Heavy sterile neutrinos: Bounds from big-bang nucleosynthesis and SN 1987A

Abstract

Cosmological and astrophysical effects of heavy (10 - 200 MeV) sterile Dirac neutrinos, mixed with the active ones, are considered. The bounds on mass and mixing angle from both supernovae and big-bang nucleosynthesis are presented.

Figures (3)

• Figure 1: Minimum mixing angle between sterile and active neutrinos, allowed by BBN, as a function of the heavy neutrino mass, both for an optimistic bound, $\Delta N =0.2$, and for a conservative bound, $\Delta N = 1.0$. The upper panel corresponds to $\nu_\mu \leftrightarrow \nu_s$ or $\nu_\tau \leftrightarrow \nu_s$ mixings, while the lower one to $\nu_e \leftrightarrow \nu_s$ mixing.
• Figure 2: Maximum lifetime of sterile neutrino, allowed by BBN, as a function of the heavy neutrino mass, both for an optimistic bound, $\Delta N =0.2$, and for a conservative bound, $\Delta N = 1.0$. The upper panel corresponds to $\nu_\mu \leftrightarrow \nu_s$ or $\nu_\tau \leftrightarrow \nu_s$ mixings, while the lower one to $\nu_e \leftrightarrow \nu_s$ mixing.
• Figure 3: Summary of our exclusion regions in the $\sin^2 \theta$-$M_s$-plane. SN 1987A excludes all mixing angles between two solid horizontal lines. BBN excludes the area below the two upper dashed lines if the heavy neutrinos were abundant in the early universe. These two upper dashed lines both correspond to the conservative limit of one extra light neutrino species permitted by the primordial $^4$He-abundance. The higher of the two is for $\nu_{\mu, \tau}$ mixing, and the slightly lower curve is for $\nu_e$ mixing. In the region below the lowest dashed curve the heavy neutrinos are not efficiently produced in the early universe and their impact on BBN is weak. For comparison we have also presented here the region excluded by NOMAD Collaboration nomad for the case of $\nu_s \leftrightarrow \nu_\tau$ mixing.