Cavity Microwave Searches for Cosmological Axions

TL;DR

This paper reviews microwave cavity experiments for detecting dark matter axions, discussing detection strategies, experimental results, and future challenges in improving sensitivity to axion signals.

Contribution

It provides a comprehensive overview of experimental techniques, results, and future directions for microwave cavity searches for axions, highlighting recent sensitivity achievements.

Findings

Experiments have reached sensitivity to exclude some axion models.

Two detection strategies are compared: wave-based and particle-based.

Future plans aim to improve sensitivity and explore wider axion mass ranges.

Abstract

This chapter will cover the search for dark matter axions based on microwave cavity experiments proposed by Pierre Sikivie. The topic begins with a brief overview of halo dark matter and the axion as a candidate. The principle of resonant conversion of axions in an external magnetic field will be described as well as practical considerations in optimizing the experiment as a signal-to-noise problem. A major focus of the lecture will be the two complementary strategies for ultra-low noise detection of the microwave photons - the ``photon-as-wave'' approach (i.e. conventional heterojunction amplifiers and soon to be quantum-limited SQUID devices), and ``photon-as-particle'' (i.e. Rydberg-atom single-quantum detection). Experimental results will be presented; these experiments have already reached well into the range of sensitivity to exclude plausible axion models, for limited ranges of mass. The section will conclude with a discussion of future plans and challenges for the microwave cavity experiment.