The Cosmophenomenology of Axionic Dark Radiation

TL;DR

The paper investigates cosmophenomenology of axionic dark radiation produced from modulus decays. It shows that ultra-relativistic axions can interact with the thermal plasma at high center-of-mass energies, enabling inelastic axion-photon scattering during BBN and non-thermal production of dark matter via axion scattering into superpartners. It maps axion energy deposition histories to decaying-particle constraints, derives conservative bounds on the axion decay constant $f_a$ and reheating temperature, and demonstrates that a dark matter relic can be generated non-thermally through axion-plasma scattering. Additionally, it forecasts a present-day Cosmic Axion Background flux with an energy around $\mathcal{O}(100)\ \mathrm{eV}$ and a flux near $10^6\ \mathrm{cm^{-2}\,s^{-1}}$, potentially dominating solar axion flux in some parameter ranges and offering a distinctive experimental target.

Abstract

Relativistic axions are good candidates for the dark radiation for which there are mounting observational hints. The primordial decays of heavy fields produce axions which are ultra-energetic compared to thermalised matter and inelastic axion-matter scattering can occur with $E_{CoM} \gg T_γ$, thus accessing many interesting processes which are otherwise kinematically forbidden in standard cosmology. Axion-photon scattering into quarks and leptons during BBN affects the light element abundances, and bounds on overproduction of $^4$He constrain a combination of the axion decay constant and the reheating temperature. For supersymmetric models, axion scattering into visible sector superpartners can give direct non-thermal production of dark matter at $T_γ \ll T_{freezeout}$. Most axions --- or any other dark radiation candidate from modulus decay --- still linger today as a Cosmic Axion Background with $E_{axion} \sim \mathcal{O}(100) eV$, and a flux of $\sim 10^6 cm^{-2} s^{-1}$.

Figures (8)

• Figure 1: The three distinct energy scales and their evolution with time: the energy of the relativistic axions, the energy of the thermal Standard Model plasma, and the centre of mass energy for scattering between the axions and the thermal plasma. These can be separated by several orders of magnitude.
• Figure 2: Feynman diagrams contributing to $a + \gamma \to q\bar{q}$. The third diagram involves an additional factor of $\alpha_{EM}$ and so we neglect it.
• Figure 3: Energy deposition histories $\epsilon_a(t)$ for axion inelastic scattering to bottom quarks (solid lines) as compared to the best-fit profile of $\epsilon_X(t)$ for a decaying particle species (dashed lines). For both solid curves, $f_a = 10^9$ GeV and $\Delta N_{eff} = 0.57$. In blue, $m_{\Phi}=5\cdot 10^6$ GeV, and in green $m_{\Phi}=5\cdot 10^7$ GeV.
• Figure 4: Conservative bounds on the axion decay constant as a function of the initial axion energy obtained by considering the effects of the decay channels to $b\bar{b}$ (solid), $c \bar{c}$ (dashed), and $s \bar{s}$ (dot-dashed) separately. The red, black and green curves correspond to $\Delta N_{eff} = 0.1, 0.5$ and $1$, respectively, and the areas below the curves are excluded from the constraints from Jedamzik:2006xz due to overproduction of $^4$He during BBN.
• Figure 5: Dark matter production from axion scattering. Here the axion scatters off a gluon, leading to pair production of heavy squarks which then cascade decay to produce the LSP.