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.Comment: 18 pages, 15 figure

Entanglement and collective flavor oscillations in a dense neutrino gas

“…where P (S) is the inverse of the function in Eq. (16). We observe that a substantial number of ∆P z (ω p ) values cluster around this line, particularly for N = 8.…”

“…Finally we note that, for a two-neutrino system, the results obtained for P 2 must be independent of our choice of particle to take the partial trace over; P shares a one-to-one relationship with entanglement entropy S, as per Eq. (16), which for a bipartite pure-state system does not depend on which of the two partitions is being traced over. However, this invariance is not shared by P z , which will differ between the two neutrinos' subsystems.…”

“…Of particular interest, however, is the magnitude P , since it is directly related to the entanglement entropy, as described previously with Eq. (16). First, using Eq.…”

MeV-scale reheating temperature and thermalization of oscillating neutrinos by radiative and hadronic decays of massive particles

“…That is the reason that a large expansion rate in the BBN epoch leads to a large deuterium abundance and vice versa. 9 We recall the reader that Tγ determines when light element abundances are to be created since the reaction process p + n → D + γ responsible for the deuterium production is the first step of BBN, and its backward reaction on 1 − f νe . In the low-reheating-temperature Universe, the neutrino abundance of each flavor is smaller than the case of the standard big-bang cosmology.…”

Improved constraints on lepton asymmetry from the cosmic microwave background

“…It was however shown in [10,23,24] that flavour asymmetries are likely to equilibrate before the onset of BBN due to neutrino oscillations, such that finally ξ e ≈ ξ µ ≈ ξ τ . Note that this is not generically true, but depends on the neutrino mixing angles as well as on the initial values of the neutrino chemical potentials [25,26]. Particularly, the relation between η l (1) and ∆N eff (2) does not necessarily hold any longer due to the combined effect of oscillations and collisions around the epoch of neutrino decoupling [27,28].…”