ARIADNE
The axion is a light, weakly-interacting particle originally hypothesized to explain the lack of observed CP-violating effects in quantum chromodynamics (the "strong CP problem"). Axions and axion-like particles are also common in theories of physics beyond the standard model, where they can mediate exotic, spin-dependent interactions. The axion has emerged as a leading candidate for dark matter, motivating a variety of experimental searches.
Axion searches fall into several categories depending on the axion source and coupling. For example, the well-known haloscope experiments, such as ADMX, search for dark matter (cosmic) axions via their coupling to photons: the signal is the conversion of an axion to photons in a microwave cavity immersed in a strong magnetic field.
The axion mass range is commonly thought to be limited, by astrophysical observations, to a "window" between about 10 μeV and 10 meV. Haloscope experiments are very sensitive to the lower end of this range, below 1 meV.
ARIADNE (Axion Resonant InterAction DetectioN Experiment) is sensitive to axions produced in the laboratory via the axion coupling to nuclear matter. The signal is a novel spin-dependent interaction: an induced magnetization from a non-magnetic source. ARIADNE is a "tabletop" NMR-type experiment which searches for the magnetization of a small (~ 1 mm3) sample of cryogenic, polarized helium-3 atoms as a dense, non-magnetic source mass is modulated in close proximity. The source consists of a tungsten sprocket, rotated so that its teeth subtend the sample at a rate adjusted to match the nuclear spin precession frequency. The signal is sensed with a SQUID magnetometer. The projected sensitivity of this resonant approach improves on previous magnetometry experiments by several orders of magnitude.
By sourcing the axion locally in the lab, ARIADNE complements the haloscope experiments by constraining axions independently of the cosmic axion abundance. It also has sensitivity above 1 meV, at the upper end of the mass window.
ARIADNE is an international collaboration of about 25 scientists from 8 institutes, based at Northwestern University. The UIUC group is responsible for the source mass.