Cosmological bounds on pseudo Nambu-Goldstone bosons

TL;DR

This work analyzes cosmological constraints on axion-like particles (ALPs) with a two-photon coupling, focusing on how a relic ALP population and its decays imprint on $N_{ m eff}$, BBN light-element abundances, the CMB spectrum, and the diffuse photon background. It shows that early decays are tightly constrained by neutrino dilution and $D/H$/$^4$He yields, while late decays are bounded by CMB distortions and the extragalactic background light, with direct-decay photon searches offering complementary limits. The study maps the allowed and excluded regions in the $m_\phi$–$g$ parameter space and discusses how these bounds shift if additional ALP couplings are present. Overall, cosmology provides robust, complementary constraints to stellar and laboratory searches for ALPs in the keV–GeV range.

Abstract

We review the cosmological implications of a relic population of pseudo Nambu-Goldstone bosons (pNGB) with an anomalous coupling to two photons, often called axion-like particles (ALPs). We establish constraints on the pNGB mass and two-photon coupling by considering big bang nucleosynthesis, the physics of the cosmic microwave background, and the diffuse photon background. The bounds from WMAP7 and other large-scale-structure data on the effective number of neutrino species can be stronger than the traditional bounds from the primordial helium abundance. These bounds, together with those from primordial deuterium abundance, constitute the most stringent probes of early decays.

Figures (8)

• Figure 1: ALP bounds in the $m_\phi-g$ parameter space. The labeling is described in Sec. \ref{['sec:motivation']}.
• Figure 2: ALP bounds in the $m_\phi-\tau$ parameter space. The labeling is described in Sec. \ref{['sec:motivation']}.
• Figure 3: Contour plots of the number of effective neutrinos $N_{\rm eff}$ as a function of the ALP mass and lifetime (left) and of the coupling parameter $g$ and lifetime $\tau$ (right). ALP cosmologies leading to $N_{\rm eff}<2.11$ can be safely excluded.
• Figure 4: Examples of the evolution of the comoving energy of all species of neutrinos (solid), ALPs (dashed) and electrons (double-dashed), as functions of the temperature of the universe. All energies are normalized to one thermal bosonic degree of freedom.
• Figure 5: Isocontours of the primordial abundance of deuterium normalized to protons (D/H) (left) and helium $Y_p$ (right) in the decaying ALP cosmology, as a function of the ALP mass and lifetime.