Flavor solitons in dense neutrino gases

Understand

We consider a dense neutrino gas in the "fast-flavor limit" (vanishing neutrino masses).

• For the first time, we identify exact solutions of the nonlinear wave equation in the form of solitons.
• They can propagate with both sub- or superluminal speed, the latter not violating causality.
• The soliton with infinite speed is a homogeneous solution and coincides with the usual fast-flavor pendulum except that it swings only once instead of being periodic.