Triggering collective oscillations by three-flavor effects

Collective flavor transformations in supernovae, caused by neutrino-neutrino interactions, are essentially a two-flavor phenomenon driven by the atmospheric mass difference and the small mixing angle theta(13). In the two-flavor approximation, the initial evolution depends logarithmically on theta(13) and the system remains trapped in an unstable fixed point for theta(13) = 0. However, any effect breaking exact nu(mu) - nu(tau) equivalence triggers the conversion. Such three-flavor perturbations include radiative corrections to weak interactions, small differences between the nu(mu) and nu(tau) fluxes, or nonstandard interactions. Therefore, extremely small values of theta(13) are in practice equivalent, the fate of the system depending only on the neutrino spectra and their mass ordering.