Cosmological bounds on neutrino degeneracy improved by flavor oscillations

Abstract

We study three-flavor neutrino oscillations in the early universe in the presence of neutrino chemical potentials. We take into account all effects from the background medium, i.e. collisional damping, the refractive effects from charged leptons, and in particular neutrino self-interactions that synchronize the neutrino oscillations. We find that effective flavor equilibrium between all active neutrino species is established well before the big-bang nucleosynthesis (BBN) epoch if the neutrino oscillation parameters are in the range indicated by the atmospheric neutrino data and by the large mixing angle (LMA) MSW solution of the solar neutrino problem. For the other solutions of the solar neutrino problem, partial flavor equilibrium may be achieved if the angle θ 13 is close to the experimental limit tan 2 θ 13 < ∼ 0.065. In the LMA case, the BBN limit on the ν e degeneracy parameter, |ξ ν | < ∼ 0.07, now applies to all flavors. Therefore, a putative extra cosmic radiation contribution from degenerate neutrinos is limited to such low values that it is neither observable in the large-scale structure of the universe nor in the anisotropies of the cosmic microwave background radiation. Existing limits and possible future measurements, for example in KATRIN, of the absolute neutrino mass scale will provide unambiguous information on the cosmic neutrino mass density, essentially free of the uncertainty of the neutrino chemical potentials.

Bulk QCD thermodynamics and sterile neutrino dark matter

“…Because of the near bi-maximal mixing paradigm emerging for the neutrino mass matrix, more stringent constraints can be placed on the neutrino lepton number since asymmetries in the more poorly constrained mu and tau flavors can be converted to an asymmetry in the more stringently constrained electron neutrinos through synchronized flavor transformation [17]. However, even the strongest constraint available, on the electron neutrino lepton number, allows |L e | 1 MeV < ∼ 0.1 at the epoch of nucleosynthesis.…”

Introduction to Cosmology

“…The efficiency of redistribution of initially large lepton asymmetry by the oscillations was calculated in ref. [29], for more related references and analytical calculations see review [12]. It has been shown that for weak mixing (LOW solution for the solar neutrino anomaly) the redistribution of leptonic numbers is insignificant, as is presented in fig.…”

“…3. As a result, the following quite restrictive bound on asymmetry of all neutrino flavours can be obtained [29]:…”

Mass limits for dark clusters of degenerate fermions

“…Cosmological constraints on the neutrino degeneracy parameter began to be analyzed at the end of the past century (see, for example: Lesgourgue & Pastor 1999;Pastor & Lesgourgue 2000;Lesgourgue & Liddle 2001;Orito et al 2002;Dolgov et al 2002b). A relic neutrino degeneracy leads to a different amount of neutrinos and antineutrinos.…”

“…In this work we will deal with fully degenerate fermionic dark matter. Semidegenerate massive neutrinos, with different degrees of degeneracy, have been extensively addressed; see, for example, Freese et al (1983); Kang & Steigman (1992); Lesgourgues & Pastor (1999); Lesgourgues & Liddle (2001); Dolgov et al (2002b);and Dolgov (2002a).…”