Fast neutrino flavor conversion modes in multidimensional core-collapse supernova models: The role of the asymmetric neutrino distributions

Abstract

A dense neutrino gas, such as the one anticipated in the supernova environment, can experience fast neutrino flavor conversions on scales much shorter than those expected in vacuum probably provided that the angular distributions of νe andνe cross each other. We perform a detailed investigation of the neutrino angular distributions obtained by solving the Boltzmann equations for fixed matter profiles of some representative snapshots during the post-bounce phase of core-collapse supernovae in multidimensional calculations of an 11.2M and a 27M progenitor models. Although the 11.2M model features νe −νe angular crossings and the associated fast modes at different time snapshots, the 27M model does not show any crossings within the decoupling region. We show that this can be understood by studying the multipole components of the neutrino distributions. In fact, there is a higher chance for the occurrence of νe −νe angular crossings for the zones where the multipole components of the neutrino distributions are strong enough. We also show that there can exist more than one crossings between the angular distributions of νe andνe. In addition, apart from the crossings within the neutrino decoupling region, there is a class of νe −νe angular crossings which appears very deep inside the proto-neutron star.

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

“…Our findings are reminiscent of the individual points of ELN crossings that were identified deep inside the PNS by [35] in the 2D simulation of this 11.2 M model and by [36] in the 3D simulations of the same 11.2 M progenitor and of a 27 M model. In the following section we will argue that the regions of fast-flavor instability form thin 2D layers in 3D space rather than isolated points, and the identification of individual points in our analysis is an artifact connected with the discretization of the physical variables in the numerical treatment.…”

“…[31] as well as Refs. [36,50] have reported such locations in 2D and 3D simulations of an 11.2 M star. In this context, however, it is important to keep in mind that our moment-based criterion employs differences of angle and energy integrals of the neutrino distribution functions.…”

“…This conclusion does probably also apply to the results of the previous investigations in Refs. [35,36].…”

“…The authors of [35] speculated that the appearance of light nuclei (among them α particles as the dominant species) is causal for the development of such instability conditions. Just as Delfan Azari et al [35], also Abbar et al [36] diagnosed ELN crossings in deep regions inside the PNS only in a small number of isolated points at the analysed post-bounce moments. They also correlated their occurrence with locations where the chemical potential of electron neutrinos nearly vanishes and pointed out that the electron fraction Y e is relatively low there and the temperature is close to maximal values.…”

“…Recently, two publications, based again on the SN models considered in [31,32], reported positive detections of locations of ELN crossings deep inside the protoneutron star (PNS) when investigating the selfconsistent 2D core-collapse simulation of an 11.2 M star computed with Boltzmann neutrino transport [35] and 3D Boltzmann neutrino-transport results for fixed matter background at some instants during the post-bounce evolution of 11.2 M and 27 M progenitors [36]. Because the diffusive conditions for neutrinos in the deep PNS interior imply that the angular distributions of both ν e and ν e are nearly isotropic, ELN crossings were found only in regions where the "asymmetry parameter" Γ = nν e /n νe , i.e.…”