Is Dark Matter Made of Axions?

Last week, when scientists at an Italian laboratory announced that unexpected blips in their detector could be from long-sought subatomic particles known as axions, their colleagues were cautiously optimistic: In physics, alleged detections of new particles often fade to insignificance as researchers gather more data. And there are other, more prosaic explanations for the blips. Conversely, the theoretical case for axions’ existence is compelling to many physicists. And the hypothetical particles are one of the leading candidates for dark matter, the mysterious substance that makes up the majority of the material universe. Confirming that axions are real would be a breakthrough for particle physics—and a discovery with far-reaching implications for our understanding of the universe’s composition and history.

The axion story begins in the 1970s, when physicists developing the Standard Model—the framework that describes the known particles and their interactions—noticed something odd about the strong nuclear force, which binds quarks together to form the protons and neutrons within the nuclei of atoms. This force somehow regulates the structure of neutrons to make them perfectly symmetrical. Put another way, although the neutron is neutral, the quarks within it carry charge—and for reasons unknown, this charge is spread out incredibly uniformly (at least to within one part in a billion, according to the latest measurements). In the language of particle physics, the neutron is said to have charge-parity (CP) symmetry: inverting all its charges from positive to negative, while also viewing its behavior in a mirror, would have no discernible effect. The question of why the particle has this arrangement became known as the “strong CP problem.”

Then, in 1977 Helen Quinn and the late Roberto Peccei, both then at Stanford University, proposed a solution: perhaps there is a hitherto unknown field that pervades all of space and suppresses the neutron’s asymmetries. Later, theoretical physicists Frank Wilczek and Steven Weinberg deduced that if the Standard Model were tweaked to allow such a field, it would imply the existence of a new particle, dubbed the axion. (Wilczek got the idea for the name from a brand of laundry detergent.) The axion would have no quantum mechanical “spin,” making it a boson. Its mass, though not zero, would be incredibly small.

Despite their vanishingly miniscule weight, axions would exist in such vast numbers that physicists soon realized they could account for much of the mass “missing” from the universe: Astronomical observations going back to the 1930s suggest that visible matter—galaxies, stars, planets, and so on—represents less than one sixth of the total mass of all matter in the cosmos, with dark matter making up the rest. The nature of this dark matter has been the subject of intense debate ever since.

“The axion actually makes a really good dark matter candidate,” says Peter Graham of Stanford. Beyond the expectation that the cosmos should be awash with the particles, axions would be naturally “dark,” meaning they would hardly interact with ordinary matter at all. “The universe likes to produce axions,” Graham says, “and it likes to produce them in such a way that they would act like the cold dark matter we know is out there.”

“Cold” is an important caveat: The axions that researchers purportedly detected with the XENON1T experiment at Italy’s Gran Sasso National Laboratory would probably have been produced inside our sun. They would be highly energetic and thus unlikely to be a dark matter component. Dark matter axions would have to be slow-moving, or cold, so they could clump together to gravitationally guide the evolution of galaxies—as dark matter is believed to do. Theorists suspect such axions may have been produced in the early universe. Moreover, because the processes thought to create cold axions may be related to the universe’s early growth spurt—an extraordinary ballooning in size known as inflation—finding and further studying these elusive particles could help physicists understand the very first moments following the big bang. Although the discovery of axions would not prove that inflation happened, Graham says, it would provide a valuable glimpse into the physics of that era. “For me, that’s the exciting thing about axions,” he adds.

Yet scientists are reacting with caution—including those on the XENON1T team. All they are sure of is that they have seen a surprisingly large number of “recoils” of electrons in the huge vat of liquid xenon that is the experiment’s heart. What made the electrons jump is open for debate. If subatomic particles called neutrinos have unforeseen magnetic properties, this arrangement could account for the observed results. Or the explanation could be more mundane: the xenon could merely be contaminated with tritium—a heavier form of hydrogen whose natural radiation could have muddied the signal seen at XENON1T. Additionally, the confidence level associated with the anomalous signal is only “3.5 sigma”—meaning there is a one in 5,000 chance the “signal” is actually just noise, the product of statistical fluctuations rather than genuine new physics. Those odds may sound good, but they are well below the one-in-3.5-million, or “five sigma,” standard traditionally linked to legitimate discoveries in particle physics.

Beyond accumulating more data and upgrading their experiment, the XENON1T researchers will look for any annual changes in the apparent signal. Solar axions should cause that signal to fluctuate as Earth orbits the sun. Meanwhile corroborating evidence could come from the Axion Dark Matter Experiment (ADMX) at the University of Washington or an experiment known as CAST (CERN Axion Solar Telescope) at CERN near Geneva. ADMX has already managed to place new constraints on the axion’s mass, and CAST has been hunting for solar axions since 2003.

If axions turn out to be real, it would be “a triumph of theoretical physics—to have made this kind of aesthetic argument, and then nature says, ‘Yup, that’s right,’” says Wilczek, who is based at the Massachusetts Institute of Technology and was a co-recipient of the 2004 Nobel Prize in Physics for his theoretical work on the strong nuclear force. The existence of axions, he says, would point to new physics beyond the Standard Model—something he and his colleagues have been anticipating for decades. New kinds of antennas could be built to look for axions created in the early universe, Wilczek suggests. If these axions can be successfully measured, it “would open up a new chapter in astronomy,” he adds, because the behavior of the particles could shed light on galaxy formation and “possibly other surprising things.”

While such developments would likely be Nobel-worthy, Wilczek is not clearing space on his shelf for a second medal. But if another Nobel were to come his way, he says, he “wouldn’t turn it down.”



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