Light sterile neutrinos after BICEP-2

TL;DR

The paper reevaluates light sterile neutrinos in light of BICEP-2, showing that sub-eV sterile masses can be compatible with current cosmological data if thermalisation is not complete. It adopts a 3 active + 1 sterile framework with a thermally distributed sterile, characterized by $m_s$ and $N_{eff}$, and performs a Bayesian joint analysis with Planck, BICEP-2, LSS, H0, CFHTLenS, PSZ, and SBL priors. The results show Planck+WP+high-ℓ and BICEP-2 yield $m_s < 0.85$ eV (2σ), tightening to $<0.48$ eV when including $H_0$, while lensing/CFHTLenS/PSZ data favor a non-zero $m_s$ around $0.44$ eV with low $N_{eff}$. Short-baseline oscillations prefer $m_s o 1.27$ eV, but a fully thermalised state ($N_{eff}=1$) is strongly disfavoured cosmologically; partial thermalisation can reconcile the datasets, depending on the data combination.

Abstract

The recent discovery of B-modes in the polarization pattern of the Cosmic Microwave Background by the BICEP2 experiment has important implications for neutrino physics. We revisit cosmological bounds on light sterile neutrinos and show that they are compatible with all current cosmological data provided that the mass is relatively low. Using CMB data, including BICEP-2, we find an upper bound of $m_s < 0.85$ eV ($2σ$ Confidence Level). This bound is strengthened to 0.48 eV when HST measurements of $H_0$ are included. However, the inclusion of SZ cluster data from the Planck mission and weak gravitational measurements from the CFHTLenS project favours a non-zero sterile neutrino mass of $0.44^{+0.11}_{-0.16}$ eV. Short baseline neutrino oscillations, on the other hand, indicate a new mass state around 1.2 eV. This mass is highly incompatible with cosmological data if the sterile neutrino is fully thermalised ($Δχ^2>10$). However, if the sterile neutrino only partly thermalises it can be compatible with all current data, both cosmological and terrestrial.

Figures (4)

• Figure 1: $1\sigma$ and $2\sigma$ marginalized contours for different combinations of CMB data sets.
• Figure 2: $1\sigma$ and $2\sigma$ marginalized contours in the plane $(m_s,\Delta N_{\rm {eff}})$. The banana shaped regions allowed by cosmology indicate a sub-eV mass and an excess in $N_{\rm {eff}}$, while the inclusion of SBL data forces the mass around 1eV, moving the contours towards the warm dark matter limit, which implies a lower value of $\Delta N_{\rm {eff}}$ because of the strong correlation between the two parameters.
• Figure 3: $1\sigma$, $2\sigma$ and $3\sigma$ confidence level limits for $\Delta N_{\rm {eff}}$, for different dataset combinations. The circles indicate the mean value.
• Figure 4: $1\sigma$, $2\sigma$ and $3\sigma$ confidence level limits for $m_s$, for different dataset combinations. The circles indicate the mean value.