A New Target for Cosmic Axion Searches

TL;DR

This paper examines how future cosmic microwave background measurements can detect or tightly constrain light pseudo-Nambu-Goldstone bosons—axions, familons, and majorons—through their impact on the relativistic energy density, parametrized by $\Delta N_{ m eff}$. By equating Goldstone production rates in the early universe to the Hubble expansion and considering both freeze-out and freeze-in scenarios, the authors translate non-detections into lower bounds on symmetry-breaking scales $\Lambda$ that couple these bosons to photons, gluons, fermions, and neutrinos. They show that cosmology can surpass existing laboratory and astrophysical bounds by many orders of magnitude for plausible reheating temperatures $T_R$, with sensitivity concentrated around the target $\Delta N_{ m eff}=0.027$ and extending to arbitrarily high decoupling temperatures. The results highlight the potential of CMB-S4 and related cosmological data to probe new physics beyond the Standard Model and to constrain the thermal history of the early universe, providing complementary information to direct searches. The work connects measurements of $\Delta N_{ m eff}$ to the reheating temperature and the nature of pNGB couplings, offering a compelling cosmological target for axion searches and a framework for interpreting future detections.

Abstract

Future CMB experiments have the potential to probe the density of relativistic species at the sub-percent level. Sensitivity at this level allows light thermal relics to be detected up to arbitrarily high decoupling temperatures. Conversely, the absence of a detection would require extra light species never to have been in equilibrium with the Standard Model. In this paper, we exploit this feature to demonstrate the sensitivity of future cosmological observations to the couplings of axions to all of the Standard Model degrees of freedom. In many cases, the constraints achievable from cosmology will surpass existing bounds from laboratory experiments and astrophysical observations by orders of magnitude.

Figures (6)

• Figure 1: Contribution of a single thermally-decoupled Goldstone boson to the effective number of neutrinos, $\Delta N_{\rm eff}$, as a function of the freeze-out temperature $T_F$. Shown are also the current $2\sigma$ sensitivity of the Planck satellite Ade:2015xua and an (optimistic) estimate of the sensitivity of a future CMB-S4 mission Baumann:2015rya.
• Figure 2: Comparison between current constraints on the axion-photon coupling and the sensitivity of a future CMB-S4 mission (figure adapted from Carosi:2013rla). Future laboratory constraints (IAXO and ADMX) are shown as shaded regions. The yellow band indicates a range of representative models for the QCD axion (not assuming that it provides all of the dark matter). The future CMB bound is a function of the reheating temperature $T_R$ and the displayed constraint conservatively assumes that the photon coupling derives only from the coupling to $U(1)_Y$ above the electroweak scale. Specific axion models typically also involve a coupling to $SU(2)_L$ in which case the bound would strengthen by an order of magnitude or more (see Appendix \ref{['app:rates']}). We note that ADMX assumes that the axion is all of the dark matter, while all other constraints do not have this restriction.
• Figure 3: Comparison between current constraints on the axion-gluon coupling and the sensitivity of a future CMB-S4 mission (figure adapted from Graham:2013gfaBlum:2014vsa). The dotted lines are the projected sensitivities of the NMR experiment CASPEr Budker:2013hfa. We note that CASPEr, the static EDM Graham:2013gfa and BBN constraints Blum:2014vsa assume that the axion is all of the dark matter, while SN 1987A Raffelt:1996wa and the future CMB constraint do not have this restriction.
• Figure 4: Feynman diagrams for the dominant Goldstone production via the gluon coupling. For gluon fusion, there are $t$- and $u$-channel diagrams in addition to the presented $s$-channel diagram. Similar diagrams apply for the couplings to the electroweak gauge bosons.
• Figure 5: Left: Axion production rate associated with the coupling to gluons as parametrized by $\gamma_g(T)$ in (\ref{['equ:Gg']}). Right: Constraint on the axion-gluon coupling $\Lambda_g$ as parametrized by $\lambda_g(T_R)$ in (\ref{['equ:Lambg']}).