Sterile neutrinos, lepton asymmetries, primordial elements: how much of each?

TL;DR

This work quantifies how a primordial leptonic asymmetry $L_\nu$ can suppress active-sterile neutrino oscillations in the early universe, thereby relaxing BBN and LSS bounds on light sterile neutrinos. Using a momentum-averaged density-matrix framework that includes vacuum oscillations, matter potentials (with $L_\nu$), and collision terms, the authors compute the evolution of neutrino densities and the neutron-to-proton ratio to predict primordial element yields, while translating sterile production into cosmological energy density via $\Omega_\nu h^2$. They show that increasing negative $L_\nu$ moves BBN/LSS exclusion regions to higher $\Delta m^2_{14}$ and/or lower $\tan^2\theta_s$, with the electron-channel generally more constraining due to $\nu_e$-driven helium production. In the LSND-like scenario, they find that an asymmetry of order $|L_\nu|\sim 10^{-4}$ can reopen the region around $\Delta m^2_{\rm LSND} \sim 1$ eV$^2$ and $\sin^22\theta_{\rm LSND}\sim$ a few $\times 10^{-3}$, though full momentum dependence and upcoming experimental results will refine these conclusions.

Abstract

We investigate quantitatively the extent to which having a primordial leptonic asymmetry (n_nu \neq n_nubar) relaxes the bounds on light sterile neutrinos imposed by BBN and LSS. We adopt a few assumptions that allow us to solve the neutrino evolution equations over a broad range of mixing parameters and asymmetries. For the general cases of sterile mixing with the electron or muon neutrino, we identify the regions that can be reopened. For the particular case of a LSND-like sterile neutrino, soon to be rejected or confirmed by MiniBooNE, we find that an asymmetry of the order of 10^-4 is needed to lift the conflicts with cosmology.

Figures (2)

• Figure 1: The bounds from BBN and LSS on sterile neutrinos mixing with electron neutrinos (left panel) and with muon neutrinos (right panel), for different values of the primordial asymmetry $L_\nu$. The regions above and to the right of the thin lines are excluded by BBN because they correspond to $Y_p \ge 0.258$. The regions above and to the right of the thick lines are excluded by LSS because they correspond to $\Omega_\nu \ge 0.8\ 10^{-2}$. The gray dotted lines enclose the excluded regions from a combination of reactor and accelerator experiments (see Sec.\ref{['sec:LSND']}). Solar and atmospheric experiments also exclude similar regions, reported in CMSV.
• Figure 2: The allowed LSND region at 99% C.L. (yellow/light shaded area) compared to the cosmological bounds from BBN and LSS in the presence of primordial asymmetries. The darker shaded area is already excluded at 99% C.L. by other experiments. The regions below and to the left of the thin lines are allowed by BBN because they correspond to $Y_p \le 0.258$. The regions below and to the left of the thick lines are allowed by LSS because they correspond to $\Omega_\nu h^2 \le 0.8\ 10^{-2}$.