Abstract
The axion is a dark matter candidate arising from the spontaneous breaking of the Peccei–Quinn symmetry, introduced to solve the strong CP problem. It has been shown that radio/microwave radiation sent out to space is backscattered in the presence of axion dark matter due to stimulated axion decay. This backscattering is a feeble and narrow echo signal centered at an angular frequency very close to one-half of the axion mass. In this article, we summarize all the relevant results found so far, including analytical formulas for the echo signal, as well as sensitivity prospects for possible near-future experiments.
Highlights
The nature of dark matter is one of the most intriguing puzzles of modern science.The lack of signals from thermally produced WIMP dark matter suggests the exploration of new routes
The QCD axion is a hypothetical particle that was originally postulated as a solution to the strong CP problem of the Standard Model of particle physics, i.e., the puzzle of why the strong interactions conserve the P and CP symmetries [3]
We presented a description of the echo method for axion dark matter detection proposed in [21]
Summary
The nature of dark matter is one of the most intriguing puzzles of modern science. The lack of signals from thermally produced WIMP dark matter suggests the exploration of new routes. One possibility is hypothetical particles produced in the early universe by nonthermal means Such is the case for sterile neutrinos [1], dark photons [2], and QCD axions [3,4,5] (or axion-like particles (ALPs) [6]), which have became very popular in the last few years. The QCD axion and ALPs are leading dark matter candidates in the sub-eV mass range [7,8,9,10] Their nonthermal production mechanisms include the realignment and decay of topological defects. The atmosphere’s transparency mainly constrains the frequency range for which the echo method is realistic At sea level, this corresponds approximately to the range 30 MHz–30 GHz, equivalent to axion masses between 2.5 × 10−7 eV and 2.5 × 10−4 eV.
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