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.
The Casimir force, which is the attraction of two uncharged material bodies due to modification of the zero-point energy associated with the electromagnetic modes in the space between them, has been measured with per cent-level accuracy in a number of recent experiments. A review of the theory of the Casimir force and its corrections for real materials and finite temperature are presented in this report. Applications of the theory to a number of practical problems are discussed.
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.
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