502 research outputs found
Numerical Simulation of Radio Signal from Extended Air Showers
The burst of radio emission by the extensive air shower provides a promising
alternative for detecting ultra-high energy cosmic rays.We have developed an
independent numerical program to simulate these radio signals. Our code is
based on a microscopic treatment, with both the geosynchrotron radiation and
charge excess effect included. Here we make a first presentation of our basic
program and its results. The time signal for different polarizations are
computed, we find that the pulses take on a bipolar pattern, the spectrum is
suppressed towards the lower frequencies.We investigate how the shower at
different heights in atmosphere contribute to the total signal, and examine the
signal strength and distribution at sites of different elevations. We also
study the signal from showers of different inclination angles and azimuth
directions. In all these cases we find the charge excess effect important.Comment: 23 pages, 14 figure
Particle decays during the cosmic dark ages
We consider particle decays during the cosmic dark ages with two aims: (1) to explain the high optical depth reported by the Wilkinson Microwave Anisotropy Probe (WMAP), and (2) to provide new constraints to the parameter space for decaying particles. We delineate the decay channels in which most of the decay energy ionizes and heats the intergalactic medium gas [and thus affects the cosmic microwave background (CMB)], and those in which most of the energy is carried awayâe.g. photons with energies 100 keV<~E<~1 TeVâand thus appears as a contribution to diffuse x-ray and gamma-ray backgrounds. The new constraints to the decay-particle parameters from the CMB power spectrum thus complement those from the cosmic x-ray and gamma-ray backgrounds. Although decaying particles can indeed produce an optical depth consistent with that reported by WMAP, in so doing they produce new fluctuations in the CMB temperature and polarization power spectra. For decay lifetimes less than the age of the Universe, the induced power spectra generally violate current constraints, while the power spectra are usually consistent if the lifetime is longer than the age of the Universe
Three-Body Annihilation of Neutralinos Below Two-Body Thresholds
We calculate the cross section for s-wave neutralino annihilation to
three-body final states below the W^+ W^- and t-bar t thresholds. Such
three-body channels may dominate the annihilation cross section if the
neutralino mass is not too much less than m_t and m_W respectively.
Furthermore, because neutrinos produced in these channels are much more
energetic than those from the or tau^+ tau^- channels, they can
dominate the energetic-neutrino fluxes from neutralino annihilation in the Sun
or Earth far below these thresholds and significantly enhance the neutrino
signal in certain regions of the supersymmetric parameter space.Comment: 19 pages, 8 postscript figures, use JHEP.cl
Relic Abundance of Asymmetric Dark Matter in Quintessence
We investigate the relic abundance of asymmetric Dark Matter particles in
quintessence model with a kination phase. The analytic calculation of the
asymmetric Dark Matter in the standard cosmological scenario is extended to the
nonstandard cosmological scenario where we specifically discuss the
quintessence model with a kination phase. We found that the enhancement of
Hubble rate changes the relic density of particles and anti--particles. We use
the present day Dark Matter abundance to constrain the Hubble rate in
quintessence model with a kination phase for asymmetric Dark Matter.Comment: 13 pages, 9 figure
Type Ia supernovae as speed sensors at intermediate redshifts
The application of large scale peculiar velocity (LSPV), as a crucial probe
of dark matter, dark energy and gravity, is severely limited by measurement
obstacles. We show that fluctuations in type Ia supernovae (SNe Ia) fluxes
induced by LSPV offer a promising approach to measure LSPV at intermediate
redshifts. In the 3D Fourier space, gravitational lensing, the dominant
systematical error, is well suppressed, localized and can be further corrected
effectively. Advance in SN observations can further significantly reduce shot
noise induced by SN intrinsic fluctuations, which is the dominant statistical
error. Robust mapping on the motion of the dark universe through SNe Ia is thus
feasible to .Comment: 6 pages, 1 figure. v2: expanded discussions. Accepted to PRD. Also
refer to the news report at Physics world
http://physicsworld.com/cws/article/news/3509
TeV cosmic-ray proton and helium spectra in the myriad model II
Recent observations show that the cosmic ray nuclei spectra start to harden
above 100 GeV, in contradiction with the conventional steady-state cosmic ray
model. We had suggested that this anomaly is due to the propagation effect of
cosmic rays released from local young cosmic ray sources, the total flux of the
cosmic ray should be computed with the myriad model, where contribution from
sources in local catalog is added to the background. However, while the
hardening could be elegantly explained in this model, the model parameters
obtained from the fit skew toward a region with fast diffusion and low
supernova rate in the Galaxy, in tension with other observations. In this
paper, we further explore this model in order to set up a concordant picture.
Two possible improvements related to the cosmic ray sources have been
considered. Firstly, instead of the usual axisymmetric disk model, we
considered a spiral model of source distribution. Secondly, for the nearby and
young sources which are paramount to the hardening, we allow for an
energy-dependent escape time. We find that major improvement comes from the
energy-dependent escape time of the local sources, and with both modifications,
not only the cosmic ray proton and helium anomalies are solved, but also the
parameters attain reasonable range values compatible with other analysis.Comment: 13 pages, 7 figures, 1 table, accepted for publication in RA
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