Dark Matter Eludes

Dark matter continues to be in good form and elusive as ever. Dark matter is the stuff that makes up most of the mass of the universe, but it doesn’t interact much with regular matter, if at all. Earlier this week the LUX experiment posted a paper showing their first results – the strongest constraints yet on how much dark matter doesn’t interact. At this point physicists have no direct detection at all of dark matter so no reasonable avenue of discovery is ignored. Several floors below me right now (yes, literally, as I sit in the physics building on the University of Washington campus) is a detector that is gunning at dark matter from an entirely different direction. Dark Matter’s dark horse is axions. Axions are hypothetical particles that would solve the strong CP problem in physics and they could be just the right stuff to be dark matter, that is if they exist at all.

The ADMX magnet in the ground photo courtesy of Christian BoutanThe challenge is to detect it. In principle, the task is simple. As well as feeling the strong force, axions should also interact with the electromagnetic force responsible for light and other radiation. When an axion passes through a magnetic field, it should sometimes reveal itself by turning into a photon. Given the axion’s tiny mass, the photons should be low-energy radio waves. So to hunt for axions, ADMX physicists search for radio signals of a fixed frequency emanating from a strong magnetic field.


In practice, the experiment requires a herculean effort. The chances that an axion will turn into a photon are tiny, so to have a shot at producing a signal, researchers must use a huge magnet. ADMX employs a 6-tonne superconducting coil a meter long and half a meter wide that produces a field 152,000 times as strong as Earth’s field. To further enhance the signal, researchers slide inside the magnet a cylindrical “resonant cavity,” in which radio waves of a specific frequency will resonate just as sound of a specific pitch resonates in an organ pipe. The cavity should amplify the production of photons 100,000-fold, and its resonant frequency can be changed by moving metal or insulating rods within it.

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