Four decades after its prediction, the axion remains the most compelling solution to the Strong-CP problem and a well-motivated dark matter candidate, inspiring a host of elegant and ultrasensitive experiments based on axion-photon mixing. This report reviews the experimental situation on several fronts. The microwave cavity experiment is making excellent progress in the search for dark matter axions in the microelectronvolt range and may be plausibly extended up to 100 µeV. Within the past several years however, it has been realized that axions are pervasive throughout string theories, but with masses that fall naturally in the nanoelectronvolt range, for which a NMR-based search is under development. Searches for axions emitted from the Sun's burning core, and purely laboratory experiments based on photon regeneration have both made great strides in recent years, with ambitious projects proposed for the coming decade. Each of these campaigns has pushed the state of the art in technology, enabling large gains in sensitivity and mass reach. Furthermore each modality has also been exploited to search for more generalized axion-like particles, that will also be discussed in this report. We are hopeful, even optimistic, that the next review of the subject will concern the discovery of the axion, its properties, and its exploitation as a probe of early universe cosmology and structure formation.

We report on the first results from a new microwave cavity search for dark matter axions with masses above 20  μeV. We exclude axion models with two-photon coupling g_{aγγ}≳2×10^{-14}  GeV^{-1} over the range 23.55

We report on the results from a search for dark matter axions with the HAYSTAC experiment using a microwave cavity detector at frequencies between 5.6 and 5.8 GHz. We exclude axion models with two photon coupling g aγγ ≳ 2 × 10 −14 GeV −1 , a factor of 2.7 above the benchmark KSVZ model over the mass range 23.15 < m a < 24.0 μeV. This doubles the range reported in our previous paper. We achieve a nearquantum-limited sensitivity by operating at a temperature T < hν=2k B and incorporating a Josephson parametric amplifier (JPA), with improvements in the cooling of the cavity further reducing the experiment's system noise temperature to only twice the standard quantum limit at its operational frequency, an order of magnitude better than any other dark matter microwave cavity experiment to date. This result concludes the first phase of the HAYSTAC program utilizing a conventional copper cavity and a single JPA.

The axion+ hypothetical elementary particle, emerged from a compelling solution to the Strong-CP problem in &CD. Subsequently, the axion wee recognized to be a good Cold Dark Matter candidate. Although dark matter axions have only feeble couplings to matter end radiation, extremely sensitive searches are underway around the world. We review the state of experiments searching for the axion through its coupling to photons. Besides answering an outstanding question in particle physics, the discovery of dark matter exions may provide a detailed history of the formation of our Milky Way galaxy through the structure of the exion signal.We dedicate this review to the memory of Professor David N Schrannn, who commissioned this work but whose untimely death prevented him from seeing its completion He was an extraordinary figure in modern astrophysics and cosmology; his loss to the field is incalculable. Most importantly, he will be re membered as a wonderful human being, a great friend and a model to younger scientists.

We describe in detail the analysis procedure used to derive the first limits from the Haloscope at Yale Sensitive to Axion CDM (HAYSTAC), a microwave cavity search for cold dark matter (CDM) axions with masses above 20 µeV. We have introduced several significant innovations to the axion search analysis pioneered by the Axion Dark Matter eXperiment (ADMX), including optimal filtering of the individual power spectra that constitute the axion search dataset and a consistent maximum likelihood procedure for combining and rebinning these spectra. These innovations enable us to obtain the axion-photon coupling |gγ| excluded at any desired confidence level directly from the statistics of the combined data.