New Developments in Flavor Evolution of a Dense Neutrino Gas

TL;DR

This paper reviews the phenomenon of fast pairwise neutrino flavor conversion in dense neutrino gases, emphasizing its potential impact on astrophysical processes like supernovae and neutron-star mergers.

Contribution

It provides a comprehensive review of the physics behind fast pairwise neutrino flavor conversion and discusses its implications for astrophysical environments.

Findings

Fast pairwise conversion can occur near neutrino decoupling regions.

It may significantly influence supernova dynamics and element synthesis.

The phenomenon has been historically overlooked but is now recognized as important.

Abstract

Neutrino-neutrino refraction dominates the flavor evolution in core-collapse supernovae, neutron-star mergers, and the early universe. Ordinary neutrino flavor conversion develops on timescales determined by the vacuum oscillation frequency. However, when the neutrino density is large enough, collective flavor conversion may arise because of pairwise neutrino scattering. Pairwise conversion is deemed to be fast as it is expected to occur on timescales that depend on the neutrino-neutrino interaction energy (i.e., on the neutrino number density) and is regulated by the angular distributions of electron neutrinos and antineutrinos. The enigmatic phenomenon of fast pairwise conversion has been overlooked for a long time. However, because of the fast conversion rate, pairwise conversion may possibly occur in the proximity of the neutrino decoupling region with yet to be understood implications for the hydrodynamics of astrophysical sources and the synthesis of the heavy elements. We review the physics of this fascinating phenomenon and its implications for neutrino-dense sources.