Abstract
Neutrino–neutrino refraction dominates the flavor evolution in core-collapse supernovae, neutron star mergers, and the early Universe. Ordinary neutrino flavor conversions develop on timescales determined by the vacuum oscillation frequency. However, when the neutrino density is large enough, collective flavor conversions may arise because of pairwise neutrino scattering. Pairwise conversions are deemed fast because they are expected to occur on timescales that depend on the neutrino–neutrino interaction energy (i.e., on the neutrino number density) and are regulated by the angular distributions of electron neutrinos and antineutrinos. The enigmatic phenomenon of fast pairwise conversions has been overlooked for a long time. However, because of the fast conversion rate, pairwise conversions could occur in the proximity of the neutrino decoupling region with yet-to-be-understood implications for the hydrodynamics of astrophysical sources and the synthesis of the heavy elements. We review the physics of this fascinating phenomenon and its implications for neutrino-dense sources.
Highlights
Schematic representation of the neutrino flavor conversion regions in the envelope of a core-collapse supernova; indicative distances from the supernova core are displayed
Spatial region whereneutrinos decouple, as shown schematically in Figure 1, and they may lead to characteristic signatures in theneutrino energy distributions, such as the so-called spectral splits [16, 17]
Spectral splits were considered a characteristic imprint of neutrino–neutrino interactions
Summary
We introduce the equations of motion for neutrinos in the mean-field approximation, many-body descriptions can be adopted [28,29,30,31,32,33,34,35,36]. The onset of flavor conversions can be examined through a linear stability analysis applied to the linearized equations of motion. To grasp the development of fast pairwise conversions, we introduce the linear stability analysis technique, discuss the conditions under which flavor conversions develop, and classify the types of flavor instability
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