186 research outputs found
Breaking the symmetries in self-induced flavor conversions of neutrino beams from a ring
Self-induced flavor conversions of supernova (SN) neutrinos have been
characterized in the spherically symmetric "bulb" model, reducing the neutrino
evolution to a one dimensional problem along a radial direction. We lift this
assumption, presenting a two-dimensional toy-model where neutrino beams are
launched in many different directions from a ring. We find that
self-interacting neutrinos spontaneously break the spatial symmetries of this
model. As a result the flavor content and the lepton number of the neutrino gas
would acquire seizable direction-dependent variations, breaking the coherent
behavior found in the symmetric case. This finding would suggest that the
previous results of the self-induced flavor evolution obtained in
one-dimensional models should be critically re-examined.Comment: v2 (8 pages, 9 eps figures): Revised version. Major changes. Model
improved and clarified. Calculations and figures updated. Matches the version
accepted for publication on PR
Another look at synchronized neutrino oscillations
In dense neutrino backgrounds present in supernovae and in the early Universe
neutrino oscillations may exhibit complex collective phenomena, such as
synchronized oscillations, bipolar oscillations and spectral splits and swaps.
We consider in detail possible decoherence effects on the simplest of these
phenomena -- synchronized neutrino oscillations that can occur in a uniform and
isotropic neutrino gas. We develop an exact formalism of spectral moments of
the flavour spin vectors describing such a system and then apply it to find
analytical approaches that allow one to study decoherence effects on its
late-time evolution. This turns out to be possible in part due to the existence
of the (previously unknown) exact conservation law satisfied by the quantities
describing the considered neutrino system. Interpretation of the decoherence
effects in terms of neutrino wave packet separation is also given, both in the
adiabatic and non-adiabatic regimes of neutrino flavour evolution.Comment: LaTeX, 32 pages, 3 figures. V2: minor textual changes, references and
an acknowledgement added, results and conclusions unchange
Temporal Instability Enables Neutrino Flavor Conversions Deep Inside Supernovae
We show that a self-interacting neutrino gas can spontaneously acquire a
non-stationary pulsating component in its flavor content, with a frequency that
can exactly cancel the "multi-angle" refractive effects of dense matter. This
can then enable homogeneous and inhomogeneous flavor conversion instabilities
to exist even at large neutrino and matter densities, where the system would
have been stable if the evolution were strictly stationary. Large flavor
conversions, especially close to a supernova core, are possible via this novel
mechanism. This may have important consequences for the explosion dynamics,
nucleosynthesis, as well as for neutrino observations of supernovae.Comment: v3: Improved Fig.1 and fixed typos. Matches version published in PR
Multi-azimuthal-angle instability for different supernova neutrino fluxes
It has been recently discovered that removing the axial symmetry in the
"multi-angle effects" associated with the neutrino-neutrino interactions for
supernova (SN) neutrinos, a new multi-azimuthal-angle (MAA) instability would
trigger flavor conversions in addition to the ones caused by the bimodal and
multi-zenith-angle (MZA) instabilities. We investigate the dependence of the
MAA instability on the original SN neutrino fluxes, performing a stability
analysis of the linearized neutrino equations of motion. We compare these
results with the numerical evolution of the SN neutrino non-linear equations,
looking at a local solution along a specific line of sight, under the
assumption that the transverse variations of the global solution are small. We
also assume that self-induced conversions are not suppressed by large matter
effects. We show that the pattern of the spectral crossings (energies where
F_{\nu_e} = F_{\nu_x}, and F_{\bar\nu_e} = F_{\bar\nu_x}) is crucial in
determining the impact of MAA effects on the flavor evolution. For neutrino
spectra with a strong excess of \nu_e over \bar\nu_e, presenting only a
single-crossing, MAA instabilities would trigger new flavor conversions in
normal mass hierarchy. In our simplified flavor evolution scheme, these would
lead to spectral swaps and splits analogous to what produced in inverted
hierarchy by the bimodal instability. Conversely, in the presence of spectra
with a moderate flavor hierarchy, having multiple crossing energies, MZA
effects would produce a sizable delay in the onset of the flavor conversions,
inhibiting the growth of the MAA instability. In this case the splitting
features for the oscillated spectra in both the mass hierarchies are the ones
induced by the only bimodal and MZA effects.Comment: (v2: 13 pages, 9 eps figures. Revised version. Accepted for
publication in PRD. Major changes: Stability analysis added. Results
unchanged
Self-induced flavor instabilities of a dense neutrino stream in a two-dimensional model
We consider a simplifed model for self-induced flavor conversions of a dense
neutrino gas in two dimensions, showing new solutions that spontaneously break
the spatial symmetries of the initial conditions. As a result of the symmetry
breaking induced by the neutrino-neutrino interactions, the coherent behavior
of the neutrino gas becomes unstable. This instability produces large spatial
variations in the flavor content of the ensemble. Furthermore, it also leads to
the creation of domains of different net lepton number flux. The transition of
the neutrino gas from a coherent to incoherent behavior shows an intriguing
analogy with a streaming flow changing from laminar to turbulent regime. These
finding would be relevant for the self-induced conversions of neutrinos
streaming-off a supernova core.Comment: (v2: revised version: 8 pages, 7 eps figures. To appear on Physical
Review D as Rapid Communication. Discussion enlarged. Two Appendices added.
Fast neutrino flavor conversions near the supernova core with realistic flavor-dependent angular distributions
It has been recently pointed out that neutrino fluxes from a supernova can
show substantial flavor conversions almost immediately above the core. Using
linear stability analyses and numerical solutions of the fully nonlinear
equations of motion, we perform a detailed study of these fast conversions,
focussing on the region just above the supernova core. We carefully specify the
instabilities for evolution in space or time, andfind that neutrinos travelling
towards the core make fast conversions more generic, i.e., possible for a wider
range of flux ratios and angular asymmetries that produce a crossing between
the zenith-angle spectra of and . Using fluxes and angular
distributions predicted by supernova simulations, we find that fast conversions
can occur within tens of nanoseconds, only a few meters away from the putative
neutrinospheres. If these fast flavor conversions indeed take place, they would
have important implications for the supernova explosion mechanism and
nucleosynthesis.Comment: 18 pages, 7 figures (Improved presentation and new panel in Fig.6
Testing Lorentz invariance with neutrino bursts from supernova neutronization
Quantum-gravity (QG) effects might generate Lorentz invariance violation by
the interaction of energetic particles with the foamy structure of the
space-time. As a consequence, particles may not travel at the universal speed
of light. We propose to constrain Lorentz invariance violation for energetic
neutrinos exploiting the neutronization burst from the next galactic
supernova (SN). This prompt signal is expected to produce a sharp peak in the
SN light curve with a duration of ms. However presence of
energy-dependent Lorentz invariance violation would significantly spread out
the time structure of this signal. We find that the detection the SN
burst from a typical galactic explosion at kpc in a Mton-class water
Cerenkov detector, would be sensitive to a quantum-gravity mass scale GeV ( GeV) for the linear (quadratic) energy
dependence of Lorentz invariance violation. These limits are rather independent
of the neutrino mass hierarchy and whether the neutrino velocity is super or
subluminal.Comment: 4 pages, 3 figures, Revised version. Minor changes. Matches published
versio
Damping the neutrino flavor pendulum by breaking homogeneity
The most general case of self-induced neutrino flavor evolution is described
by a set of kinetic equations for a dense neutrino gas evolving both in space
and time. Solutions of these equations have been typically worked out assuming
that either the time (in the core-collapse supernova environment) or space (in
the early universe) homogeneity in the initial conditions is preserved through
the evolution. In these cases one can gauge away the homogeneous variable and
reduce the dimensionality of the problem. In this paper we investigate if small
deviations from an initial postulated homogeneity can be amplified by the
interacting neutrino gas, leading to a new flavor instability. To this end, we
consider a simple two flavor isotropic neutrino gas evolving in time, and
initially composed by only and with equal densities. In the
homogeneous case, this system shows a bimodal instability in the inverted mass
hierarchy scheme, leading to the well studied flavor pendulum behavior. This
would lead to periodic pair conversions . To break space homogeneity, we introduce small amplitude
space-dependent perturbations in the matter potential. By Fourier transforming
the equations of motion with respect to the space coordinate, we then
numerically solve a set of coupled equations for the different Fourier modes.
We find that even for arbitrarily tiny inhomogeneities, the system evolution
runs away from the stable pendulum behavior: the different modes are excited
and the space-averaged ensemble evolves towards flavor equilibrium. We finally
comment on the role of a time decaying neutrino background density in weakening
these results.Comment: (7 pages, 5 eps figures. Figure improved. Final version appeared in
PRD
Self-induced temporal instability from a neutrino antenna
It has been recently shown that the flavor composition of a self-interacting
neutrino gas can spontaneously acquire a time-dependent pulsating component
during its flavor evolution. In this work, we perform a more detailed study of
this effect in a model where neutrinos are assumed to be emitted in a
two-dimensional plane from an infinite line that acts as a neutrino antenna. We
consider several examples with varying matter and neutrino densities and find
that temporal instabilities with various frequencies are excited in a cascade.
We compare the numerical calculations of the flavor evolution with the
predictions of linearized stability analysis of the equations of motion. The
results obtained with these two approaches are in good agreement in the linear
regime, while a dramatic speed-up of the flavor conversions occurs in the
non-linear regime due to the interactions among the different pulsating modes.
We show that large flavor conversions can take place if some of the temporal
modes are unstable for long enough, and that this can happen even if the matter
and neutrino densities are changing, as long as they vary slowly.Comment: v2: revised version, 15 pages, 6 figures. Minor changes. Typos
removed, figures improved. Matches the version accepted on JCA
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