Dark Matter

This May Be the Experiment That Finally Discovers the Dark Matter Particle

“If dark matter axions exist within the frequency band we will be probing for the next few years, then it’s only a matter of time before we find them.”

Daniel Oberhaus

New results from the Axion Dark Matter Experiment (ADMX) at the University of Washington suggest that it is now well-tuned enough to detect axions, a theoretical low-mass particle that many physicists believe may account for dark matter.

The ADMX is over 20 years old and first came online at the Lawrence Livermore National Laboratory in 1995. In 2010, it was moved to the University of Washington where it has been in the process of being upgraded ever since. As detailed this week in Physical Review Letters, the ADMX is finally sensitive enough to register the weak interactions caused by the theoretical axion particle.

“This experiment heralds a new era of ultrasensitive probes of low mass axionic dark matter,” the researchers wrote in the paper.

This is where the ADMX experiment is located at the University of Washington. The actual detector is underground. Image: Mark Stone/University of Washington

The ADMX is technically known as an axion haloscope, which University of Washington physicist Gray Rybka likened to a large radio receiver.

“If you think of an AM radio, it’s exactly like that,” Rybka said in a statement. “We’ve built a radio that looks for a radio station, but we don’t know its frequency. We turn the knob slowly while listening. Ideally we will hear a tone when the frequency is right.”

The ADMX is basically a large refrigerator that is cooled to around -460 degrees Fahrenheit and contains a strong magnetic field generated by a large superconducting magnet that is 800 times stronger than the fields generated by magnets you put on your refrigerator at home. Receivers in the system are waiting to detect the telltale electromagnetic radiation produced when an axion passing through the system interacts with the magnetic field and converts its energy into microwave photons.

Since axions are only theoretical at this point and might not even exist, Rybka and his colleagues essentially need to work their way through a spectrum of millions of possible wavelengths that might be produced during the axion’s interaction with the magnetic field. Until now, the problem was making a system sensitive enough to detect the incredibly weak electromagnetic radiation produced by this conversion.

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The ADMX was limited in its sensitivity due to ambient heat and electromagnetic ‘noise’ produced by the system’s transistor amplifiers, which basically boost the strength of an electronic signal. The refrigeration system solved the thermal radiation problem, and to reduce the noise Rybka and his colleagues swapped the transistor amplifiers for custom “quantum limited amplifiers” developed by UC Berkeley physicist John Clarke. These amplifiers use superconducting materials instead of regular conductors to reduce the amount of electromagnetic noise produced by the system by a factor of 30.

“The initial versions of this experiment, with transistor-based amplifiers would have taken hundreds of years to scan the most likely range of axion masses,” said Gianpaolo Carosi, a physicist at Lawrence Livermore National Laboratory also involved with the ADMX experiments. “With the new superconducting detectors we can search the same range on timescales of only a few years.”

Based on cosmological observations and measurements of galaxy movement, physicists have concluded that there ought to be way more mass in the universe than we can actually see. This missing mass is called dark matter and is thought to make up nearly 70 percent of the universe compared with the roughly 5 percent we can actually see (the rest is thought to be dark energy). It’s generally thought that dark matter is a particle (although other physicists have sought alternative explanations, such as revising the theory of gravity), but so far no one has been able to find it.

Some leading particle candidates for dark matter include axions, neutrinos, and weakly interacting massive particles (WIMPs), but so far every experiment that tries to find them has come up empty handed. In this respect, the unprecedented sensitivity of ADMX may be just the breakthrough that is needed to finally detect axions. If no axion is found, researchers may have to fundamentally revise some of their theories of dark matter—but the ADMX researchers remain confident that they’re on the cusp of a breakthrough.

“We have the sensitivity, and have a very good shot at finding the axion,” University of Washington physicist Leslie Rosenberg said. “No new technology is needed. We don’t need a miracle anymore, we just need time.”