Axion dark matter: How to see it?

Abstract

The axion is a highly motivated elementary particle that could address two fundamental questions in physics—the strong charge-parity (CP) problem and the dark matter mystery. Experimental searches for this hypothetical particle started reaching theoretically interesting sensitivity levels, particularly in the micro–electron volt (gigahertz) region. They rely on microwave resonators in strong magnetic fields with signals read out by quantum noise limited amplifiers. Concurrently, there have been intensive experimental efforts to widen the search range by devising various techniques and to enhance sensitivities by implementing advanced technologies. These orthogonal approaches will enable us to explore most of the parameter space for axions and axion-like particles within the next decades, with the 1- to 25-gigahertz frequency range to be conquered well within the first decade. We review the experimental aspects of axion physics and discuss the past, present, and future of the direct search programs.

General relativity effects in precision spin experimental tests of fundamental symmetries

“…The question of the lower limit on the axion mass remains open. In the focus of our discussion will be the minimal axion model with the Weinberg relation (2.24) between the axion mass and the nucleon coupling constant, and we do not dwell on more speculative axion-like particles, for example, in supersymmetric models, see reviews [58,113,380,[386][387][388]. Quite naturally, in the U (1) P Q current, in addition to the chromodynamic anomaly, there is also an electromagnetic anomaly which generates, by analogy with the axion-gluon L a (2.22), the axion-photon interaction…”

“…CAPP [395,401,402], and RADES [403] is beyond the scope of this article, and we refer readers to the detailed discussion of the issue and planned new experiments in the reviews [57,58,113,380,[386][387][388]404]. Still another application of the direct and inverse Primakoff effect is to the "shining the laser light through the wall" approach [394], when an intermediate axion is produced in a magnetic field by the Primakoff mechanism, then penetrates through a wall opaque to the light, and subsequently regenerates back into a photon in the magnetic field [405].…”

Propagating Quantum Microwaves: Towards Applications in Communication and Sensing

“…The dark photon, on the other hand, is constrained by observation to have only extremely weak coupling with electromagnetism and axion haloscopes can therefore be used to search for dark photons, as well. Haloscopes have been used to search for axionic dark matter in the 1.8-24 µeV mass range [192][193][194], and the results have also been applied to constrain dark photons in this mass range [195].…”

Structure of Axion Miniclusters

“…The axion is a theoretical elementary particle that can resolve the charge-parity (CP) problem [1] of quantum chromodynamics (QCD) [2][3][4], and provide a candidate to explain the observed [5] cosmological dark matter (DM) [6][7][8]. Axion DM is increasingly of interest both theoretically and experimentally [9,10], with a number of constraints on its allowed properties [11][12][13].…”