Collective Flavor Conversions Are Interactions of Neutrinos with Quantized Flavor Waves

Collective oscillations in dense neutrino gases (flavor waves) are notable for their instabilities that cause fast flavor conversion. We develop a quantum theory of interacting neutrinos and flavor wave quanta, which are analogous to plasmons but also carry flavor. The emission or absorption of such flavor plasmons ψ, or "flavomons," changes the neutrino flavor. When an angular crossing occurs, the process ν_{μ}→ν_{e}+ψ is more rapid than its inverse along the direction of the crossing, triggering stimulated ψ emission and fast instability. Calculating the rate via Feynman diagrams matches the fast instability growth rate. Our novel ν and ψ kinetic equations, corresponding to quasilinear theory, describe instability evolution without resolving the small scales of the flavomon wavelength, potentially overcoming the main challenge of fast flavor evolution.