Abstract
Existing searches for cosmic axions relics have relied heavily on the axion being non-relativistic and making up dark matter. However, light axions can be copiously produced in the early Universe and remain relativistic today, thereby constituting a Cosmic $\textit{axion}$ Background (C$a$B). As prototypical examples of axion sources, we consider thermal production, dark-matter decay, parametric resonance, and topological defect decay. Each of these has a characteristic frequency spectrum that can be searched for in axion direct detection experiments. We focus on the axion-photon coupling and study the sensitivity of current and future versions of ADMX, HAYSTAC, DMRadio, and ABRACADABRA to a C$a$B, finding that the data collected in search of dark matter can be repurposed to detect axion energy densities well below limits set by measurements of the energy budget of the Universe. In this way, direct detection of relativistic relics offers a powerful new opportunity to learn about the early Universe and, potentially, discover the axion.
Highlights
The existence of an axion with mass well below the electroweak scale could resolve the strong CP puzzle [1,2,3,4], and is entirely in line with UV expectations given the ubiquity of axions in string theory, where they arise from the deconstruction of extra-dimensional gauge fields [5,6,7]
We focus on the axion-photon coupling and study the sensitivity of current and future versions of ADMX, HAYSTAC, DMRadio, and ABRACADABRA to a Cosmic axion Background (CaB), finding that the data collected in search of dark matter can be repurposed to detect axion energy densities well below limits set by measurements of the energy budget of the Universe
This motivates scenarios where the CaB is produced in the late Universe, where ρa can be larger than ργ, and in particular we discuss dark matter decaying to two relativistic axions, χ → aa, with ma ≪ mχ=2
Summary
The existence of an axion with mass well below the electroweak scale could resolve the strong CP puzzle [1,2,3,4], and is entirely in line with UV expectations given the ubiquity of axions in string theory, where they arise from the deconstruction of extra-dimensional gauge fields [5,6,7]. We demonstrate that DMRadio will be sensitive to scenarios where ΩaðωÞ ≲ 5 × 10−5, roughly corresponding to ρa < ργ (a target cavity instruments may reach in the future) and further the H0 preferred parameter space discussed earlier The sensitivity to such small energy densities suggests that the data collected by axion direct detection experiments can be repurposed to probe a range of cosmic sources of axions beyond nonrelativistic dark matter. While ADMX is close, no existing instrument is currently sensitive to cosmological relics This motivates scenarios where the CaB is produced in the late Universe, where ρa can be larger than ργ, and in particular we discuss dark matter decaying to two relativistic axions, χ → aa, with ma ≪ mχ=2.
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