Neutrino flavor transformation in the lepton-asymmetric universe

Abstract

We investigate neutrino flavor transformation in the early universe in the presence of a lepton asymmetry, focusing on a two-flavor system with 1 - 3 mixing parameters. We identify five distinct regimes that emerge in an approximate treatment neglecting collisions as the initial lepton asymmetry at high temperature is varied from values comparable to current constraints on the lepton number down to values at which the neutrino-neutrino forward-scattering potential is negligible. The characteristic phenomena occurring in these regimes are (1) large synchronized oscillations, (2) minimal flavor transformation, (3) asymmetric (neutrino- or antineutrino-only) MSW, (4) partial MSW, and (5) symmetric MSW. We examine our numerical results in the framework of adiabaticity, and we illustrate how they are modified by collisional damping. Finally, we point out the existence of matter-neutrino resonances in the early universe and show that they suffer from non-adiabaticity.

18 pages, 15 figures

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• Resurrection of large lepton number asymmetries from neutrino flavor oscillations
• Primordial neutrino asymmetry evolution with full mean-field effects and collisions
• Lepton asymmetry, neutrino spectral distortions, and big bang nucleosynthesis
• Constraints on primordial lepton asymmetries with full neutrino transport
• Self-consistency in models of neutrino scattering and fast flavor conversion
• Effect of collisions on neutrino flavor inhomogeneity in a dense neutrino gas
• Prospects for Neutrino Spin Coherence in Supernovae
• Geometric phases in neutrino oscillations with nonlinear refraction
• The Universe at the MeV era: neutrino evolution and cosmological observables