Fast flavor oscillations of astrophysical neutrinos with 1, 2, extellipsis{}, ensuremath{infty} crossings

Soumya Bhattacharyya and Basudeb Dasgupta 2021 Journal of Cosmology and Astroparticle Physics 2021 023

In the early Universe, as well as in supernovae and merging neutron stars, neu-
trinos have such high densities that they affect each other and exhibit collective flavor oscil-
lations. A crucial ingredient for fast collective flavor oscillations is that the electron lepton
number (ELN) distribution changes its sign as a function of direction, i.e., has a zero cross-
ing. We present a study in two spatial dimensions and time, focussing on the fast and linear
regime, and show how fast flavor oscillations depend on the ELN and its crossings. We
show that a large number of crossings can inhibit flavor oscillations. This may be a natural
self-limiting mechanism that stabilizes the flavor content of the dense neutrino gas in a vast
majority of scenarios, especially the early Universe, where the angular distributions for all
flavors are very similar and crossings occur mainly due to fluctuations.