Abstract
Neutrinos from a supernova (SN) might undergo fast flavor conversions near the collapsed stellar core. We perform a detailed study of this intriguing possibility, analyzing time-dependent state-of-the-art 3D SN models of 9 and 20 Msun. Both models were computed with multi-D three-flavor neutrino transport based on a two-moment solver, and both exhibit the presence of the lepton-number emission self-sustained asymmetry (LESA). The transport solution does not provide the angular distributions of the neutrino fluxes, which are crucial to track the fast flavor instability. To overcome this limitation, we use a recently proposed approach based on the angular moments of the energy-integrated electron lepton-number distribution. With this method we find the possibility of fast neutrino flavor instability at radii <~20 km, which is well interior to the neutrinosphere. Our results confirm recent observations in a 2D SN model and in 2D/3D models with fixed matter background, which were computed with Boltzmann neutrino transport. However, the flavor unstable locations are not isolated points as discussed previously, but thin skins surrounding volumes where electron antineutrinos are more abundant than electron neutrinos. These volumes grow with time and appear first in the convective layer of the proto-neutron star (PNS), where a decreasing electron fraction (Ye) and high temperatures favor the occurrence of regions with negative neutrino chemical potential. Since Ye remains higher in the LESA dipole direction, where convective lepton-number transport out from the nonconvective PNS core slows down the deleptonization, flavor unstable conditions become more widespread in the opposite hemisphere. This interesting phenomenon deserves further investigation, since its impact on SN modeling and possible consequences for SN dynamics and neutrino observations are presently unclear. (abridged)
Highlights
The deepest supernova (SN) regions provide a unique laboratory to probe neutrino flavor conversions in a nonlinear regime, where the neutrino evolution is determined mainly by their mutual interactions
We use a recently proposed approach based on the angular moments of the energy-integrated electron lepton-number distribution up to second order, i.e., angle-energy integrals of the difference between νe and νe phase-space distributions multiplied by corresponding powers of the unit vector of the neutrino velocity. With this method we find the possibility of fast neutrino flavor instability at radii smaller than ∼20 km, which is well interior to the neutrinosphere where neutrinos are still in the diffusive and near-equilibrium regime
We performed a detailed investigation of 3D state-of-theart SN models for the presence of fast neutrino flavor instability as well as to study the favorable conditions
Summary
The deepest supernova (SN) regions provide a unique laboratory to probe neutrino flavor conversions in a nonlinear regime, where the neutrino evolution is determined mainly by their mutual interactions. Just as Delfan Azari et al [35], Abbar et al [36] diagnosed ELN crossings in deep regions inside the PNS only in a small number of isolated points at the analyzed postbounce moments They correlated their occurrence with locations where the chemical potential of electron neutrinos nearly vanishes and pointed out that the electron fraction Ye is relatively low there and the temperature is close to maximal values. Some of us have recently proposed an alternative method to diagnose the possibility of fast instabilities in the absence of detailed knowledge of the ELN distributions [47] This recipe is based on identifying a specific Fourier mode of the flavor instability field called the “zero mode,” which has an calculable growth rate depending only on the angular moments of the ELN up to second order.
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