690 research outputs found
New analysis of the SN 1987A neutrinos with a flexible spectral shape
We analyze the neutrino events from the supernova (SN) 1987A detected by the
Kamiokande II (KII) and Irvine-Michigan-Brookhaven (IMB) experiments. For the
time-integrated flux we assume a quasi-thermal spectrum of the form
where plays the role of a
spectral index. This simple representation not only allows one to fit the total
energy emitted in and the average energy
, but also accommodates a wide range of shapes, notably
anti-pinched spectra that are broader than a thermal distribution. We find that
the pile-up of low-energy events near threshold in KII forces the best-fit
value for $\alpha$ to the lowest value of any assumed prior range. This applies
to the KII events alone as well as to a common analysis of the two data sets.
The preference of the data for an ``unphysical'' spectral shape implies that
one can extract meaningful values for and only
if one fixes a prior value for . The tension between the KII and IMB
data sets and theoretical expectations for is not resolved by
an anti-pinched spectrum.Comment: to appear in PRD (6 pages, 6 eps figures
Red giant bound on the axion-electron coupling reexamined
If axions or other low-mass pseudoscalars couple to electrons (``fine
structure constant'' ) they are emitted from red giant stars by the
Compton process and by bremsstrahlung .
We construct a simple analytic expression for the energy-loss rate for all
conditions relevant for a red giant and include axion losses in evolutionary
calculations from the main sequence to the helium flash. We find that
\alpha_a\lapprox0.5\mn(-26) or m_a\lapprox 9\,\meV/\cos^2\beta lest the red
giant core at helium ignition exceed its standard mass by more than
0.025\,\MM_\odot, in conflict with observational evidence. Our bound is the
most restrictive limit on , but it does not exclude the possibility
that axion emission contributes significantly to the cooling of ZZ~Ceti stars
such as G117--B15A for which the period decrease was recently measured.Comment: 11 pages, uuencoded and compressed postscript fil
Core-collapse supernova simulations: Variations of the input physics
Spherically symmetric simulations of stellar core collapse and post-bounce
evolution are used to test the sensitivity of the supernova dynamics to
different variations of the input physics. We consider a state-of-the-art
description of the neutrino-nucleon interactions, possible lepton-number
changing neutrino reactions in the neutron star, and the potential impact of
hydrodynamic mixing behind the supernova shock.Comment: 6 pages, 6 ps figures (in color), to appear in W. Hillebrandt and E.
Mueller, eds., Proceedings of the 11th Workshop on "Nuclear Astrophysics"
held at Ringberg Castle, February 11-16, 200
Comment on "Cherenkov Radiation by Neutrinos in a Supernova Core"
Mohanty and Samal have shown that the magnetic-moment interaction with
nucleons contributes significantly to the photon dispersion relation in a
supernova core, and with an opposite sign relative to the usual plasma effect.
Because of a numerical error they overestimated the magnetic-moment term by two
orders of magnitude, but it is still of the same order as the plasma effect. It
appears that the Cherenkov processes gamma+nu -> nu and nu -> nu+gamma remain
forbidden, but a final verdict depends on a more detailed investigation of the
dynamical magnetic susceptibility of a hot nuclear medium.Comment: 2 pages, REVTEX. Submitted as a Comment to PR
On some singularities of the correlation functions that determine neutrino opacities
Certain perturbation graphs in the calculation of the effects of the medium
on neutrino scattering in supernova matter have a nonintegrable singularity in
a physical region. A number of papers have addressed the apparent pathology
through an ansatz that invokes higher order (rescattering) effects. Taking the
Gamow-Teller terms as an example, we display an expression for the spin-spin
correlation function that determines the cross-sections. It is clear from the
form that there are no pathologies in the order by order perturbation
expansion. Explicit formulae are given for a simple case, leading to an answer
that is very different from one given by other authors.Comment: 8 page
Supernova neutrinos and antineutrinos: ternary luminosity diagram and spectral split patterns
In core-collapse supernovae, the nu_e and anti-nu_e species may experience
collective flavor swaps to non-electron species nu_x, within energy intervals
limited by relatively sharp boundaries ("splits"). These phenomena appear to
depend sensitively upon the initial energy spectra and luminosities. We
investigate the effect of generic variations of the fractional luminosities
(l_e, l_{anti-e}, l_x) with respect to the usual "energy equipartition" case
(1/6, 1/6, 1/6), within an early-time supernova scenario with fixed thermal
spectra and total luminosity. We represent the constraint l_e+l_{anti-e}+4l_x=1
in a ternary diagram, which is explored via numerical experiments (in
single-angle approximation) over an evenly-spaced grid of points. In inverted
hierarchy, single splits arise in most cases, but an abrupt transition to
double splits is observed for a few points surrounding the equipartition one.
In normal hierarchy, collective effects turn out to be unobservable at all grid
points but one, where single splits occur. Admissible deviations from
equipartition may thus induce dramatic changes in the shape of supernova
(anti)neutrino spectra. The observed patterns are interpreted in terms of
initial flavor polarization vectors (defining boundaries for the single/double
split transitions), lepton number conservation, and minimization of potential
energy.Comment: 24 pages, including 14 figures (1 section with 2 figures added).
Accepted for publication in JCA
Astrophysical Axion Bounds
Axion emission by hot and dense plasmas is a new energy-loss channel for
stars. Observational consequences include a modification of the solar
sound-speed profile, an increase of the solar neutrino flux, a reduction of the
helium-burning lifetime of globular-cluster stars, accelerated white-dwarf
cooling, and a reduction of the supernova SN 1987A neutrino burst duration. We
review and update these arguments and summarize the resulting axion
constraints.Comment: Contribution to Axion volume of Lecture Notes in Physics, 20 pages, 3
figure
Electron-, Mu-, and Tau-Number Conservation in a Supernova Core
We study if the neutrino mixing parameters suggested by the atmospheric
neutrino anomaly imply chemical equilibrium between mu- and tau-flavored
leptons in a supernova (SN) core. The initial flavor-conversion rate would
indeed be fast if the nu_mu-nu_tau-mixing angle were not suppressed by
second-order refractive effects. The neutrino diffusion coefficients are
different for nu_mu, anti-nu_mu, nu_tau and anti-nu_tau so that neutrino
transport will create a net mu and tau lepton number density. This will
typically lead to a situation where the usual first-order refractive effects
dominate, further suppressing the rate of flavor conversion. Altogether,
neutrino refraction has the nontrivial consequence of guaranteeing the separate
conservation of e, mu, and tau lepton number in a SN core on the infall and
cooling time scales, even when neutrino mixing angles are large.Comment: Slightly expanded version with improved presentation, no changes of
substanc
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