No Arabic abstract
The axion, originated from the Peccei-Quinn mechanism proposed to solve the strong-CP problem, is a well motivated and popular dark matter candidate. Experimental searches for this hypothetical particle are starting to reach theoretically interesting sensitivity levels. However, only a small fraction of the allowed parameter space has been explored so far, mostly in the $mu$eV (GHz) region, relying on large volume solenoid magnetic fields and microwave resonators with signals read out by quantum noise limited amplifiers. There have been intensive experimental efforts to widen the search range by devising various techniques as well as to enhance sensitivities by implementing advanced technologies. The developments and improvements in these orthogonal approaches will enable us to explore most of the parameter space of the axion and axion-like particles within the next five to ten years. We review the experimental aspects of axion physics and discuss the past, present and future of the individual search programs.
A network of synchronized detectors can increase the likelihood of discovering the QCD axion, within the Axion Quark Nugget (AQN) dark matter model. A similar network can also discriminate the X-rays emitted by the AQNs from the background signal. These networks can provide information on the directionality of the dark matter flux (if any), as well as its velocity distribution, and can therefore test the Standard Halo Model. We show that the optimal configuration to detect AQN-induced axions is a triangular network of stations 100 km apart. For X-rays, the optimal network is an array of tetrahedral units.
The axion has emerged in recent years as a leading particle candidate to provide the mysterious dark matter in the cosmos, as we review here for a general scientific audience. We describe first the historical roots of the axion in the Standard Model of particle physics and the problem of charge-parity invariance of the strong nuclear force. We then discuss how the axion emerges as a dark matter candidate, and how it is produced in the early Universe. The symmetry properties of the axion dictate the form of its interactions with ordinary matter. Astrophysical considerations restrict the particle mass and interaction strengths to a limited range, which facilitates the planning of experiments to detect the axion. A companion review discusses the exciting prospect that the axion could indeed be detected in the near term in the laboratory.
We discuss the possibility to predict the QCD axion mass in the context of grand unified theories. We investigate the implementation of the DFSZ mechanism in the context of renormalizable SU(5) theories. In the simplest theory, the axion mass can be predicted with good precision in the range $m_a = (2-16)$ neV, and there is a strong correlation between the predictions for the axion mass and proton decay rates. In this context, we predict an upper bound for the proton decay channels with antineutrinos, $tau(pto K^+ bar{ u}) lesssim 4 times 10^{37} text{ yr}$ and $tau(p to pi^+ bar{ u}) lesssim 2 times 10^{36}text{ yr}$. This theory can be considered as the minimal realistic grand unified theory with the DFSZ mechanism and it can be fully tested by proton decay and axion experiments.
We propose a new strategy to search for dark matter axions in the mass range of 40--400 $mu$eV by introducing dielectric haloscopes, which consist of dielectric disks placed in a magnetic field. The changing dielectric media cause discontinuities in the axion-induced electric field, leading to the generation of propagating electromagnetic waves to satisfy the continuity requirements at the interfaces. Large-area disks with adjustable distances boost the microwave signal (10--100 GHz) to an observable level and allow one to scan over a broad axion mass range. A sensitivity to QCD axion models is conceivable with 80 disks of 1 m$^2$ area contained in a $10$ Tesla field.
We propose an approach to search for axion dark matter with a specially designed superconducting radio frequency cavity, targeting axions with masses $m_a lesssim 10^{-6} text{ eV}$. Our approach exploits axion-induced transitions between nearly degenerate resonant modes of frequency $sim$ GHz. A scan over axion mass is achieved by varying the frequency splitting between the two modes. Compared to traditional approaches, this allows for parametrically enhanced signal power for axions lighter than a GHz. The projected sensitivity covers unexplored parameter space for QCD axion dark matter for $10^{-8} text{ eV} lesssim m_a lesssim10^{-6} text{ eV}$ and axion-like particle dark matter as light as $m_a sim 10^{-14} text{ eV}$.