120 research outputs found
On the Occurrence of Crossings Between the Angular Distributions of Electron Neutrinos and Antineutrinos in the Supernova Core
Neutrino fast pairwise conversions have been postulated to occur in the dense
core of a core-collapse supernova (SN), possibly having dramatic consequences
on the SN mechanism and the observable neutrino signal. One crucial condition
favoring pairwise conversions is the presence of crossings between the electron
neutrino and antineutrino angular distributions (i.e., electron neutrino lepton
number crossings, ELN crossings). A stationary and spherically symmetric SN
toy-model is constructed to reproduce the development of the neutrino angular
distributions in the dense SN core in the absence of perturbations induced by
hydrodynamical instabilities. By iteratively solving the neutrino Boltzmann
equations including the collisional term, our model predicts that ELN crossings
can develop only in the proximity of the decoupling region and for a sharp
radial evolution of the baryon density, when the electron neutrino and
antineutrino number densities are comparable. Such conditions are likely to
occur only in the late SN stages. Interestingly, flavor instabilities induced
by spatial or temporal perturbations are unlikely to generate ELN crossings
dynamically within our simplified setup.Comment: 11 pages, 8 figures, Version accepted in APJ. Results unchange
Diffuse emission of high-energy neutrinos from gamma-ray burst fireballs
Gamma-ray bursts (GRBs) have been suggested as possible sources of the
high-energy neutrino flux recently detected by the IceCube telescope. We
revisit the fireball emission model and elaborate an analytical prescription to
estimate the high-energy neutrino prompt emission from pion and kaon decays,
assuming that the leading mechanism for the neutrino production is
lepto-hadronic. To this purpose, we include hadronic, radiative and adiabatic
cooling effects and discuss their relevance for long- (including high- and
low-luminosity) and short-duration GRBs. The expected diffuse neutrino
background is derived, by requiring that the GRB high-energy neutrino
counterparts follow up-to-date gamma-ray luminosity functions and redshift
evolutions of the long and short GRBs. Although dedicated stacking searches
have been unsuccessful up to now, we find that GRBs could contribute up to a
few % to the observed IceCube high-energy neutrino flux for sub-PeV energies,
assuming that the latter has a diffuse origin. Gamma-ray bursts, especially
low-luminosity ones, could however be the main sources of the IceCube
high-energy neutrino flux in the PeV range. While high-luminosity and
low-luminosity GRBs have comparable intensities, the contribution from the
short-duration component is significantly smaller. Our findings confirm the
most-recent IceCube results on the GRB searches and suggest that larger
exposure is mandatory to detect high-energy neutrinos from high-luminosity GRBs
in the near future.Comment: 28 pages, including 8 figures. Matches version published in JCA
Synchronization vs. decoherence of neutrino oscillations at intermediate densities
We study collective oscillations of a two-flavor neutrino system with
arbitrary but fixed density. In the vacuum limit, modes with different energies
quickly de-phase (kinematical decoherence), whereas in the limit of infinite
density they lock to each other (synchronization). For intermediate densities,
we find different classes of solutions. There is always a phase transition in
the sense of partial synchronization occurring only above a density threshold.
For small mixing angles, partial or complete decoherence can be induced by a
parametric resonance, introducing a new time scale to the problem, the final
outcome depending on the spectrum and mixing angle. We derive an analytic
relation that allows us to calculate the late-time degree of coherence based on
the spectrum alone.Comment: 11 pages, including 13 figures. Clarifying paragraphs and 2 figures
added; results unchanged. Matches published version in PR
Student activation and active learning
Student activation and active learning have been proposed as toolsthrough which students acquire knowledge and develop problem-solving skills. On the other hand, traditional teaching is still preferred in Physics courses with abstract and theoretical content. By employing student activation, I intend to explore whether the latter improves the student performance in theoretical courses
Tomographic Constraints on High-Energy Neutrinos of Hadronuclear Origin
Mounting evidence suggests that the TeV-PeV neutrino flux detected by the
IceCube telescope has mainly an extragalactic origin. If such neutrinos are
primarily produced by a single class of astrophysical sources via hadronuclear
() interactions, a similar flux of gamma-ray photons is expected. For the
first time, we employ tomographic constraints to pinpoint the origin of the
IceCube neutrino events by analyzing recent measurements of the cross
correlation between the distribution of GeV gamma rays, detected by the Fermi
satellite, and several galaxy catalogs in different redshift ranges. We find
that the corresponding bounds on the neutrino luminosity density are up to one
order of magnitude tighter than those obtained by using only the spectrum of
the gamma-ray background, especially for sources with mild redshift evolution.
In particular, our method excludes any hadronuclear source with a spectrum
softer than as a main component of the neutrino background, if its
evolution is slower than . Starburst galaxies, if able to accelerate
and confine cosmic rays efficiently, satisfy both spectral and tomographic
constraints.Comment: 6 pages, 3 figures; accepted for publication in Physical Review
Letter
Star-forming galaxies as the origin of diffuse high-energy backgrounds: Gamma-ray and neutrino connections, and implications for starburst history
Star-forming galaxies have been predicted to contribute considerably to the
diffuse gamma-ray background as they are guaranteed reservoirs of cosmic rays.
Assuming that the hadronic interactions responsible for high-energy gamma rays
also produce high-energy neutrinos and that O(100) PeV cosmic rays can be
produced and confined in starburst galaxies, we here discuss the possibility
that star-forming galaxies are also the main sources of the high-energy
neutrinos observed by the IceCube experiment. First, we compute the diffuse
gamma-ray background from star-forming galaxies, adopting the latest Herschel
PEP/HerMES luminosity function and relying on the correlation between the
gamma-ray and infrared luminosities reported by Fermi observations. Then we
derive the expected intensity of the diffuse high-energy neutrinos from
star-forming galaxies including normal and starburst galaxies. Our results
indicate that starbursts, including those with active galactic nuclei and
galaxy mergers, could be the main sources of the high-energy neutrinos observed
by the IceCube experiment. We find that assuming a cosmic-ray spectral index of
2.1-2.2 for all starburst-like galaxies, our predictions can be consistent with
both the Fermi and IceCube data, but larger indices readily fail to explain the
observed diffuse neutrino flux. Taking the starburst high-energy spectral index
as free parameter, and extrapolating from GeV to PeV energies, we find that the
spectra harder than E^(-2.15) are likely to be excluded by the IceCube data,
which can be more constraining than the Fermi data for this population.Comment: 26 pages, including 7 figures. Discussion expanded, new figures
added. Accepted by JCA
Fast Pairwise Conversion of Supernova Neutrinos: A Dispersion-Relation Approach
Collective pair conversion
by forward scattering, where or , may be generic for supernova
neutrino transport. Depending on the local angular intensity of the electron
lepton number carried by neutrinos, the conversion rate is "fast," i.e., of the
order of . We present a novel approach to understand these phenomena: A
dispersion relation for the frequency and wave number of
disturbances in the mean field of flavor coherence. Run-away
solutions occur in "dispersion gaps," i.e., in "forbidden" intervals of
and/or where propagating plane waves do not exist. We stress
that the actual solutions also depend on the initial and/or boundary conditions
which need to be further investigated.Comment: 6 pages, 3 figures. Minor changes in the text, references added and
discussion of figure 3 extended. Matches published PRL versio
Triggering collective oscillations by three-flavor effects
Collective flavor transformations in supernovae, caused by neutrino-neutrino
interactions, are essentially a two-flavor phenomenon driven by the atmospheric
mass difference and the small mixing angle theta_13. In the two-flavor
approximation, the initial evolution depends logarithmically on theta_13 and
the system remains trapped in an unstable fixed point for theta_13 = 0.
However, any effect breaking exact nu_mu-nu_tau equivalence triggers the
conversion. Such three-flavor perturbations include radiative corrections to
weak interactions, small differences between the nu_mu and nu_tau fluxes, or
non-standard interactions. Therefore, extremely small values of theta_13 are in
practice equivalent, the fate of the system depending only on the neutrino
spectra and their mass ordering.Comment: 6 pages, including 3 figures (slightly modified to match the
published version). Accepted for publication in PRD
- …