Axion Dark Matter

Axions and axion-like particles (ALPs) are a compelling class of theoretical pseudo-scalar particles to serve as solutions to the dark matter problem as well as lingering problems in particle physics. As candidates for the dark matter, axions aid in the formation of cosmological structure and are expected to permeate the Galaxy and our solar system. Our groups id interested in the role of axion dark matter in structure formation and methods for its detection, using both direct and indirect techniques.

Direct Detection

Evidence for the existence of dark matter has so far been limited to its interactions via gravity. More direct probes of the dark matter stand to tell us a great deal more about its composition, including its mass, spin, and relation to the Standard Model. Our group is currently involved with the axion direct detection searches ADMX and is the primary institution of the TOORAD collaboration. These searches function through an inverse-Primakov process, converting dark matter axions into microwaves in a resonant cavity threaded by a strong magnetic field. ADMX is currently taking data, searching for axions in the micro- to milli-electron-volt range and operating at landmark DFSZ sensitivity. TOORAD is currently in design, and will use revolutionary topological anti-ferromagnetic materials to probe higher axion masses.

Our group activities in direct detection involve design, discovery potential forecasting, operation, and data analysis. Our work in direct detection is also complemented by our structure formation research, which can provide models of potential axion signals to searches. See the page on Structure formation for more details.

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Statistical Analysis and Axion Phenomenology

There exist numerous experimental searches (e.g. this paper) and astrophysical and cosmological probes for QCD axions and axionlike particles. It is important to study the observables associated with different axion models to allow for a consistent combination of all of the experimental data into a global statistical analysis.

Doing so, we can extract the maximum amount of information from the avialable data, constrain the axion model parameter space, and provide a guidance for future activities by identifying the most promising regions of parameter space. Since there are many ways to realise axion models -- even in case of the QCD axion -- the combination of complementary information from different experiments is crucial in determining the most likely nature of the axion.

In statistical terms, we perform parameter estimation for a given axion model and a model comparison between different axions models. These tasks can be addressed using either classical or Bayesian methods, which usually requires the use of high-performance computing and ideally a modern and flexible software suite that links models and data with statistical tools. An example for such a toolkit is the publicly available code GAMBIT, the Global And Modular BSM Inference Tool.

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Cosmological Probes

The cosmos provides numerous extraterrestrial observables which can act as sensitive probes of the constituents and the history of the universe. Notable examples are the Cosmic Microwave Background (CMB), the large scale structure, the abundance of light elements or the expansion rate. Very often these observables probe conditions which are out of reach in human-built experiments, like for example colliders. Therefore, they provide unique and complementary insights to the fundamental questions in physics.

We are especially interested in the information on the nature of dark matter that can be extracted from these probes. In particular, axions and axion-like particles (ALPs) often leave a specific imprint on many cosmological probes. This gives us the opportunity to constrain axion dark matter by identifying and modeling these imprints.

One type of those probes are upcoming 21 cm intensity mapping (IM) surveys of the late-time universe. To model the signal we exploited a semi-analytical halo model and included ALPs by comparison to massive neutrinos. Forecasts conducted with this model for future 21 cm IM surveys reveal an exciting picture: Strong constraints on the axion density are accessible, making it possible to test certain predictions by grand unified theories. These results also call for further refinement and calibration of the model with numerical simulations.

A different set of probes are galaxy surveys, which measure the position, the redshift, and the shape of a large number of galaxies. These surveys are sensitive to the large scale distribution of matter, and in particular to its largest fraction, dark matter, via a physical effect called "gravitational lensing". It occurs when the path of a light ray is modified by large amounts of mass according to the laws of general relativity. In galaxy surveys, gravitational lensing manifests itself as a variation in the shape and alignment of the recorded galaxies and is statistically measurable. The amount of gravitational lensing is correlated with the distribution of matter and hence allows for the reconstruction of the large scale structure of the universe. We compare the result to predictions from axion and ALP dark matter models, using again semi-analytical methods to infer the distribution of matter within our models. We expect to constrain the available parameter space significantly.
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Constraints on the axion density parameter for different axion masses from a combination of CMB temperature, polarisation, and lensing