Building a dark matter radio
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Craig Hogan
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Craig Hogan, head of the Center for Particle Astrophysics, wrote this column.
You may have heard lately that the famous cosmic dark matter — the mysterious new kind of stuff that makes up most of the gravitational mass of the universe — may not, in fact, be completely dark, but may actually emit small amounts of light. That would be very exciting, because we might detect the light and use it to help figure out what the stuff is made of.
For example, the Fermi Gamma-Ray Space Telescope detects light, in the form of photons from the center of our galaxy, that may be caused by massive dark matter particles annihilating each other. Such high-energy photons can be created if the individual dark matter particles themselves are massive— much more massive than any known stable particle.
But it's also possible that the dark matter particles have extraordinarily low mass — even smaller than the tiny masses associated with neutrinos. In that case, the light emitted by dark matter, if any, would not show up as high-energy gamma rays; instead, it would show up as radio waves. Indeed, even the dark matter itself acts more like a coherent oscillating wave field than a collection of individual particles. In this situation, the best way to search for them may not be a traditional particle detector but a receiver more like a radio.
A leading candidate for this kind of dark matter is called an axion. The existence of such a field was predicted long ago, not from a need for dark matter, but as a way to explain why strong interactions (the quantum chromodynamics of the Standard Model that control the structure of atomic nuclei) do not appear to distinguish the past from the future as the other interactions do. In standard cosmology, if such a particle has a low mass, roughly in the microwave range of radio frequencies, it could be produced in sufficient abundance to be some, or even all, of the cosmic dark matter.
If so, we might find them in the laboratory. It turns out that if cosmic axions from our galaxy pass through a strong magnet, they give off a small amount of radio light at exactly the frequency corresponding to their tiny mass. To detect them, we want to build a radio tuned to that mass. The radio in this case uses a highly resonant cavity, similar to those that Fermilab uses all the time to accelerate particles. We don't know the mass of the axion exactly, so to search for the axion, we have to tune the radio — the cavity — until we get a signal.
The Axion Dark Matter Experiment has started a search like this at the University of Washington. (Because the experiment is not sensitive to cosmic rays, the actual apparatus does not have to be deep underground, but is on campus.) The tuning starts at low frequencies, searching for axions of relatively low mass, where it can use relatively large cavities. But there is a long way to go: Theory provides only a rough guess about the mass of the axion, and nobody yet knows exactly how to build smaller cavities that can efficiently search for higher-mass axions.
Fermilab scientists and engineers are planning to make unique contributions to state-of-the-art higher-frequency cavity designs for the higher-mass search, drawing on their years of experience with radio-frequency cavities in accelerators. This unique fusion of accelerator science and dark matter science is an exciting example of the synergy that happens at Fermilab.
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