Sterile Neutrinos: Cosmology vs Short-BaseLine Experiments

TL;DR

The study jointly tests sterile-neutrino scenarios (3+1 and 3+2) against cosmological data and short-baseline oscillation results, allowing partial thermalization via multiplicities $N_i\in[0,1]$ and incorporating SBL $\chi^2$ as priors. Across data sets, one or two sterile states with masses near $1$ eV are consistent with cosmology if not fully thermalized, with Planck and lensing considerations modulating $N_{ m eff}$ and $\sum m_\nu$ but not ruling out these scenarios. The joint analyses find that $m_4$ typically lies in the $1.0$–$1.3$ eV range for (3+1) and that two sterile masses $m_4\sim1.0$–$2.0$ eV and $m_5\sim1.6$–$2.1$ eV are compatible under (3+2), provided the sterile states are partially thermalized ($N_4,N_5<1$). Overall, the results support a cosmological-viable interpretation of eV-scale sterile neutrinos when their cosmological energy density contribution is suppressed relative to full thermalization.

Abstract

Cosmology and short baseline neutrino oscillation data both hint at the existence of light sterile neutrinos with masses in the 1 eV range. Here we perform a detailed analysis of the sterile neutrino scenario using both cosmological and SBL data. We have additionally considered the possibility that the extra neutrino degrees of freedom are not fully thermalised in the early universe. Even when analyzing only cosmological data we find a preference for the existence of massive sterile neutrinos in both (3+1) and (3+2) scenarios, and with the inclusion of SBL data the evidence is formally at the 3.3sigma level in the case of a (3+1) model. Interestingly, cosmological and SBL data both point to the same mass scale of approximately 1 eV. In the (3+1) framework WMAP9+SPT provide a value of the sterile mass eigenstate m_4 = (1.72 \pm 0.65) eV: this result is strenghtened by adding the prior from SBL posterior to m_4 = (1.27 \pm 0.12) eV (m_4 = (1.23 \pm 0.13) eV when SDSS is also considered in the cosmological analysis). In the (3+2) scheme, two additional, non--fully thermalized, neutrinos are compatible with the whole set of cosmological and SBL data, leading to mass values of m_4 = (0.95 \pm 0.30) eV and m_5 = (1.59 \pm 0.49) eV. The inclusion of Planck data does not change our considerations about the mass scale; concerning the extra neutrino degrees of freedom, invoking a partial thermalisation the 3+1 model is still consistent with the latest data.

Figures (11)

• Figure 1: Allowed $3\sigma$ regions (99.73% CL) in the $\sin^{2}2\vartheta_{e\mu}$--$\Delta{m}^{2}_{41}$ plane in the 3+1 model obtained from $\nu_{e}$ and $\bar{\nu}_{e}$ disappearance data (left of the dark red curve), $\nu_{\mu}$ and $\bar{\nu}_{\mu}$ disappearance data (left of the dark green curve), combined disappearance data (left of the dark orange curve), $\nu_{\mu}\to\nu_{e}$ and $\bar{\nu}_{\mu}\to\bar{\nu}_{e}$ appearance data (inside the blue curve) and from the global fit (inside the red curves). The best-fit points in the last two cases are indicated by crosses.
• Figure 2: Case (B): $N'_{\rm eff}$ massive neutrinos with total mass $\sum m_{\nu}$ -- Two dimensional marginalized 68% and 95% confidence level regions in the plane $\sum m_{\nu}$ vs $N'_{\rm eff}$ . Leftmost (red) contours refer to CMB--only data (WMAP9+SPT+ACT), while rightmost (blue) contours include also BAO and HST. The larger (green) area denotes the results for WMAP9+SPT datasets.
• Figure 3: Case (B): $N'_{\rm eff}$ massive neutrinos with total mass $\sum m_{\nu}$ -- One dimensional marginalized posteriors for $N'_{\rm eff}$ (left panel) and $\sum m_{\nu}$ (right panel). Solid (red) lines refer to CMB--only data (WMAP9+SPT+ACT), while dashed (blue) lines include also BAO and HST. The dot--dashed (green) curve denotes the results for WMAP9+SPT datasets.
• Figure 4: (3+1) analysis -- One dimensional marginalized posterior for $m_{4}$. The thick (green) and thin (blue) lines refer to the case of Table \ref{['tab:sbl1']} with and without the SBL prior, respectively. Solid lines stand for the analysis on CMB--only (WMAP9+SPT) data; dot--dashed lines refer to the inclusion of information from the matter power spectrum. The (red) dashed line shows the 3+1 SBL posterior. 95% C.L. upper bounds on the mass for the different cases are reported as vertical lines.
• Figure 5: (3+1) analysis -- Two dimensional marginalized 68% and 95% confidence level regions in the plane $N_4$ -- $m_{4}$ for the different combinations of datasets reported in Tables \ref{['tab:sbl1']}.