Art by Lynn Dai. Photography by Max Watzky.
Deep within the winding corridors of Yale’s Wright Laboratory, a machine converses with the universe. Through the soft murmur of circuitry, the gentle hum of coolers, and the low drone of spinning motors, the machine calls out to the cosmos, waiting for a faint reply. It has repeated this routine daily for almost twelve years, shutting down only when a hurricane threatens its power source or when a part needs replacing. The machine does not mind the long wait—it stands resolute, working patiently and meticulously, and will keep searching for decades more if it must. Tune, scan, wait. Repeat.
This is the HAYSTAC experiment. At first glance, its day-to-day operations might seem like monotonous work. But for the Yale scientists who tend this machine, its work could not be more exciting. Without exaggeration, HAYSTAC is looking for what the scientists deem the most important kind of matter in the universe—a tiny, elusive particle called the axion. Although it was hypothesized nearly fifty years ago, the axion has recently enjoyed a resurgence of attention and research. Indeed, it may be scientists’ last, best hope to solve some of the most pressing problems in modern physics.
Now, HAYSTAC, which stands for the Haloscope at Yale Sensitive to Axion Dark Matter, is just one part of a massive institutional search for the axion at Wright Laboratory. Yale’s Axion Dark Matter group is enormous, spearheaded by six professors and comprising dozens of staff scientists, postdoctoral researchers, PhD students, and undergraduates—and that’s to say nothing of their many assistants and collaborators around the globe. Aside from HAYSTAC, the group is making rapid progress on two new initiatives, called ALPHA and RAY. ALPHA, which stands for the Axion Longitudinal Plasma Haloscope, is a new experiment to search for axions with greater speed, and RAY, which stands for Rydberg Atoms at Yale, is a technological effort to improve the efficiency of the axion-detecting instruments that underlie HAYSTAC and ALPHA.
Symmetry and Conservation
The story of the axion, like many other stories in fundamental physics, is one of symmetry. We all feel intuitively how beautiful symmetry can be, like the perfectly mirrored wings of a butterfly. But in physics, symmetries are beautiful for a different reason: they reveal something profound about the fabric of reality. When a system exhibits some kind of symmetry, it indicates that some physical quantity must be conserved. Imagine a hockey puck sliding across an ice rink. Now, imagine the rink magically shifts ten feet to the right. Nothing about the ice has changed—it still looks the same in every direction, a quality called translational symmetry, so something must be conserved. In this case, the symmetry of the space implies that the puck’s momentum remains constant. Even though the ice has moved, the puck keeps sliding along at the same speed and in the same direction as before.
But what happens when we see conservation without symmetry to match? This was exactly the problem facing particle theorists Roberto Peccei and Helen Quinn in the 1970s. They were exploring a symmetry called charge conjugation parity (CP) and its corresponding conservation law: specifically, if a particle’s charge and location in space is flipped, it should obey the laws of physics identically as before. Peccei and Quinn noticed that the existing theory actually broke this symmetry, but experiment after experiment repeatedly showed that the conservation law was preserved!
So how did Peccei and Quinn solve this discrepancy? They proposed the existence of a new particle that would dynamically cancel out the CP asymmetry. “This axion particle was added to the standard model to explain why there’s no CP violation. In the long history of physics, many particles have been introduced successfully based on observed symmetries and conservation laws,” said Steve Lamoreaux, the principal investigator of HAYSTAC.
A Dark Twist
At the same time Peccei and Quinn proposed their resolution to the CP problem, another problem was brewing in fundamental physics—the problem of dark matter. Stars and galaxies in space move according to the gravitational pull they exert on one another, which is proportional to their masses. But astronomers noticed that the motion of these objects was completely inconsistent with the masses they could see. “Basically, visible matter does not explain how these galaxies move. There seems to be some missing matter that has some gravitational effect, and this matter was called dark matter,” said Karsten Heeger, Director of Wright Laboratory.
Today, most physicists agree that dark matter is composed of tiny yet-undiscovered particles, perhaps many times smaller than protons, neutrons, or electrons. Dark matter must also interact very weakly with regular matter or light, since we cannot see or touch it. As our understanding of dark matter has grown over the decades, the list of viable particle candidates has shrunk precipitously, leaving few options on the table. What particle might actually fit the bill?
“The axion is the perfect particle, because it’s tiny and barely interacts with anything,” Lamoreaux said. The axion could not be a better candidate: it had the potential to resolve both the CP asymmetry problem and the paradox of dark matter, killing two birds with one stone. There was just one small problem: the same things that make the axion the perfect candidate for dark matter, its minute size and refusal to interact with other matter, also make it a nightmare to measure.
Today, physicists know of only one viable mechanism to observe the axion. In the presence of a strong magnetic field, an axion can convert into a photon, a tiny packet of light. But this process is incredibly rare, making the photon signal extremely hard to detect. “To get a sense for how much power we’d get, if you lit a match on the surface of the Earth […] that would be the energy rate for photons on the pupil of your eye if you’re on the surface of the Moon,” Lamoreaux said.
To complicate matters, physicists aren’t sure what frequency of light the axion would convert into. “There’s a huge parameter space, from ten microelectronvolts up to one hundred electronvolts,” Lamoreaux said. That’s a span of eight orders of magnitude—if the axion is real, actually finding the signal it emits would require an immense amount of grit and ingenuity. That’s where HAYSTAC, ALPHA, and RAY come in.
Loud and Clear?
HAYSTAC and ALPHA are effectively ultra-sensitive radios, searching through each possible axion frequency through a process of tuning. The radio receiver is a metal cavity exposed to an extremely strong magnet. The magnetic field converts axions into photons, but these photons are too dim to be observed directly. Instead, they must be amplified through resonance with the cavity. Think of plucking the strings on a violin: the shorter the string, the higher the pitch of the note. Similarly, adjusting the length of the cavity changes the frequency of light it amplifies—if the cavity is small, it resonates at a higher frequency, and if the cavity is large, it resonates at a lower frequency.
But even with this technique, searching over such a vast array of frequencies with only one device would take many, many years. “We can do a couple [frequencies] in a day, but it will take months to scan over a given frequency range,” said Claire Laffan (YC ’21), a PhD student working on ALPHA. And for higher frequencies, the scanning process runs even slower. “To resonantly enhance a very high frequency photon, you need a small cavity. However, the smaller the volume of the cavity, the fewer axions go in and out of it, and therefore there’s a lower probability you’ll detect anything,” Laffan said.
Today, the Yale axion team is focused on accelerating the search. The ALPHA team is experimenting with special synthetic materials in order to tune the resonant cavity more efficiently. “The cool thing about our new resonator is that its frequency range is not dependent on its volume, so we can make an arbitrarily large volume cavity while still being sensitive to these really high mass axions,” Laffan said. Meanwhile, the RAY team is engineering new, more sensitive detectors to measure the light from axion conversion. Their technology takes advantage of Rydberg atoms, a special class of matter that is extremely sensitive to photons. By measuring how the electrons in a Rydberg atom become energized when exposed to light, the RAY team can measure the effect of a single photon at a time. “Right now, we’re testing to see if our atoms are being transitioned to some other Rydberg state via the axion interaction,” said Tyler Johnson, a postdoctoral researcher working on RAY.
The Axion Alliance
However, despite their trailblazing innovations, the Yale team knows they can’t do it alone. They see their mission as aiding the worldwide scientific community. “One experiment is not going to do it in anybody’s lifetime. We need to have a harmonized effort with people working on different frequencies and all sharing technologies,” Lamoreaux said. “If we can demonstrate this proof of concept, it would change the way that other experiments work, and we could all scan the parameter space faster and look for axions in places that we haven’t before,” Laffan said.
And what if we actually find the axion? It would be a revolution in physics, potentially resolving the now age-old mysteries of CP symmetry and dark matter. “I never get my hopes up that it’s going to be there—it could be the WiFi signal, or someone walking in the laboratory,” Laffan said. But rather than mark the end of an era, the detection of the axion would mean a beautiful beginning for a brand new era of questions and discovery. “I think that it is one of the most human things we can do, to ask questions and try to find answers regardless of what those answers might tangibly give us […] I think building these experiments is a beautiful expression of our curiosity,” Laffan said.