Flavor versus mass eigenstates in neutrino asymmetries: implications for cosmology

TL;DR

This paper clarifies that neutrino flavor and mass eigenstate asymmetries are distinct, affecting cosmological constraints from BBN and CMB, and highlights the importance of understanding their differences for accurate cosmological bounds.

Contribution

It demonstrates that flavor and mass eigenstate asymmetries are different and that cosmological bounds on them must be considered separately, impacting neutrino asymmetry constraints.

Findings

Flavor and mass eigenstate asymmetries are distinct.

Cosmological bounds on flavor asymmetries can be stronger.

Constraints depend on the asymmetry of the heaviest neutrino.

Abstract

We show that, if they exist, lepton number asymmetries ($L_\alpha$) of neutrino flavors should be distinguished from the ones ($L_i$) of mass eigenstates, since Big Bang Nucleosynthesis (BBN) bounds on the flavor eigenstates cannot be directly applied to the mass eigenstates. Similarly, Cosmic Microwave Background (CMB) constraints on mass eigenstates do not directly constrain flavor asymmetries. Due to the difference of mass and flavor eigenstates, the cosmological constraint on the asymmetries of neutrino flavors can be much stronger than conventional expectation, but not uniquely determined unless at least the asymmetry of the heaviest neutrino is well constrained. Cosmological constraint on $L_i$ for a specific case is presented as an illustration.