Hidden away in a remote, modest-looking North Campus nuclear physics lab is a treasure trove of extraordinary minds and remarkable devices. One of these is an apparatus powerful enough to make a decent bomb, magnetic enough to raise a tower of wrenches stacked end-to-end, and sensitive enough to hypothetically detect cellphone signals from Pluto.
But for the team of Axion Dark Matter eXperiment (ADMX) astrophysicists in charge of it, this device represents something much more exciting: the key to finally unlocking one of modern physics’ greatest mysteries, dark matter.
Dark matter explains a lot of the elusive why’s and how’s of our universe, from the gravitational interactions within and between galaxies to the fact that the matter we can see makes up only 4 percent of what the math says is actually out there — a “minority component,” as Leslie Rosenberg, professor of physics and the leading researcher behind ADMX, explained.
Dark energy is thought to make up roughly 75 percent of the universe, and dark matter the remaining 21 percent. It’s called “dark” because it neither emits nor absorbs radiation of any kind, something that makes it nearly impossible to definitively detect. This, of course, means that no one really knows what dark matter actually is.
The purpose of the ADMX’s device is to search for the axion, a hypothetical particle that many astrophysicists consider an extremely attractive candidate for dark matter’s identity. Other suggestions include WIMPS — weakly interacting massive particles — and the now-obsolete MACHOs — massive compact halo objects. Axions are about 500 million times lighter than electrons, and, assuming they exist, can only be detected when they decay into microwave photons — something that happens naturally once every 10^50 years.
Overcoming a time gap that huge obviously presents an equally large problem for anyone hunting axions. The ADMX team has managed to do it, using what to the untrained eye looks like nothing more than a gleaming, 11-foot-tall stainless-steel tube tucked underneath the belly of an ion accelerator in a shadowy corner of the lab. And though, at the moment, that’s nearly an apt description, in two months the tube will unite three pieces of technology that together form the heart of ADMX: a superconducting magnet, a resonant microwave cavity, and what graduate student Christian Boutan described as “the most sensitive microwave receiver on earth,” the Superconducting Quantum Interference Device (SQUID) amplifier.
A good way to visualize how all these parts work together is to imagine all the dark-matter axions in the universe as a giant swarm of fruit flies, and the magnetic field produced by the superconducting magnet as an electric fly-swatter. As the axion-flies pass through the magnetic field surrounding the microwave cavity, they encounter the fly-swatter, and zzt! — dead, in a sudden burst of light. Now, imagine that instead of leaving their bodies behind in the bottom of the cavity, the axion-flies actually turn into the light the collisions produce, and that different-sized flies turn into different-sized flashes of light.
If the size of the flash corresponds to the wavelength of the photon the axion decays into, and the wavelength of the photon in turn corresponds to the mass of the axion, then ADMX researchers can tune their microwave receiver to look for the flashes indicative of one predicted axion weight at a time, and thus systematically search all possible hiding places of the particle.
“We just keep looking,” said Andrew Wagner, an ADMX research associate. “It’s like having a flashlight — you know what you’re looking for is between here and here, and you just move the flashlight. That’s all you can do.”
The key to getting all of this to work is some serious refrigeration. Extreme cold keeps the magnet superconducting as well as eliminating background “thermal noise” emitted by various electronic processes, which muddles the SQUID amplifier’s detection of an axion/photon wavelength. The cooling process begins with the liquefaction of the cryogen (coolant), helium-3. The liquid helium is then transferred into the 11-foot cylinder, where it fills chambers created by smaller, nested cylinders, growing colder and colder until it reaches both the SQUID amplifier and the microwave cavity, which the superconducting magnet surrounds.
In previous versions of ADMX, the temperature at this point in the process was 1.2 K. The recent addition of a refrigeration component has made it possible to get it down even lower, to 300 mK; the ADMX team’s ultimate goal is to reach temperatures of 100 mK or less.
The resonant microwave cavity is a big, hollow metal tube; its purpose is to keep the axions-turned-microwave photons in place long enough for the SQUID’s antenna to detect them. In order for that to happen, the cavity needs to be “tuned” to the photon frequency the ADMX team is investigating. Wagner described it as “a pipe organ for photons.”
“At this point, the rest of the experiment’s just an AM radio, and you’re tuning very slowly over frequencies looking for the one that corresponds exactly to the axion mass,” explained Gray Rybka, research assistant professor. “If there’s a certain frequency with a whole lot more power than you expect, then that could be an axion.”
The real heart of the experiment is the SQUID amplifier — a device that could pick up a phone call from Pluto, were it not for the heat of the sky, Rybka said.
It’s about four times the area of a quarter, and its job is to both pick up and amp up the signal the photon produces, and then down-convert it to a frequency that can easily be read and saved.
“It’s a little more complicated than a hand-held radio,” Rybka said, indicating the shoulder-high cart full of equipment that the team uses to analyze the SQUID’s output.
“We’re really the only game in town when it comes to dark-matter axions,” Wagner said. “There aren’t any other experiments of this scale.” And together, the addition of the SQUID amplifier — which allows the entire range of possible axion masses to be scanned — and the refrigeration component that makes scanning of that magnitude possible, has upgraded ADMX to the level of a “definitive experiment”: an experiment designed to answer a question once and for all. This means that if dark-matter axions exist, whatever their mass, ADMX researchers will find them. And if they don’t?
“If we don’t, it’s disappointing, but it eliminates a very good candidate [for dark matter], so it would make people really stretch their heads and think, if not that, then what the hell could it be?” Wagner said. “It would make dark matter even more of a hot topic.”
ADMX is expected to start “listening” for axions again in January or February, once all the upgraded pieces come back together in that outwardly simple 11-foot metal tube. The whole world will be tuning in to hear their verdict.
Reach contributing writer Cara Skalisky at email@example.com. Twitter: @caraskalisky
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