There are two audiences for our experimental results. Particle physicists would like to know what axion models have been explored and what axion masses have been studied. Astrophysicists would like to know what fraction of our local dark matter halo density has been probed, and whether this density can then be made up of axions. Currently, we have not yet found an axion signature; so we provide limits as to a) the coupling strength of axions to photons, and b) the halo density that can be made up of axions.
There are two benchmark models that we consider. One is the KSVZ axion, and the other is the DFSZ axion (in reality, there are a host of different KSVZ axion models, as well as a host of different DFSZ axion models; we choose one from each family).
The figure below is a particle physics result. We assume that the dark matter halo density is entirely made up of KSVZ axions, and then search for microwave power. If we do not find an axion signal with this expected power, then we exclude the searched region and then move on. To greater than 90% confidence, we did not find a KSVZ axion in the following mass (frequency) region. The exclusion plot is shown below.
The figure below is the astrophysics result. Assuming a particle model, we determine what dark matter halo density composed of axions would be necessary to be seen in our experiment. If we do not find an axion signal given our experiment sensitivity, then we exclude a dark matter halo density in this mass (frequency) region and then search elsewhere. To greater than 90% confidence, we did not find a KSVZ axion with a realistic dark matter halo density (about half a proton mass per cc), and can only weakly exclude a dark matter halo density composed of DFSZ axions. The exclusion plot is shown below.
Our experiment is the world's first (and currently, only) experiment to probe realistic dark matter axion models with sufficient sensitivity. The figure below shows the sensitivity of many different experiment families. Should the dark matter in our galaxy consist of axions, the axion mass is expected to be in the range 10-6 eV to 10-4 eV. The horizontal axis of the figure shows axion mass, while the vertical axis shows coupling strength. There are two lines marked KSVZ and DFSZ, which represent two different benchmark axion models. In the expected mass region, the microwave cavity experiments are most sensitive. The sliver at the bottom which has probed the KSVZ axion model is us.
To conclude: dark matter axions are difficult to detect. Most experiments do not have sufficient sensitivity to find realistic axions. However, microwave cavity experiments actually do, and are therefore the most promising family of experiments. Of these, the ADMX experiment is the only one that is sensitive to realistic dark matter axions. Our search continues ...