Flavor instabilities in the neutrino line model

A dense neutrino medium can experience collective flavor oscillations through nonlinear neutrino-neutrino refraction. To make this multi-dimensional flavor transport problem more tractable, all existing studies have assumed certain symmetries (e.g., the spatial homogeneity and directional isotropy in the early universe) to reduce the dimensionality of the problem. In this work we show that, if both the directional and spatial symmetries are not enforced in the neutrino line model, collective oscillations can develop in the physical regimes where the symmetry-preserving oscillation modes are stable. Our results suggest that collective neutrino oscillations in real astrophysical environments (such as core-collapse supernovae and black-hole accretion discs) can be qualitatively different from the predictions based on existing models in which spatial and directional symmetries are artificially imposed.Comment: 5 pages, 1 figur

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

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.Comment: 11 pages, 8 figures, Version accepted in APJ. Results unchange

Effect of collisions on neutrino flavor inhomogeneity in a dense neutrino gas

We investigate the stability, with respect to spatial inhomogeneity, of a two-dimensional dense neutrino gas. The system exhibits growth of seed inhomogeneity due to nonlinear coherent neutrino self-interactions. In the absence of incoherent collisional effects, we observe a dependence of this instability growth rate on the neutrino mass spectrum: the normal neutrino mass hierarchy exhibits spatial instability over a larger range of neutrino number density compared to that of the inverted case. We further consider the effect of elastic incoherent collisions of the neutrinos with a static background of heavy, nucleon-like scatterers. At small scales, the growth of flavor instability can be suppressed by collisions. At large length scales we find, perhaps surprisingly, that for inverted neutrino mass hierarchy incoherent collisions fail to suppress flavor instabilities, independent of the coupling strength.Comment: 10 pages, 6 figures Version accepted in PLB. Minor changes. Title change

Do Neutrinos Become Flavor Unstable Due to Collisions with Matter in the Supernova Decoupling Region?

In core-collapse supernovae, the neutrino density is so large that neutrino flavor instabilities, leading to flavor conversion, can be triggered by the forward scattering of neutrinos among each other, if a crossing between the angular distributions of electron neutrino and antineutrinos exists (fast instability) or in the presence of perturbations induced by the neutrino vacuum frequency (slow instability). Recently, it has been advanced the conjecture that neutrino collisions with the medium could be another mean to kickstart flavor change (collisional instability). We rely on a spherically symmetric core-collapse supernova model with mass $18.6\ M_\odot$, compute the neutrino angular distributions solving the kinetic equations and investigate the occurrence of flavor instabilities at different post-bounce times, ranging from the accretion phase to the early cooling phase. We find that fast and slow flavor instabilities largely dominate over the collisional ones in the decoupling region for all post-bounce times. While more work is needed to assess the relevance of collisional instabilities in neutrino-dense environments, our findings suggest that neutrino collisions with matter affect the flavor evolution in the decoupling region, but are not responsible for triggering flavor conversion.Comment: 17 pages, 9 figures, revtex4-1 two-column forma

The three flavor revolution in fast pairwise neutrino conversion

The modeling of fast flavor evolution of neutrinos in dense environments has been traditionally carried out by relying on a two flavor approximation for simplicity. In addition, vacuum mixing has been deemed negligible. For the first time, we highlight that the fast flavor evolution in three flavors is intrinsically different from the one obtained in the two flavor approximation. This is due to the exponential growth of flavor mixing in the $e$--$\mu$ and $e$--$\tau$ sectors generated by the vacuum term in the Hamiltonian. As a result, substantially larger flavor mixing is found in three flavors. Our findings highlight that the two flavor approximation is not justified for fast pairwise conversion, even if the angular distributions of non-electron type neutrinos are initially identical.Comment: 2 column revtex4-1, 9 pages, 7 figures. Version accepted in PR