The three flavor revolution in fast pairwise neutrino conversion

TL;DR

This paper demonstrates that modeling fast neutrino flavor conversion with three flavors reveals significantly different and larger flavor mixing effects than the traditional two-flavor approximation, especially due to vacuum terms.

Contribution

It shows for the first time that three-flavor modeling fundamentally alters the understanding of fast neutrino flavor evolution compared to two-flavor models.

Findings

Three-flavor effects lead to larger flavor mixing.

Vacuum terms cause exponential growth in $e$--$ au$ and $e$--$$ sectors.

Two-flavor approximation is insufficient for accurate modeling.

Abstract

The modeling of fast flavor evolution of neutrinos in dense environments has been traditionally carried out by relying on a two flavor approximation for simplicity. In addition, vacuum mixing has been deemed negligible. For the first time, we highlight that the fast flavor evolution in three flavors is intrinsically different from the one obtained in the two flavor approximation. This is due to the exponential growth of flavor mixing in the $e$--$\mu$ and $e$--$\tau$ sectors generated by the vacuum term in the Hamiltonian. As a result, substantially larger flavor mixing is found in three flavors. Our findings highlight that the two flavor approximation is not justified for fast pairwise conversion, even if the angular distributions of non-electron type neutrinos are initially identical.