Fast flavor conversions at the edge of instability in a two-beam model

Understand

A dense neutrino gas exhibiting angular crossings in the electron lepton number is unstable and develops fast flavor conversions.

• Instead of assuming an unstable configuration from the onset, we imagine that the system is externally driven toward instability.
• We use the simplest model of two neutrino beams initially of different flavor that either suddenly appear or one or both slowly build up.
• Flavor conversions commence well before the putative unstable state is fully attained, and the final outcome depends on how the system is driven.