The Fast Flavor Instability as a Catalyst for Neutrino Spin Oscillations in Neutron Star Mergers

Recent studies have shown that in anisotropic environments neutrinos can present coherent helicity oscillations which depend directly on the absolute mass scale and Majorana phases. These oscillations are usually too transient to produce important effects in high-energy astrophysical phenomena. However, we demonstrate that a fast flavor instability can substantially increase the characteristic energy of spin oscillations for neutrinos travelling in specific directions, implying a fast flavor instability could 'catalyze' helicity transformation and make it measurable. We find several locations in a neutron star merger simulation where the conditions for a fast flavor instability and for resonant spin oscillations are met simultaneously. We run a particle-in-cell simulation of the fast flavor instability using data from these locations and show the energy of spin oscillations along the direction of maximal neutrino flux is raised by 4 or more orders of magnitude over the course of the instability. We directly solve the Schrodinger Equation using the spin-flavor Hamiltonian from before and after the Instability (setting it to be constant) to qualitatively observe the change in spin oscillations caused by a fast flavor instability. We find that the oscillations are still too transient for all but the lowest energy neutrinos to experience significant spin-flip effects. We conclude nonlinear resonance is likely necessary for spin flip to have an important influence on supernova or merger dynamics, though our proposed mechanism could work in conjuction with other effects to sufficiently raise the energy of spin oscillations.