1,348 research outputs found
Prolongation of Friction Dominated Evolution for Superconducting Cosmic Strings
This investigation is concerned with cosmological scenarios based on particle
physics theories that give rise to superconducting cosmic strings (whose
subsequent evolution may produce stable loop configurations known as vortons).
Cases in which electromagnetic coupling of the string current is absent or
unimportant have been dealt with in previous work. The purpose of the present
work is to provide quantitative estimates for cases in which electromagnetic
interaction with the surrounding plasma significantly affects the string
dynamics. In particular it will be shown that the current can become
sufficiently strong for the initial period of friction dominated string motion
to be substantially prolonged, which would entail a reinforcement of the short
length scale end of the spectrum of the string distribution, with potentially
observable cosmological implications if the friction dominated scenario lasts
until the time of plasma recombination.Comment: 10 pages Late
Self-induced decoherence in dense neutrino gases
Dense neutrino gases exhibit collective oscillations where "self-maintained
coherence" is a characteristic feature, i.e., neutrinos of different energies
oscillate with the same frequency. In a non-isotropic gas, however, the flux
term of the neutrino-neutrino interaction has the opposite effect of causing
kinematical decoherence of neutrinos propagating in different directions, an
effect that is at the origin of the "multi-angle behavior" of neutrinos
streaming off a supernova core. We cast the equations of motion in a form where
the role of the flux term is manifest. We study in detail the symmetric case of
equal neutrino and antineutrino densities where the evolution consists of
collective pair conversions ("bipolar oscillations"). A gas of this sort is
unstable in that an infinitesimal anisotropy is enough to trigger a run-away
towards flavor equipartition. The "self-maintained coherence" of a perfectly
isotropic gas gives way to "self-induced decoherence."Comment: Revtex, 16 pages, 12 figure
Numerical Toy-Model Calculation of the Nucleon Spin Autocorrelation Function in a Supernova Core
We develop a simple model for the evolution of a nucleon spin in a hot and
dense nuclear medium. A given nucleon is limited to one-dimensional motion in a
distribution of external, spin-dependent scattering potentials. We calculate
the nucleon spin autocorrelation function numerically for a variety of
potential densities and distributions which are meant to bracket realistic
conditions in a supernova core. For all plausible configurations the width of
the spin-density structure function is found to be less than the temperature.
This is in contrast with a naive perturbative calculation based on the one-pion
exchange potential which overestimates the width and thus suggests a large
suppression of the neutrino opacities by nucleon spin fluctuations. Our results
suggest that it may be justified to neglect the collisional broadening of the
spin-density structure function for the purpose of estimating the neutrino
opacities in the deep inner core of a supernova. On the other hand, we find no
indication that processes such as axion or neutrino pair emission, which depend
on nucleon spin fluctuations, are substantially suppressed beyond the
multiple-scattering effect already discussed in the literature. Aside from
these practical conclusions, our model reveals a number of interesting and
unexpected insights. For example, the spin-relaxation rate saturates with
increasing potential strength only if bound states are not allowed to form by
including a repulsive core. There is no saturation with increasing density of
scattering potentials until localized eigenstates of energy begin to form.Comment: 14 latex pages in two-column format, 15 postscript figures included,
uses revtex.sty and epsf.sty. Submitted to Physical Review
Cosmic Magnetic Fields and Their Influence on Ultra-High Energy Cosmic Ray Propagation
We discuss the influence of large scale cosmic magnetic fields on the
propagation of hadronic cosmic rays above 10^19 eV based on large scale
structure simulations. Our simulations suggest that rather substantial
deflection up to several tens of degrees at 10^20 eV are possible for nucleon
primaries. Further, spectra and composition of cosmic rays from individual
sources can depend on magnetic fields surrounding these sources in
intrinsically unpredictable ways. This is true even if deflection from such
individual sources is small. We conclude that the influence of large scale
cosmic magnetic fields on ultra-high energy cosmic ray propagation is currently
hard to quantify. We discuss possible reasons for discrepant results of
simulations by Dolag et al. which predict deflections of at most a few degrees
for nucleons. We finally point out that even in these latter simulations a
possible heavy component would in general suffer substantial deflection.Comment: 10 latex pages, 9 ps figues, for the proceedings of the Cosmic Ray
International Seminar (CRIS), May 31 - June 4 200
Supernova Neutrino Opacity from Nucleon-Nucleon Bremsstrahlung and Related Processes
Elastic scattering on nucleons, \nu N -> N \nu, is the dominant supernova
(SN) opacity source for \mu and \tau neutrinos. The dominant energy- and
number-changing processes were thought to be \nu e^- -> e^- \nu and \nu\bar \nu
e^+ e^- until Suzuki (1993) showed that the bremsstrahlung process \nu\bar
\nu NN NN was actually more important. We find that for energy exchange,
the related ``inelastic scattering process'' \nu NN NN \nu is even more
effective by about a factor of 10. A simple estimate implies that the \nu_\mu
and \nu_\tau spectra emitted during the Kelvin-Helmholtz cooling phase are much
closer to that of \nu\bar_e than had been thought previously. To facilitate a
numerical study of the spectra formation we derive a scattering kernel which
governs both bremsstrahlung and inelastic scattering and give an analytic
approximation formula. We consider only neutron-neutron interactions, we use a
one-pion exchange potential in Born approximation, nonrelativistic neutrons,
and the long-wavelength limit, simplifications which appear justified for the
surface layers of a SN core. We include the pion mass in the potential and we
allow for an arbitrary degree of neutron degeneracy. Our treatment does not
include the neutron-proton process and does not include nucleon-nucleon
correlations. Our perturbative approach applies only to the SN surface layers,
i.e. to densities below about 10^{14} g cm^{-3}.Comment: 36 pages, LaTeX, 6 postscript figs included, matches version accepted
for publication in Astrophysical Journa
Mu-tau neutrino refraction and collective three-flavor transformations in supernovae
We study three-flavor collective neutrino transformations in the
dense-neutrino region above the neutrino sphere of a supernova core. We find
that two-flavor conversions driven by the atmospheric mass difference and the
13-mixing angle capture the full effect if one neglects the second-order
difference between the muon and tau neutrino refractive index. Including this
"mu-tau matter term" provides a resonance at a density of approximately 3 x
10^7 g cm^-3 that typically causes significant modifications of the overall
electron neutrino and antineutrino survival probabilities. This effect is
surprisingly sensitive to deviations from maximal 23-mixing, being different
for each octant.Comment: 9 pages, 7 figures. New presentation of results, version to be
published in PR
Constructing Dirac linear fermions in terms of non-linear Heisenberg spinors
We show that the massive (or massless) neutrinos can be described as special
states of Heisenberg nonlinear spinors. As a by-product of this decomposition a
particularly attractive consequence appears: the possibility of relating the
existence of only three species of mass-less neutrinos to such internal
non-linear structure. At the same time it allows the possibility that neutrino
oscillation can occurs even for massless neutrinos
Astrophysical interpretation of the medium scale clustering in the ultra-high energy sky
We compare the clustering properties of the combined dataset of ultra-high
energy cosmic rays events, reported by the AGASA, HiRes, Yakutsk and Sugar
collaborations, with a catalogue of galaxies of the local universe (redshift
z<~0.06). We find that the data reproduce particularly well the clustering
properties of the nearby universe within z <~0.02. There is no statistically
significant cross-correlation between data and structures, although
intriguingly the nominal cross-correlation chance probability drops from ~50%
to ~10% using the catalogue with a smaller horizon. Also, we discuss the impact
on the robustness of the results of deflections in some galactic magnetic field
models used in the literature. These results suggest a relevant role of
magnetic fields (possibly extragalactic ones, too) and/or possibly some heavy
nuclei fraction in the UHECRs. The importance of a confirmation of these hints
by Auger data is emphasized.Comment: 10 pages, 7 figures; one reference adde
Prompt neutrinos from atmospheric charm in the general-mass variable-flavor-number scheme
We present predictions for the prompt-neutrino flux arising from the decay of
charmed mesons and baryons produced by the interactions of high-energy cosmic
rays in the Earth's atmosphere, making use of a QCD approach on the basis of
the general-mass variable-flavor-number scheme for the description of charm
hadroproduction at NLO, complemented by a consistent set of fragmentation
functions. We compare the theoretical results to those already obtained by our
and other groups with different theoretical approaches. We provide comparisons
with the experimental results obtained by the IceCube Collaboration in two
different analyses and we discuss the implications for parton distribution
functions.Comment: 43 pages, 21 figures, updated version, to be published in JHE
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