Light Sterile Neutrinos in Cosmology and Short-Baseline Oscillation Experiments

TL;DR

The paper investigates whether a light sterile neutrino at the eV scale can explain short-baseline oscillation anomalies while remaining consistent with cosmological data. It analyzes Planck, WMAP, ACT/SPT, BAO, and local $H_0$ measurements, allowing $N_{ ext{eff}}$ and $m_s^{\text{eff}}$ to vary and testing two sterile-production models: thermal and Dodelson-Widrow. It also incorporates a local $H_0$ prior and a short-baseline (SBL) 3+1 prior on $m_s$ to assess cosmology–SBL compatibility. The results show that the $H_0$ prior raises $N_{ ext{eff}}$ and disfavors large $m_s^{\text{eff}}$, while including local cluster data (LGC) can relieve tensions and favors an eV-scale $m_s$ with $m_s^{\text{eff}}$ bounds around a few tenths of an eV; the DW model yields slightly better cosmology–SBL compatibility than the thermal case. However, full cosmology–SBL compatibility generally requires suppressed sterile production in the early Universe (e.g., via a lepton asymmetry), with the 2σ limits around $m_s^{\text{eff}} \lesssim 0.3$ eV (or up to ~0.5–0.6 eV depending on model and data).

Abstract

We analyze the most recent cosmological data, including Planck, taking into account the possible existence of a sterile neutrino with a mass at the eV scale indicated by short-baseline neutrino oscillations data in the 3+1 framework. We show that the contribution of local measurements of the Hubble constant induces an increase of the value of the effective number of relativistic degrees of freedom above the Standard Model value, giving an indication in favor of the existence of sterile neutrinos and their contribution to dark radiation. Furthermore, the measurements of the local galaxy cluster mass distribution favor the existence of sterile neutrinos with eV-scale masses, in agreement with short-baseline neutrino oscillations data. In this case there is no tension between cosmological and short-baseline neutrino oscillations data, but the contribution of the sterile neutrino to the effective number of relativistic degrees of freedom is likely to be smaller than one. Considering the Dodelson-Widrow and thermal models for the statistical cosmological distribution of sterile neutrinos, we found that in the Dodelson-Widrow model there is a slightly better compatibility between cosmological and short-baseline neutrino oscillations data and the required suppression of the production of sterile neutrinos in the early Universe is slightly smaller.

Figures (7)

• Figure 1: Results of the analysis of cosmological data alone. The light and dark shadowed regions in the 2D plots show, respectively, the 68% and 95% marginalized posterior probability regions obtained from the analysis of the data sets indicated in the legends with corresponding color. In the bottom-left panel $m_{s}$ is constant, with the indicated value in eV, along the dashed lines in the thermal model and along the solid lines in the Dodelson-Widrow model. The four lower intervals of $H_{0}$ in the upper-right panel correspond to: Eq. (\ref{['H0planck']}) for Planck+WP+highL 1303.5076, Eq. (\ref{['H02']}) for Cepheids+SNe Ia 1103.2976, Eq. (\ref{['H04']}) for COSMOGRAIL 1208.6010, Eq. (\ref{['H0loc']}) for the $H_{0}$ prior. In all panels the labels CMB, CMB+$H_0$, CMB+$H_0$+BAO and CMB+$H_0$+BAO+LGC indicate the fits performed in this work.
• Figure 2: Comparison of the allowed intervals of $N_{\text{eff}}$ obtained from the fits of CMB, CMB+$H_0$, CMB+$H_0$+BAO and CMB+$H_0$+BAO+LGC data without (black) and with the SBL prior in the thermal (blue) and Dodelson-Widrow (red) models. The segments in each bar correspond to 68%, 95% and 99% probability. The dotted vertical line corresponds to $\Delta N_{\text{eff}} = 1$.
• Figure 3: Comparison of the allowed intervals of $m_{s}^{\text{eff}}$ obtained from the fits of CMB, CMB+$H_0$, CMB+$H_0$+BAO and CMB+$H_0$+BAO+LGC data without (black) and with the SBL prior in the thermal (blue) and Dodelson-Widrow (red) models. The segments in each bar correspond to 68%, 95% and 99% probability.
• Figure 4: Comparison of the allowed interval of $m_{s}$ obtained from the 3+1 analysis of SBL data 1308.5288 with those obtained in the fits presented in this paper. The segments in each bar correspond to 68%, 95% and 99% probability. The out-of-bounds upper limits obtained in the CMB+$H_0$+BAO+LGC analysis are: $7.4 \, \text{eV}$ (99%, TH), $4.8 \, \text{eV}$ (95%, DW), $17.1 \, \text{eV}$ (99%, DW).
• Figure 5: Results of the analysis of cosmological data with the SBL prior in the thermal model. The light and dark shadowed regions in the 2D plots show, respectively, the 68% and 95% marginalized posterior probability regions obtained from the analysis of the data sets indicated in the legends with corresponding color. In the bottom-left panel $m_{s}$ is constant along the dashed lines, with the indicated value in eV. The four lower intervals of $H_{0}$ in the upper-right panel are equal to those in Fig. \ref{['fig:nosbl']}. In all panels the labels CMB, CMB+$H_0$, CMB+$H_0$+BAO and CMB+$H_0$+BAO+LGC indicate the fits performed in this work.