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Sterile neutrino self-interactions: $H_0$ tension and short-baseline anomalies

Maria Archidiacono, Stefano Gariazzo, Carlo Giunti, Steen Hannestad, Thomas Tram

TL;DR

Sterile neutrinos in the eV range face cosmological bounds due to thermalisation, but a pseudoscalar self-interaction can reconcile CMB observations with local $H_0$ measurements. The study updates cosmological constraints, showing that high-$\ell$ polarization forces $m_s$ to be $\lesssim 1$ eV in the pseudoscalar model, while preserving compatibility with $H_0$ and SBL hints within narrow ranges. A Bayesian combination of CMB+BAO+lensing data with SBL oscillation analyses suggests viable sterile masses around $1$ eV, with higher masses strongly disfavored. Overall, the pseudoscalar scenario alleviates the $H_0$ tension more effectively than standard $\Lambda$CDM while remaining consistent with SBL anomalies in restricted mass windows.

Abstract

Sterile neutrinos with a mass in the eV range have been invoked as a possible explanation of a variety of short baseline (SBL) neutrino oscillation anomalies. However, if one considers neutrino oscillations between active and sterile neutrinos, such neutrinos would have been fully thermalised in the early universe, and would be therefore in strong conflict with cosmological bounds. In this study we first update cosmological bounds on the mass and energy density of eV-scale sterile neutrinos. We then perform an updated study of a previously proposed model in which the sterile neutrino couples to a new light pseudoscalar degree of freedom. Consistently with previous analyses, we find that the model provides a good fit to all cosmological data and allows the high value of $H_0$ measured in the local universe to be consistent with measurements of the cosmic microwave background. However, new high $\ell$ polarisation data constrain the sterile neutrino mass to be less than approximately 1 eV in this scenario. Finally, we combine the cosmological bounds on the pseudoscalar model with a Bayesian inference analysis of SBL data and conclude that only a sterile mass in narrow ranges around 1 eV remains consistent with both cosmology and SBL data.

Sterile neutrino self-interactions: $H_0$ tension and short-baseline anomalies

TL;DR

Sterile neutrinos in the eV range face cosmological bounds due to thermalisation, but a pseudoscalar self-interaction can reconcile CMB observations with local measurements. The study updates cosmological constraints, showing that high- polarization forces to be eV in the pseudoscalar model, while preserving compatibility with and SBL hints within narrow ranges. A Bayesian combination of CMB+BAO+lensing data with SBL oscillation analyses suggests viable sterile masses around eV, with higher masses strongly disfavored. Overall, the pseudoscalar scenario alleviates the tension more effectively than standard CDM while remaining consistent with SBL anomalies in restricted mass windows.

Abstract

Sterile neutrinos with a mass in the eV range have been invoked as a possible explanation of a variety of short baseline (SBL) neutrino oscillation anomalies. However, if one considers neutrino oscillations between active and sterile neutrinos, such neutrinos would have been fully thermalised in the early universe, and would be therefore in strong conflict with cosmological bounds. In this study we first update cosmological bounds on the mass and energy density of eV-scale sterile neutrinos. We then perform an updated study of a previously proposed model in which the sterile neutrino couples to a new light pseudoscalar degree of freedom. Consistently with previous analyses, we find that the model provides a good fit to all cosmological data and allows the high value of measured in the local universe to be consistent with measurements of the cosmic microwave background. However, new high polarisation data constrain the sterile neutrino mass to be less than approximately 1 eV in this scenario. Finally, we combine the cosmological bounds on the pseudoscalar model with a Bayesian inference analysis of SBL data and conclude that only a sterile mass in narrow ranges around 1 eV remains consistent with both cosmology and SBL data.

Paper Structure

This paper contains 9 sections, 2 equations, 6 figures, 2 tables.

Table of Contents

  1. Introduction
  2. Pseudoscalar sterile neutrino interactions
  3. "Vanilla" sterile neutrinos
  4. Pseudoscalar interactions
  5. Methodology and data
  6. Results
  7. CMB phenomenology in the pseudoscalar model
  8. Short-baseline neutrino oscillations
  9. Conclusions

Figures (6)

  • Figure 1: Marginalized 1D posterior of $H_0$ obtained by fitting Planck TTTEEE data.
  • Figure 2: Upper panels: Percentage relative difference in temperature (upper left panel) and polarization (upper right panel) between the Pseudoscalar best-fit of TT (blue solid lines) and of TTTEEE (orange dashed lines) and the Planck 2018 best-fit. The data points with error bars show the Planck 2018 measurements. Lower left panel: CMB lensing power spectrum of the Planck 2018 best-fit, and Pseudoscalar best-fit of TT (blue solid line), TTTEEE (orange dashed line), and TTTEEE + lensing + BAO (green dot-dashed line). Lower right panel: Ratio between the volume distance ($D_V$) divided by the size of the sound horizon at baryon drag ($r_d$) in the Pseudoscalar best-fit (same colour coding as in the plot of CMB lensing) and in Planck 2018 best-fit. The BAO measurements are: SDSS MGS Ross:2014qpa, 6DFGS Beutler:2011hx, BOSS DR12 Alam:2016hwk, WiggleZ Kazin:2014qga, DR14 LRG Bautista:2017wwp, SDSS quasar Blomqvist:2019rah. Notice that only SDSS MGS, 6DFGS, BOSS DR12 are included in the analysis.
  • Figure 3: Left panel: The effective angular sound horizon as a function of conformal time in the early Universe with $h=0.7$ for the three cases "Pseudoscalar", "Sterile" and "Massless" described in the text. The vertical lines mark the conformal time of recombination. Right panel: Hubble rate for the same models in the late Universe with fixed angular scale of the sound-horizon at recombination $100\times\theta_s=1.04$.
  • Figure 4: Relative differences in $C_\ell^{TT}$ (left panels) and $C_\ell^{EE}$ (right panels). The upper panels have $h=0.7$, and the "Sterile" case is clearly more different due to its different late-time behaviour. The lower panels show the case where the angular sound-horizon at recombination has been fixed, and this reduces the scatter by a factor 5. Now the "Massless" and the "Sterile" case are much more similar while the "Pseudoscalar" case stands out.
  • Figure 5: Marginalized 2D $1\,\sigma$ (dim) and $2\,\sigma$ (light) contours and 1D posterior for a subset of cosmological parameters $(n_s,\,\Delta N_{\rm eff},\,m_s,\,H_0)$.
  • ...and 1 more figures