Constraints on massive sterile neutrino species from current and future cosmological data

TL;DR

This work investigates constraints on massive sterile neutrinos in (3+2) models using current cosmological data and projects future sensitivities for Planck and galaxy surveys. It extends CAMB/CosmoMC to include active and sterile masses $m_{\nu}$, $m_{\nu_s}$ and sterile counts $N_{\nu_s}$, analyzing data with and without BBN priors on light-element abundances. The results show that current data allow about two sub-eV sterile species in addition to three sub-eV active neutrinos, though BBN disfavors fully thermalized (3+2) models; forecasts indicate Planck plus BOSS or Euclid can constrain $N_{\nu_s}$ to ~0.1–0.3 and $m_{\nu}$, $m_{\nu_s}$ to ~10–30% for sub-eV sterile masses, with degeneracies among parameters. The study also highlights potential biases in $H_0$ and $m_{\nu}$ if sterile neutrinos are neglected, which could be tested by independent $H_0$ measurements and KATRIN, underscoring the importance of including sterile neutrinos in cosmological analyses.

Abstract

Sterile massive neutrinos are a natural extension of the Standard Model of elementary particles. The energy density of the extra sterile massive states affects cosmological measurements in an analogous way to that of active neutrino species. We perform here an analysis of current cosmological data and derive bounds on the masses of the active and the sterile neutrino states as well as on the number of sterile states. The so-called (3+2) models with three sub-eV active massive neutrinos plus two sub-eV massive sterile species is well within the 95% CL allowed regions when considering cosmological data only. If the two extra sterile states have thermal abundances at decoupling, Big Bang Nucleosynthesis bounds compromise the viability of (3+2) models. Forecasts from future cosmological data on the active and sterile neutrino parameters are also presented. Independent measurements of the neutrino mass from tritium beta decay experiments and of the Hubble constant could shed light on sub-eV massive sterile neutrino scenarios.

Figures (3)

• Figure 1: The top, middle and bottom panels show the 68% and 95% CL constraints on the plane $m_{\nu}$-$N_{{\nu}_s}$, $m_{\nu}$-$m_{{\nu}_s}$ and $m_{{\nu}_s}$-$N_{{\nu}_s}$, respectively. The blue (red) contours denote the allowed regions by "run1" ("run2") data sets, see text for details. The masses of the sterile and active neutrinos are both in eV units.
• Figure 2: The empty (filled) contours denote the $68\%$, $95\%$ and $99.73\%$ CL regions for Planck plus BOSS (Euclid) data. The neutrino parameters in the fiducial model are $N_{\nu_s}= 1$, $m_{\nu_s}=0.3$ eV and $m_\nu=0.1$ eV.
• Figure 3: The empty (filled) contours denote the $68\%$, $95\%$ and and $99.73\%$ CL regions for Planck plus BOSS (Euclid) data. The neutrino parameters in the fiducial model are $N_{\nu_s}= 1$, $m_{\nu_s}=0.3$ eV and $m_\nu=0.1$ eV.