Fast neutrino flavor instability in the neutron-star convection layer of three-dimensional supernova models

Abstract

It has been speculated for a long time that neutrinos from a supernova (SN) may undergo fast flavor conversions near the collapsed stellar core. We perform a detailed study of this intriguing possibility, for the first time analyzing two time-dependent state-of-the-art three-dimensional (3D) SN models of 9 M and 20 M from recent papers of Glas et al. Both models were computed with multi-dimensional three-flavor neutrino transport based on a two-moment solver, and both exhibit the presence of the so-called lepton-number emission self-sustained asymmetry (LESA). The transport solution does not provide the angular distributions of the flavor-dependent 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 up to second order, i.e., angle-energy integrals of the difference between νe andνe phasespace 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. Our results confirm recent observations in a 2D (axisymmetric) SN model and in 2D and 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 whereνe are more abundant than νe. These volumes grow with time and appear first in the convective layer of the proto-neutron star (PNS), where a decreasing electron fraction and high temperatures favor the occurrence of regions with negative neutrino chemical potential. Since the electron fraction 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.

Late-time behavior of fast neutrino oscillations

“…Ray Sawyer pointed out that much more rapid "fast" flavor conversions can take place [44][45][46][47]. These have a frequency ∼μ and might have a much more drastic effect for neutrino physics as well as supernova astrophysics [69][70][71][72][73][74][75]. The criterion for fast conversions to occur appears to be related to that for slow conversions-i.e., the difference of neutrino and antineutrino flux distributions in the momentum space must have a zero crossing [20], though a more detailed understanding still remains wanting.…”

On the Time Distribution of Supernova Antineutrino Flux

“…To take the argument further, suppose that P ee = |U 2 e1 | ∼ 0.68 [82], as obtained in the case of the normal mass spectrum for antineutrinos [62], neglecting neutrino selfinteractions [59][60][61][62][63][64]66,67]. In this case, if the Φ 0 ν x flux is absent, we have a 30% decrease in the flux reaching the Earth; if it is similar, however, the effect is minor or negligible.…”

“…In addition, an intense debate about neutrino flavor transformation in the supernova has sparked off particle physics [59-61], and not yet come to a complete conclusion-for some extensive reviews of the state of the art, see, e.g., [62][63][64][65][66][67]. One of the main points of discussion is focused on the so-called self-induced neutrino collective oscillations, a phenomenon of a non-linear nature, due to the fact that neutrinos propagate in a medium containing and composed of neutrinos.…”

Accounting for the Heisenberg and Pauli principles in the kinetic approach to neutrino oscillations

“…references [50][51][52][53] and references therein. The recent works [54][55][56][57][58][59][60][61][62][63][64][65][66][67][68][69] focus primarily on the so-called fast neutrino flavor conversions put forward in references [70,71]. As has been argued in reference [72], the fast flavor conversions can be triggered by the collision term, absent in equation (4.9).…”