due to different
realizations of substructures distribution are also discussed in the work.Comment: 28 pages, 8 figures, 2 table. The detailed fomula of the propagation
and boost factor are moved to the Appendix. Accepted by JCA
North Atlantic simulations in Coordinated Ocean-ice Reference Experiments phase II (CORE-II). Part I: Mean states
Simulation characteristics from eighteen global ocean–sea-ice coupled models are presented with a focus on the mean Atlantic meridional overturning circulation (AMOC) and other related fields in the North Atlantic. These experiments use inter-annually varying atmospheric forcing data sets for the 60-year period from 1948 to 2007 and are performed as contributions to the second phase of the Coordinated Ocean-ice Reference Experiments (CORE-II). The protocol for conducting such CORE-II experiments is summarized. Despite using the same atmospheric forcing, the solutions show significant differences. As most models also differ from available observations, biases in the Labrador Sea region in upper-ocean potential temperature and salinity distributions, mixed layer depths, and sea-ice cover are identified as contributors to differences in AMOC. These differences in the solutions do not suggest an obvious grouping of the models based on their ocean model lineage, their vertical coordinate representations, or surface salinity restoring strengths. Thus, the solution differences among the models are attributed primarily to use of different subgrid scale parameterizations and parameter choices as well as to differences in vertical and horizontal grid resolutions in the ocean models. Use of a wide variety of sea-ice models with diverse snow and sea-ice albedo treatments also contributes to these differences. Based on the diagnostics considered, the majority of the models appear suitable for use in studies involving the North Atlantic, but some models require dedicated development effort
- …