Abstract
Light axion-like particles occur in many theories of beyond-Standard-Model physics, and may make up some or all of the universe's dark matter. One of the ways they can couple to the Standard Model is through the electromagnetic $F_{\mu\nu} \tilde F^{\mu\nu}$ portal, and there is a broad experimental program, covering many decades in mass range, aiming to search for axion dark matter via this coupling. In this paper, we derive limits on the absorbed power, and coupling sensitivity, for a broad class of such searches. We find that standard techniques, such as resonant cavities and dielectric haloscopes, can achieve O(1)-optimal axion-mass-averaged signal powers, for given volume and magnetic field. For low-mass (frequency $\ll$ GHz) axions, experiments using static background magnetic fields generally have suppressed sensitivity - we discuss the physics of this limitation, and propose experimental methods to avoid it, such as microwave up-conversion experiments. We also comment on the detection of other forms of dark matter, including dark photons, as well as the detection of relativistic hidden sector particles.
Highlights
Axionlike particles, in particular the QCD axion, are a well-motivated dark matter (DM) candidate
We will focus on the aFμνFμν axion-photon-photon coupling, and address the sensitivity limits on such experiments— how small a DM-Standard Model (SM) coupling could we possibly detect, given the dimensions, timescales, sensors etc. available? We choose the aFμνFμν coupling partly because, for a generic QCD axion, this coupling must lie within a fairly narrow range [4,10]; it is a generic feature of many other axionlike-particle models [11]
For low-mass axions, we review why static-background-field experiments generally have suppressed sensitivity, and point out that this suppression can be alleviated in a number of ways, potentially motivating new experimental concepts
Summary
In particular the QCD axion, are a well-motivated dark matter (DM) candidate. We choose the aFμνFμν coupling partly because, for a generic QCD axion, this coupling must lie within a fairly narrow (logarithmic) range [4,10]; it is a generic feature of many other axionlike-particle models [11]. It represents a analyzed example of the kind of sensitivity limits we are interested in. For low-mass (frequency ≪ GHz) axions, we review why static-background-field experiments generally have suppressed sensitivity (compared to their scaling at higher frequencies), and point out that this suppression can be alleviated in a number of ways, potentially motivating new experimental concepts. We comment on some of these extensions later in the paper, and in the Conclusions
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