For many years, it was believed that flavor oscillations of supernova neutrinos are described by the MSW effect, just like in the Sun. In recent years, however, it has been realized that in a supernova a novel collective mechanism is also in play. The resulting collective oscillations occur much closer to the PNS than the MSW resonances. In fact, it is valid to ask whether the oscillations exist close to, or even inside, the neutrinosphere. The latter possibility would require a significant revision of the basic neutrino transport paradigm.
Duan & Friedland have recently investigated collective oscillations during the late (cooling) stage of the explosion. They have shown that the frequently used "single-angle" treatment of the flavor evolution indeed predicts that oscillations start right at the neutrinosphere in this regime. Thus, the standard lore of the field would suggest that the basic picture of neutrino transport is in jeopardy. Upon further investigation, however, the authors uncovered a novel effect that suppresses oscillations close to the neutrinosphere. In the full "multiangle" calculation, the supernova transport framework is saved.
The paper has recently been accepted to Phys. Rev. Letters. In addition to the numerical results from the supercomputer simulations, the paper also explains the physics of the "multiangle" suppression.
The attached figures from the papers show the onset of the oscillations in single-angle and multi-angle calculations, as well as the highly nontrivial pattern of oscillations as a function of the neutrino energy and angle of emission.
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