A Cosmic Window on the Dark Axion Portal

Author:

Heejoung Hong, Ui Min, Minho Son, Tevong You

Keyword:

High Energy Physics - Phenomenology, High Energy Physics - Phenomenology (hep-ph)

journal:

KCL-PH-TH/2023-44

date:

2023-10-29 16:00:00

Abstract

Axions and dark photons are common in many extensions of the Standard Model. The dark axion portal -- an axion coupling to the dark photon and photon -- can significantly modify their phenomenology. We study the cosmological constraints on the dark axion portal from Cosmic Microwave Background (CMB) bounds on the energy density of dark radiation, $\Delta N_\text{eff}$. By computing the axion-photon-dark photon collision terms and solving the Boltzmann equations including their effects, we find that light axions are generally more constrained by $\Delta N_\text{eff}$ than from supernova cooling or collider experiments. However, with dark photons at the MeV scale, a window of parameter space is opened up above the supernova limits and below the experimental exclusion, allowing for axion decay constants as low as $f_a \sim 10^4$ GeV. This region also modifies indirectly the neutrino energy density, thus relaxing the cosmological upper bound on the sum of neutrino masses. Future CMB measurements could detect a signal or close this open window on the dark axion portal.

PDF: A Cosmic Window on the Dark Axion Portal.pdf