15 research outputs found
Flavor stability analysis of supernova neutrino fluxes compared with simulations
We apply a linearized stability analysis to simplified models of
accretion-phase neutrino fluxes streaming from a supernova. We compare the
results with recent numerical studies and find excellent agreement. This
provides confidence that a linearized stability analysis can be further applied
to more realistic models.Comment: Contribution to the Proceedings of HANSE 2011 workshop, 4 pages, 2
figure
Suppression of Self-Induced Flavor Conversion in the Supernova Accretion Phase
Self-induced flavor conversions of supernova (SN) neutrinos can strongly
modify the flavor dependent fluxes. We perform a linearized flavor stability
analysis with accretion-phase matter profiles of a 15 M_sun spherically
symmetric model and corresponding neutrino fluxes. We use realistic energy and
angle distributions, the latter deviating strongly from quasi-isotropic
emission, thus accounting for both multi-angle and multi-energy effects. For
our matter and neutrino density profile we always find stable conditions:
flavor conversions are limited to the usual MSW effect. In this case one may
distinguish the neutrino mass hierarchy in a SN neutrino signal if the mixing
angle theta_13 is as large as suggested by recent experiments.Comment: 4 pages, 5 figures; minor edits, matches the version published in PR
Supernova neutrino halo and the suppression of self-induced flavor conversion
Neutrinos streaming from a supernova (SN) core occasionally scatter in the
envelope, producing a small "neutrino halo" with a much broader angle
distribution than the primary flux originating directly from the core. Cherry
et al. (2012) have recently pointed out that, during the accretion phase, the
halo actually dominates neutrino-neutrino refraction at distances exceeding
some 100 km. However, the multiangle matter effect (which increases if the
angle distribution is broader) still appears to suppress self-induced flavor
conversion during the accretion phase.Comment: related to our previous PRL 108 (2012) 061101 [arXiv:1109.3601]; v2
with appendix on analytic treatment of halo, matches the published versio
Constraining the cosmic radiation density due to lepton number with Big Bang Nucleosynthesis
The cosmic energy density in the form of radiation before and during Big Bang
Nucleosynthesis (BBN) is typically parameterized in terms of the effective
number of neutrinos N_eff. This quantity, in case of no extra degrees of
freedom, depends upon the chemical potential and the temperature characterizing
the three active neutrino distributions, as well as by their possible
non-thermal features. In the present analysis we determine the upper bounds
that BBN places on N_eff from primordial neutrino--antineutrino asymmetries,
with a careful treatment of the dynamics of neutrino oscillations. We consider
quite a wide range for the total lepton number in the neutrino sector, eta_nu=
eta_{nu_e}+eta_{nu_mu}+eta_{nu_tau} and the initial electron neutrino asymmetry
eta_{nu_e}^in, solving the corresponding kinetic equations which rule the
dynamics of neutrino (antineutrino) distributions in phase space due to
collisions, pair processes and flavor oscillations. New bounds on both the
total lepton number in the neutrino sector and the nu_e -bar{nu}_e asymmetry at
the onset of BBN are obtained fully exploiting the time evolution of neutrino
distributions, as well as the most recent determinations of primordial 2H/H
density ratio and 4He mass fraction. Note that taking the baryon fraction as
measured by WMAP, the 2H/H abundance plays a relevant role in constraining the
allowed regions in the eta_nu -eta_{nu_e}^in plane. These bounds fix the
maximum contribution of neutrinos with primordial asymmetries to N_eff as a
function of the mixing parameter theta_13, and point out the upper bound N_eff
< 3.4. Comparing these results with the forthcoming measurement of N_eff by the
Planck satellite will likely provide insight on the nature of the radiation
content of the universe.Comment: 17 pages, 9 figures, version to be published in JCA
Updated BBN bounds on the cosmological lepton asymmetry for non-zero theta13
We discuss the bounds on the cosmological lepton number from Big Bang
Nucleosynthesis (BBN), in light of recent evidences for a large value of the
neutrino mixing angle theta13. The largest asymmetries for electron and muon or
tau neutrinos compatible with 4He and 2H primordial yields are computed versus
the neutrino mass hierarchy and mixing angles. The flavour oscillation dynamics
is traced till the beginning of BBN and neutrino distributions after decoupling
are numerically computed. The latter contains in general, non thermal
distortion due to the onset of flavour oscillations driven by solar squared
mass difference in the temperature range where neutrino scatterings become
inefficient to enforce thermodynamical equilibrium. Depending on the value of
theta13, this translates into a larger value for the effective number of
neutrinos, N_eff. Upper bounds on this parameter are discussed for both
neutrino mass hierarchies. Values for N_eff which are large enough to be
detectable by the Planck experiment are found only for the (presently
disfavoured) range sin^2(theta13)<0.01.Comment: 16 pages, 5 figure
Neutrino oscillations at high densities: cosmological and astrophysical aspects
This doctoral thesis treats the effects of neutrino oscillations in the early universe and supernovae. The main probe for the context of the early universe is big bang nucleosynthesis (BBN). I explain the general link between neutrinos and BBN with an emphasis on the degeneracy between neutrino asymmetry and extra degrees of freedom. I show how the degenerate effects of both oscillations and collisions lead to observables detectable in the near future that brake this degeneracy and draw bounds on neutrino asymmetry and their contribution to the energy density of the universe in the current epoch. Considering astrophysical aspects, I analyze the significance of a proper treatment of neutrino evolution to understand the neutrino signal from the next galactic supernova (SN). I show the type of effects collective neutrino conversions can produce and the numerical difficulties confronted. Finally, I show how that stability analysis can complement a numerical treatment and provide definitive answers in
some example cases