Role of dense matter in collective supernova neutrino transformations

For neutrinos streaming from a supernova (SN) core, dense matter suppresses self-induced flavor transformations if the electron density n_e significantly exceeds the neutrino density n_nu in the conversion region. If n_e is comparable to n_nu one finds multi-angle decoherence, whereas the standard self-induced transformation behavior requires that in the transformation region n_nu is safely above n_e. This condition need not be satisfied in the early phase after supernova core bounce. Our new multi-angle effect is a subtle consequence of neutrinos traveling on different trajectories when streaming from a source that is not point-like.

Revisiting cosmological bounds on radiative neutrino lifetime

Neutrino oscillation experiments and direct bounds on absolute masses constrain neutrino mass differences to fall into the microwave energy range, for most of the allowed parameter space. As a consequence of these recent phenomenological advances, older constraints on radiative neutrino decays based on diffuse background radiations and assuming strongly hierarchical masses in the eV range are now outdated. We thus derive new bounds on the radiative neutrino lifetime using the high precision cosmic microwave background spectral data collected by the Far Infrared Absolute Spectrophotometer instrument on board of Cosmic Background Explorer. The lower bound on the lifetime is between a few x 10^19 s and 5 x 10^20 s, depending on the neutrino mass ordering and on the absolute mass scale. However, due to phase space limitations, the upper bound in terms of the effective magnetic moment mediating the decay is not better than ~ 10^-8 Bohr magnetons. We also comment about possible improvements of these limits, by means of recent diffuse infrared photon background data. We compare these bounds with pre-existing limits coming from laboratory or astrophysical arguments. We emphasize the complementarity of our results with others available in the literature.