Sterile neutrinos with pseudoscalar self-interactions and cosmology

TL;DR

The paper tackles the tension between terrestrial hints of eV-scale sterile neutrinos and cosmological data. It proposes a pseudoscalar-mediated self-interaction that suppresses early sterile production and creates a late-time ν_s–φ fluid, reconciling N_eff and mass bounds with data. Through MCMC analyses of Planck, BAO, and HST data, the pseudoscalar model fits as well as ΛCDM and far better than ΛCDM with a fully thermalized 1 eV sterile neutrino, while offering a preferred higher H0. The framework predicts partial thermalization and strong late-time interactions, with robust implications for future cosmological surveys and hidden-sector physics.

Abstract

Sterile neutrinos in the electronvolt mass range are hinted at by a number of terrestrial neutrino experiments. However, such neutrinos are highly incompatible with data from the Cosmic Microwave Background and large scale structure. This paper discusses how charging sterile neutrinos under a new pseudoscalar interaction can reconcile eV sterile neutrinos with terrestrial neutrino data. We show that this model can reconcile eV sterile neutrinos in cosmology, providing a fit to all available data which is way better than the standard $Λ$CDM model with one additional fully thermalized sterile neutrino. In particular it also prefers a value of the Hubble parameter much closer to the locally measured value.

Figures (6)

• Figure 1: Upper panel: Relative increase in the pseudo scalar-sterile neutrino energy density compared to the energy density of one active neutrino, due to sterile neutrino annihilations. Lower panel: Temporary suppression of the pseudo scalar-sterile neutrino equation of state parameter.
• Figure 2: Linear matter power spectra for $\Lambda+1\nu_{\textrm{s}}+m_{\nu{\textrm{,s}}}$ (lower panel) and the pseudoscalar model (upper panel) for various values of the sterile neutrino mass and for $N_{\textrm{eff}}=4.046$ (in the pseudoscalar model $11/32 \times 4.046$ are strongly interacting). For comparision, the matter power spectrum obtained with Planck 2015 best-fit is also shown.
• Figure 3: Upper panel: Temperature anisotropies power spectra for various models: Pseudoscalar (red/dashed line), $\Lambda+0.06{\textrm{ eV }}{\nu_{\textrm{a}}}$ (blue/solid line), $\Lambda+1\nu_{\textrm{s}}+m_{\nu{\textrm{,s}}}$ (purple/dot-dashed line), $\Lambda+1{\textrm{ eV }}{\nu_{\textrm{s}}}$ (black/solid line). Lower panel: Relative errors for the models listed above compared to the $\Lambda+0.06{\textrm{ eV }}{\nu_{\textrm{a}}}$ model. In all cases the spectra plotted are with the best-fit parameters obtained when fitting to CMB data.
• Figure 4: One dimensional posterior for $m_{\nu{\textrm{,s}}}$ in the $\Lambda+1\nu_{\textrm{s}}+m_{\nu{\textrm{,s}}}$ model and in the pseudoscalar scenario for the combination of data PlanckTT+lowP. The vertical lines show the best fit for the sterile neutrino masses obtained by the global oscillation data analysis in the 3+1 scenario Kopp:2013vaa
• Figure 5: One dimensional posterior for $H_0$ in the $\Lambda$CDM model and in the pseudoscalar scenario for the combination of PlanckTT+lowP data. The grey region shows the $1\sigma$ confidence interval from direct measurements.