Collisional Flavor Instability in Neutrino-Dense Anisotropic Environments*

Neutrinos are produced copiously in extreme astrophysical environments like those of core-collapse supernovae (CCSNe) and binary neutron star (NS) mergers. It is hardly surprising then, that they also impact the evolution of such processes/objects. While vacuum oscillations may not be sufficient for significant flavor transformation, neutrino-neutrino refraction can substantially change this picture. Because such environments are so neutrino-dense, neutrinos can start interacting with themselves via. the neutral current interaction. This coherent forward scattering of neutrinos enhances flavor conversion significantly. Such neutrino oscillations are called collective oscillations because the neutrino-neutrino refraction Hamiltonian couples neutrinos of different energy and momenta. This makes it an interesting but difficult problem to solve.

The picture gets even more interesting when neutrino “collisions” are included. It has been observed (numerically) that the processes of neutrino emission and absorption on nucleons can aid collective oscillations. Small asymmetries in neutrino emission and absorption rates can cause a flavor instability, thereby leading to significant flavor conversion. Previously, this theory of collisional flavor instability has been applied to simulated NS merger data while assuming homogeneity of space, and an isotropic neutrino momentum distribution. Our work seeks to generalize this to include an anisotropic momentum distribution for the neutrinos.

Collisional instabilities can, in principle, develop anywhere in an accretion disk provided the region contains enough neutrinos, protons, and neutrons. This makes them different from the so-called fast instabilities restricted to develop primarily in the decoupling region. A larger proportion of the heavier muon and tau flavor neutrinos may cause faster cooling of the accretion disk. Neutrino flavor transformation can also have implications for heavy-element nucleosynthesis in such environments. Thus, it is essential to model neutrino flavor transformation as realistically as possible and, while the task is computationally difficult, incorporate all such collective neutrino oscillation effects in CCSNe and NS merger simulations.