On the Occurrence of Crossings between the Angular Distributions of Electron Neutrinos and Antineutrinos in the Supernova Core

Abstract

Neutrino fast pairwise conversions have been postulated to occur in the dense core of a core-collapse supernova (SN), possibly having dramatic consequences on the SN mechanism and the observable neutrino signal. One crucial condition favoring pairwise conversions is the presence of crossings between the electron neutrino and antineutrino angular distributions (i.e., electron neutrino lepton number crossings, ELN crossings). A stationary and spherically symmetric SN toy-model is constructed to reproduce the development of the neutrino angular distributions in the dense SN core in the absence of perturbations induced by hydrodynamical instabilities. By iteratively solving the neutrino Boltzmann equations including the collisional term, our model predicts that ELN crossings can develop only in the proximity of the decoupling region and for a sharp radial evolution of the baryon density, when the electron neutrino and antineutrino number densities are comparable. Such conditions are likely to occur only in the late SN stages. Interestingly, flavor instabilities induced by spatial or temporal perturbations are unlikely to generate ELN crossings dynamically within our simplified setup.

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

“…This can seriously hinder the occurrence of ELN crossings in the SN environment. In particular, no ELN crossings within the shock region have been found in 1D SN models so far [44,45] except in the pre-shock SN region [46]. However, the situation can be different in multi-D SN models where the neutrino distributions can be spatially asymmetric due to multi-D hydrodynamics.…”

“…Nevertheless, one-dimensional SN simulations have not shown such ELN crossings during the early stages of CCSNe within the shock region [44,45] (note, however, that they might still exist in the pre-shock SN region [46]). Although the angular distributions ofν e 's are normally more peaked than that of ν e 's in the forward direction, the large difference between the number densities of ν e andν e hinders the occurrence of ELN crossings.…”

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

“…In this context, a peculiar type of self-induced flavor conversions, called "fast" instabilities [4,8,10,[15][16][17][18][19][20][21][22][23][24][25][26][27][28], is expected to lead to flavor conversions developing on very short distances, r O(1) m. Fast flavor conversions have been associated with "crossings" in the electron lepton number (ELN) angular distribution, i.e., with a change of sign in the difference between ν e andν e number densities as a function of emission angle [16,18,24]. Conditions for crossings in the ELN were expected to be possible in the neutrino decoupling region in SN cores, where the different flavors have significantly different angular distributions.…”

“…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. the ratio of the number densities of both neutrino types, is close to unity (see also [26]). Consequently and naturally in the equilibrium diffusion regime, the chemical potential of ν e nearly vanishes in these regions.…”

Tau lepton asymmetry by sterile neutrino emission—moving beyond one-zone supernova models

“…Most importantly, current hydrodynamical SN simulations are run under the assumption that neutrino flavor conversions do not play any major role in the explosion mechanism [7][8][9]. Recent findings shed doubt on this ansatz, and an agreement remains to be found [10][11][12][13][14][15][16][17][18].…”