Is cosmology compatible with sterile neutrinos?

TL;DR

By combining data from cosmic microwave background experiments, large scale structure, and Lyman-alpha forest observations, the hypothesis of a fourth, sterile, massive neutrino is constrain and the sterile neutrinos hypothesis is excluded as an explanation of the LSND anomaly.

Abstract

By combining data from cosmic microwave background (CMB) experiments (including the recent WMAP third year results), large scale structure (LSS) and Lyman-alpha forest observations, we constrain the hypothesis of a fourth, sterile, massive neutrino. For the 3 massless + 1 massive neutrino case we bound the mass of the sterile neutrino to m_s<0.26eV (0.44eV) at 95% (99.9%) c.l.. These results exclude at high significance the sterile neutrino hypothesis as an explanation of the LSND anomaly. We then generalize the analysis to account for active neutrino masses (which tightens the limit to m_s<0.23eV 0.42eV) and the possibility that the sterile abundance is not thermal. In the latter case, the contraints in the (mass, density) plane are non-trivial. For a mass of >1eV or <0.05eV the cosmological energy density in sterile neutrinos is always constrained to be omega_nu <0.003 at 95 c.l.. However, for a sterile neutrino mass of ~0.25eV, omega_nu can be as large as 0.01.

Figures (3)

• Figure 1: The epoch of equality $a_{\rm eq}$ as a function of mass of sterile neutrino and its energy density. The non-relativistic matter density here is fixed to $\omega_m=0.15$, so that in the standard 3-neutrino model, $a_{\rm EQ}=2.82\times 10^{-4}$. Notice that, at fixed $\omega_s$, $a_{\rm eq}$ rises very rapidly for lower masses since the neutrinos behave as radiation. Thermalized neutrinos lie along the dashed curve.
• Figure 2: Effect of extra sterile neutrino on the CMB (top) and LSS (bottom) power spectra. Thin lines correspond to standard model, sterile neutrino of mass $m=1$eV (dashed) $m=0.3$eV (dot-dashed) and fixed sterile density $\omega_{s}=0.01$. These curves are normalised to large scale $C_\ell$. Thick dashed and dot-dashed curves correspond to models, which in addition to having sterile mass have had dark matter density increased to match standard $a_{\rm eq}$ and $h$ increased to match CMB peak positions and were normalised at the first peak. Dotted vertical lines on the bottom plot enclose the area where LSS experiments are currently sensitive to with thick line normalisations chosen to illustrate the fact that the $1$eV model is a poorer fit than $0.3$eV model. See text for discussion.
• Figure 3: $1$,$2$-$\sigma$ constraints on the sterile neutrino mass and abundance.