8,817 research outputs found
Instabilities in neutrino systems induced by interactions with scalars
If there are scalar particles of small or moderate mass coupled very weakly
to Dirac neutrinos, in a minimal way, then neutrino-anti-neutrino clouds of
sufficient number density can experience an instability in which helicities are
suddenly reversed. The predicted collective evolution is many orders of
magnitude faster than given by cross-section calculations. The instabilities
are the analogue of the ``flavor-angle" instabilities (enabled by the Z
exchange force) that may drive very rapid flavor exchange among the neutrinos
that emerge from a supernova. These exchanges do require a tiny seed in
addition to the scalar couplings, but the transition time is proportional to
the negative of the logarithm of the seed strength, so that the size of this
parameter is comparatively unimportant. For our actual estimates we use a tiny
non-conservation of leptons; an alternative would be a neutrino magnetic moment
in a small magnetic field. The possibility of a quantum fluctuation as a seed
is also discussed. Operating in the mode of putting limits on the coupling
constant of the scalar field, for the most minimal coupling scheme, with
independent couplings to all three , we find a rough limit on the
dimensionless coupling constant for a neutrino-flavor independent coupling of
, to avoid the effective number of light neutrinos in the early
universe being essentially six. If, on the other hand, we wish to fine-tune the
model to give a more modest excess (over three) in the effective neutrino
number, as may be needed according to recent WMAP analyses, it is easy to do
so.
\pacs{13.15.+g}Comment: 10 pages, 4 figures, comments on alternative seeds adde
The multi-angle instability in dense neutrino systems
We calculate rates of flavor exchange within clouds of neutrinos interacting
with each other through the standard model coupling, assuming a conventional
mass matrix. For cases in which there is an angular dependence in the relation
among intensity, flavor and spectrum, we find instabilities in the evolution
equations and greatly speeded-up flavor exchange. The instabilities are
categorized by examining linear perturbations to simple solutions, and their
effects are exhibited in complete numerical solutions to the system. The
application is to the region just under the neutrino surfaces in the supernova
core.Comment: 9 pages, 3 figure
Plasma effects on resonant fusion
We investigate the effects of plasma interactions on resonance-enhanced
fusion rates in stars. Starting from basic principles we derive an expression
for the fusion rate that can serve as a basis for discussion of approximation
schemes. The present state-of-the-art correction algorithms, based on the
classical correlation function for the fusing particles and the classical
energy shift for the resonant state, do not follow from this result, even as an
approximation. The results of expanding in a perturbation solution for the case
of a weakly coupled plasma are somewhat enlightening. But at this point we are
at a loss as to how to do meaningful calculations in systems with even moderate
plasma coupling strength. Examples where this can matter are: the effect of a
possible low energy C + C resonance on X-ray bursts from
accreting neutron stars or on supernova 1A simulations; and the calculation of
the triple rate in some of the more strongly coupled regions in which
the process enters, such as accretion onto a neutron star.Comment: 10 pages, 1 figur
Speed-up of neutrino transformations in a supernova environment
When the neutral current neutrino-neutrino interaction is treated completely,
rather than as an interaction among angle-averaged distributions, or as a set
of flavor-diagonal effective potentials, the result can be flavor mixing at a
speed orders of magnitude faster than that one would anticipate from the
measured neutrino oscillation parameters. It is possible that the energy
spectra of the three active species of neutrinos emerging from a supernova are
nearly identical.Comment: 8 pages, 4 figure
Evolution speed in some coupled-spin models
We investigate the time evolution of some models with N spins and pairwise
couplings, for the case of large N, in order to compare evolution times with
"speed limit" minima derived in the literature. Both in a (symmetric) case with
couplings of the same strength between each pair and in a case of broken
symmetry, the times necessary for evolution to a state in which the simplest
initial state has evolved into a nearly orthogonal state are proportional to
1/N, as is the speed limit time. However the coefficient in the broken symmetry
case comes much closer to the speed limit value. Introducing a different
criterion for evolution speed, based on macroscopic changes in occupation, we
find a corresponding enhancement in rates in the asymmetric case as compared to
the symmetric case.Comment: 6 pages, 4 figures. Correction of numerous mistakes. One model made
simpler. A superior algorithm used to solve the second exampl
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