3,451 research outputs found
Lorentz Invariance Violation and the Observed Spectrum of Ultrahigh Energy Cosmic Rays
There has been much interest in possible violations of Lorentz invariance,
particularly motivated by quantum gravity theories. It has been suggested that
a small amount of Lorentz invariance violation (LIV) could turn off photomeson
interactions of ultrahigh energy cosmic rays (UHECRs) with photons of the
cosmic background radiation and thereby eliminate the resulting sharp
steepening in the spectrum of the highest energy CRs predicted by Greisen
Zatsepin and Kuzmin (GZK). Recent measurements of the UHECR spectrum reported
by the HiRes and Auger collaborations, however, indicate the presence of the
GZK effect. We present the results of a detailed calculation of the
modification of the UHECR spectrum caused by LIV using the formalism of Coleman
and Glashow. We then compare these results with the experimental UHECR data
from Auger and HiRes. Based on these data, we find a best fit amount of LIV of
,consistent with an upper limit of . This possible amount of LIV can lead to a recovery of the cosmic ray
spectrum at higher energies than presently observed. Such an LIV recovery
effect can be tested observationally using future detectors.Comment: corrected proof version to be published in Astroparticle Physic
Corrected Table for the Parametric Coefficients for the Optical Depth of the Universe to Gamma-rays at Various Redshifts
Table 1 in our paper, ApJ 648, 774 (2006) entitled "Intergalactic Photon
Spectra from the Far IR to the UV Lyman Limit for 0 < z < 6 and the Optical
Depth of the Universe to High Energy Gamma-Rays" had erroneous numbers for the
coefficients fitting the parametric form for the optical depth of the universe
to gamma-rays. The correct values for these parameters as described in the
original text are given here in a corrected table for various redshifts for the
baseline model (upper row) and fast evolution (lower row) for each individual
redshift. The parametric approximation is good for optical depths between 0.01
and 100 and for gamma-ray energies up to ~2 TeV for all redshifts but also for
energies up to ~10 TeV for redshifts less than 1.Comment: Table 1 corrected and new gamma-ray energy range of validity give
Intergalactic Photon Spectra from the Far IR to the UV Lyman Limit for and the Optical Depth of the Universe to High Energy Gamma-Rays
We calculate the intergalactic photon density as a function of both energy
and redshift for 0 < z < 6 for photon energies from .003 eV to the Lyman limit
cutoff at 13.6 eV in a Lambda-CDM universe with and
. Our galaxy evolution model gives results which are
consistent with Spitzer deep number counts and the spectral energy distribution
of the extragalactic background radiation. We use our photon density results to
extend previous work on the absorption of high energy gamma-rays in
intergalactic space owing to interactions with low energy photons and the 2.7 K
cosmic background radiation. We calculate the optical depth of the universe,
tau, for gamma-rays having energies from 4 GeV to 100 TeV emitted by sources at
redshifts from ~0 to 5. We also give an analytic fit with numerical
coefficients for approximating . As an example of the
application of our results, we calculate the absorbed spectrum of the blazar
PKS 2155-304 at z = 0.117 and compare it with the spectrum observed by the
H.E.S.S. air Cherenkov gamma-ray telescope array.Comment: final version to be published in Ap
Quantum tunneling through vacuum-multiparticle induced potentials
The vacuum cavity mode induces a potential barrier and a well when an
ultra-slow excited atom enters the interaction region so that it can be
reflected or transmitted with a certain probability. We demonstrate here that a
slow-velocity excited particle tunnels freely through a vacuum electromagnetic
field mode filled with ground state atoms. The reason for this is the
trapping of the moving atom into its upper state due to multiparticle
influences and the corresponding decoupling from the interaction with the
environment such that the emitter does not {\it see} the induced potentials.Comment: Multiparticle samples, quantum tunneling, vacuum induced potential
Investigating Biometric Response for Information Retrieval Applications
Current information retrieval systems make no measurement of the userâs response to the searching process or the information itself. Existing psychological studies show that subjects exhibit measurable physiological responses
when carrying out certain tasks, e.g. when viewing images, which generally result in heightened emotional states. We find that users exhibit measurable biometric behaviour in the form of galvanic skin response when watching movies, and engaging in interactive tasks. We examine how this data might be exploited in the indexing of data for search and within the search process itself
Interacting double dark resonances in a hot atomic vapor of helium
We experimentally and theoretically study two different tripod configurations
using metastable helium (He*), with the probe field polarization
perpendicular and parallel to the quantization axis, defined by an applied weak
magnetic field. In the first case, the two dark resonances interact
incoherently and merge together into a single EIT peak with increasing coupling
power. In the second case, we observe destructive interference between the two
dark resonances inducing an extra absorption peak at the line center.Comment: 7 pages, 7 figure
Storage and retrieval of light pulses in atomic media with "slow" and "fast" light
We present experimental evidence that light storage, i.e. the controlled
release of a light pulse by an atomic sample dependent on the past presence of
a writing pulse, is not restricted to small group velocity media but can also
occur in a negative group velocity medium. A simple physical picture applicable
to both cases and previous light storage experiments is discussed.Comment: 4 pages, 3 figures, submitted to Physical Review Letter
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