A search for sterile neutrinos with the latest cosmological observations

TL;DR

The paper tests the existence of sterile neutrinos using the latest cosmological observations by comparing a massless sterile neutrino model ($\\Lambda$CDM+$N_{ m eff}$) and a massive sterile neutrino model ($\$\\Lambda$CDM+$N_{ m eff}$+$m_{ u,{ m sterile}}^{\rm eff}$) against Planck 2015 data plus BAO, $H_0$, SZ, lensing, and cosmic shear. It finds a mild hint for an extra relativistic species with $N_{ m eff}=3.29^{+0.11}_{-0.17}$ (68.3% CL) when low-redshift data are included, corresponding to a $1.44\sigma$ excess over the standard value, and a slight improvement in fit; in contrast, the massive sterile neutrino is tightly constrained ($m_{ u,{ m sterile}}^{\rm eff}<0.2417$ eV with all data) and does not improve the overall fit, leading to no strong evidence for a massive sterile neutrino. The results are broadly consistent with neutrino oscillation experiments (Daya Bay, MINOS) and IceCube cosmic-ray data, hinting at dark radiation but disfavoring a fully thermalized eV-scale sterile neutrino. These findings help address the $H_0$ tension by allowing extra relativistic degrees of freedom, while keeping the standard active neutrino mass sum fixed.

Abstract

We report the result of a search for sterile neutrinos with the latest cosmological observations. Both cases of massless and massive sterile neutrinos are considered in the $Λ$CDM cosmology. The cosmological observations used in this work include the Planck 2015 temperature and polarization data, the baryon acoustic oscillation data, the Hubble constant direct measurement data, the Planck Sunyaev-Zeldovich cluster counts data, the Planck lensing data, and the cosmic shear data. We find that the current observational data give a hint of the existence of massless sterile neutrino (as dark radiation) at the 1.44$σ$ level, and the consideration of an extra massless sterile neutrino can indeed relieve the tension between observations and improve the cosmological fit. For the case of massive sterile neutrino, the observations give a rather tight upper limit on the mass, which implies that actually a massless sterile neutrino is more favored. Our result is consistent with the recent result of neutrino oscillation experiment done by the Daya Bay and MINOS collaborations, as well as the recent result of cosmic ray experiment done by the IceCube collaboration.

Figures (4)

• Figure 1: Constraint results for the $\Lambda$CDM+$N_{\rm eff}$ model from the CMB+BAO data combination and the CMB+BAO+other data combination. Left panel: two-dimensional marginalized posterior contours (68.3% and 95.4% CL) in the $N_{\rm eff}$--$H_0$ plane. Right panel: one-dimensional marginalized posterior distributions for $N_{\rm eff}$.
• Figure 2: The two-dimensional marginalized contours (68.3% and 95.4% CL) in the $\Omega_m$--$H_0$ plane, for the $\Lambda$CDM and $\Lambda$CDM+$N_{\rm eff}$+$m_{\nu,{\rm{sterile}}}^{\rm{eff}}$ models, from the constraints of CMB+BAO ( left) and CMB+BAO+other ( right).
• Figure 3: The one-dimensional posterior distributions of $H_0$ in the $\Lambda$CDM and $\Lambda$CDM+$N_{\rm eff}$+$m_{\nu,{\rm{sterile}}}^{\rm{eff}}$ models, from the constraints of CMB+BAO ( left) and CMB+BAO+other ( right).
• Figure 4: The one-dimensional posterior distributions and two-dimensional marginalized contours (68.3% and 95.4% CL) for the $\Lambda$CDM+$N_{\rm eff}$+$m_{\nu,{\rm{sterile}}}^{\rm{eff}}$ model, from the constraints of the CMB+BAO and CMB+BAO+other data combinations.