Flavor Conversion Research Articles

Solar Neutrinos and Lepton Mixing

With latest experimental data the solar neutrino problem enters new phase when crucial aspects of the problem can be formulated in an essentially (solar) model independent way. Original neutrino fluxes can be considered as free parameters to be found from the solar neutrino experiments. Resonance flavour conversion gives the best fit of all experimental results. Already existing data allow one to constraint both the neutrino parameters and the original neutrino fluxes. The reconciliation of the solution of the solar neutrino problem with other neutrino mass hints (atmospheric neutrino problem, hot dark matter etc.) may require the existence of new very light singlet fermion. Supersymmetry can provide a framework within which the desired properties of such a light fermion follow naturally. The existence of the fermion can be related to axion physics, mechanism of $\mu$-term generation etc..

Neutrino mixing constraints and supernova nucleosynthesis.

We reexamine the constraints on mixing between electron and muon or [tau] neutrinos from shock-reheating and [ital r]-process nucleosynthesis in supernovas. To this end neutrino flavor evolution is described by nonlinear equations following from a quantum kinetic approach. This takes into account neutrino forward scattering off the neutrino background itself. In contrast with other claims in the literature it is shown that a sound self-consistent analytical approximation can in the first place only be performed in the adiabatic limit where phase terms are suppressed. In the cosmologically interesting mass range below about 25 eV the resulting mixing parameter bounds are between 1 and 2 orders of magnitude less restrictive than limits neglecting neutrino contributions. Extensions to the nonadiabatic regime derived in the literature usually neglect coherence effects from phases. To check their importance numerical simulations of the evolution equations were performed in this regime. They indicate that analytical approximations for the flavor conversion efficiencies can indeed be extended by neglecting phase terms. This allows more stringent bounds similar to the ones derived in earlier work. These bounds depend to some extent on the adopted supernova model and tend to be somewhat less restrictive in the mixing angle but simultaneously extendmore » to smaller mixing masses compared to limits neglecting the neutrino-induced potential.« less

Part IV. Neutrino astrophysics

We discuss how the neutrino material-heating rate and electron fraction in the region above the neutrino-sphere in a nascent type II supernova become very sensitive to neutrino flavor mixings. Matter-enhancement effects would ensure that a v μ or v τ (hereafter, v τ) neutrino with a vacuum mass of roughly 10 to 100 eV would have a mass-level-crossing with a light v e in this region. Coincidentaly, this is the neutrino mass range which is sometimes conjectured to provide the closure density for the universe. Since the supernova ν τ neutrinos have considerably higher average energies than do the ν e, transformations between these species at the level crossings result in more energetic electron neutrinos. This would imply an enhanced ν e capture rate which, in turn, would result in a higher heating rate and would tend to increase the electron fraction, reducing the neutron excess. For vacuum mixing angle between ν τ and ν e of θ>10 −4 the increase in early heating results in roughly a factor of two increase in supernova explosion energy, which may help solve the explosion-mechanism problem. However, late neutrino flavor conversion will drive the ejected material proton-rich and a promising site for r-process nucleosynthesis may be lost unless it is also true that θ < 10 −3.

Neutrino flavor conversion in a supernova core

If ν μ or ν τ mix with ν e, neutrino oscillations and collisions in a supernova (SN) core allow these flavors effectively to participate in β equilibrium and thus to obtain a large chemical potential. If a sterile species mixes with ν e, these effects lead to an anomalous loss of energy and lepton number. We study flavor conversion in a SN core on the basis of a new kinetic equation which rigorously includes neutrino interference and degeneracy effects. Our discussion serves as an example and illustration of the properties of this “non-Abelian Boltzmann equation”.

Resonant enhancement of flavor-changing neutrino interactions

The resonant amplification of neutrino oscillations in the presence of flavor-changing neutrino interactions with matter is analyzed. It is shown that a significant {mu}-flavor conversion can take place even in the absence of neutrino mixing in vacuum. To account for the solar neutrino deficit, the strength of the new interactions should be {approximately} 10{sup {minus}2}G{sub F} and the resulting neutrino suppression and spectrum is similar to that in the ordinary MSW effect. I discuss some extensions of the standard model where these interactions can be present, taking into account the experimental constraints that arise mainly from the induced leptonic rare decays.

Direct tests for solar-neutrino mass, mixing, and Majorana magnetic moment.

The present data on solar neutrinos may suggest matter effects resulting in (i) flavor conversion (Mikheyev-Smirnov-Wolfenstein effect) due to a nonzero neutrino mass and/or mixing and (ii) spin-flavor conversion due to a Majorana magnetic moment. Both possibilities can be explicitly tested in upcoming experiments (i) via the effect of Earth matter on {sup 7}Be neutrinos and (ii) by the detection of {sup 8}B {ital antineutrinos} during solar activity.

E6 leptoquarks and the solar-neutrino problem.

The possibility that non-conventional neutrino oscillations take place in the superstring inspired E sub 6 models is considered. In this context, the influence of leptoquark mediated interactions of the neutrinos with nucleons in the resonant flavor conversion is discussed. It is shown that this effect can be significant for v sub e - v sub tau oscillations if these neutrinos have masses required in the ordinary Mikheyev-Smirnov-Wolfenstein (MSW) effect, and may lead to a solution of the solar neutrino problem even in the absence of vacuum mixings. On the other hand, this model cannot lead to a resonant behavior in the sun if the neutrinos are massless.