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New cosmological mass limit on thermal relic axions

S. Hannestad, A. Mirizzi, G. Raffelt

TL;DR

This work extends cosmological neutrino mass techniques to thermal relic axions, focusing on hadronic models where axions couple to pions. By computing the axion freeze-out temperature through the axion–pion interaction and the expansion history, and by performing a likelihood analysis against diverse cosmological data (LSS, CMB, SN Ia, Lyα, and Hubble constant), the authors obtain a stringent bound on hadronic axions: $m_a<1.05$ eV (95% CL), corresponding to $f_a>5.7\times10^6$ GeV. They show that the constraint is driven by the early decoupling and its imprint on small-scale structure, with Lyα data particularly constraining high $g_*$ scenarios; non-hadronic models can evade some limits if the axion–pion coupling is suppressed. The result provides a cosmological complement to laboratory and astrophysical searches (e.g., CAST) and closes portions of the hadronic axion parameter space that were previously allowed by SN1987A and stellar cooling arguments.

Abstract

Observations of the cosmological large-scale structure provide well-established neutrino mass limits. We extend this argument to thermal relic axions. We calculate the axion thermal freeze-out temperature and thus their cosmological abundance on the basis of their interaction with pions. For hadronic axions we find a new mass limit $m_a<1.05$ eV (95% CL), corresponding to a limit on the axion decay constant of $f_a>5.7\times 10^6$ GeV. For other models this constraint is significantly weakened only if the axion-pion coupling is strongly suppressed. For comparison we note that the same approach leads to $\sum m_ν<0.65$ eV (95% CL) for neutrinos.

New cosmological mass limit on thermal relic axions

TL;DR

This work extends cosmological neutrino mass techniques to thermal relic axions, focusing on hadronic models where axions couple to pions. By computing the axion freeze-out temperature through the axion–pion interaction and the expansion history, and by performing a likelihood analysis against diverse cosmological data (LSS, CMB, SN Ia, Lyα, and Hubble constant), the authors obtain a stringent bound on hadronic axions: eV (95% CL), corresponding to GeV. They show that the constraint is driven by the early decoupling and its imprint on small-scale structure, with Lyα data particularly constraining high scenarios; non-hadronic models can evade some limits if the axion–pion coupling is suppressed. The result provides a cosmological complement to laboratory and astrophysical searches (e.g., CAST) and closes portions of the hadronic axion parameter space that were previously allowed by SN1987A and stellar cooling arguments.

Abstract

Observations of the cosmological large-scale structure provide well-established neutrino mass limits. We extend this argument to thermal relic axions. We calculate the axion thermal freeze-out temperature and thus their cosmological abundance on the basis of their interaction with pions. For hadronic axions we find a new mass limit eV (95% CL), corresponding to a limit on the axion decay constant of GeV. For other models this constraint is significantly weakened only if the axion-pion coupling is strongly suppressed. For comparison we note that the same approach leads to eV (95% CL) for neutrinos.

Paper Structure

This paper contains 16 sections, 13 equations, 8 figures, 3 tables.

Table of Contents

  1. Introduction
  2. Axions
  3. Axion decoupling
  4. Pionic process
  5. Expansion rate
  6. Freeze-out conditions
  7. Likelihood analysis
  8. Theoretical predictions
  9. Cosmological Data
  10. Large Scale Structure.
  11. Cosmic Microwave Background.
  12. Supernova luminosity distances.
  13. The Lyman-$\alpha$ forest.
  14. Hubble parameter.
  15. Limits on axion parameters
  16. ...and 1 more sections

Figures (8)

  • Figure 1: Dimensionless axion absorption rate $h(\mu)$ as defined by Eq. (\ref{['eq:hdef']}).
  • Figure 2: Effective number of thermal degrees of freedom in the early universe during the post-QCD epoch, assuming the particle content listed in Table \ref{['tab:particles']}. Upper panel:$g_*$. Lower Panel:$g_{*S}/g_*$.
  • Figure 3: Average axion absorption rate for $f_a=10^7$ GeV and cosmic expansion rate as a function of the cosmic temperature. Rates are in units of $T^2/m_{\rm Pl}$.
  • Figure 4: Axion freeze-out, assuming the $a$-$\pi$ coupling of Eq. (\ref{['eq:axionpioncoupling']}). Top: Decoupling temperature. Middle: Corresponding $g_*$. Bottom: Present-day axion density.
  • Figure 5: Likelihood contours for the allowed axion parameters. Everything to the right of the dark shaded region is excluded at the 68% CL, and everything to the right of the light shaded region is excluded at the 95% CL. Contours of equal axion mass with the indicated $m_a$ values in eV are shown as dashed lines. Upper panel: All data included. Lower panel: Lyman-$\alpha$ data excluded.
  • ...and 3 more figures