The stars were still faintly visible on the morning four years ago that Scott Kravitz first drove out to the salt mines in southeastern New Mexico. Then a second-year graduate student in physics, Kravitz was preparing to journey half a mile below the Earth’s surface.
When he arrived at the facility, Kravitz passed through security, donned coveralls and safety equipment (including a portable air purifier in case of fire), and boarded a mesh-walled container called the cage. The double-decker structure — which fits no more than six people on each level — is the only method for descending to the tunnels below.
It was unlike any elevator Kravitz had ridden. Within seconds, he and his fellow passengers were swallowed by darkness. No one spoke. No one turned on a headlamp. The soft sounds of air whistling past and the rattling of the cage were all he could hear.
PARTICLE PASSAGE: The subterranean tunnel in a New Mexico salt mine that now houses the EXO-200 experiment. (Photo: The EXO-200 Collaboration)
“It was like a meditation,” Kravitz recalls. The trip felt longer than its five-minute duration. Kravitz was eager to disembark, but not out of nervousness. More than anything, he was excited about what lay below.
When the doors opened, a wave of hot, stale air thick with salt dust greeted him. Headlamps and overhead lighting illuminated the excavated rose-gray walls and floor of the 250-million-year-old mineral reserve.
What brought Kravitz to this remote spot beneath the desert was neither geology nor earth science. Nor was it waste disposal — though the nearly impermeable salt bed is primarily used for that purpose, housing drums of radioactive refuse. Kravitz, PhD ’17, had come as a member of Stanford physics professor Giorgio Gratta’s research team, which studies elusive subatomic particles called neutrinos. Down in the mines, the scientists are trying to detect an unusual event that could unlock mysteries about the makeup of everything around us.
Little Neutral Ones
The neutrino is a fundamental (or indivisible) particle of matter, significantly smaller than an atom. It is just one of 17 fundamental particles that physicists have discovered to date. Others include the electron, familiar from high school chemistry class, and the photon, or particle of light, which is the only fundamental particle our eyes can detect.
Each of these particles has special properties, and the neutrino is no exception. For one, it is the most abundant particle of matter. Its name comes from the Italian for “little neutral one,” encapsulating both the fact that it is very tiny — even for a fundamental particle — and that it has no positive or negative electric charge.
Photo: Giorgio Gratta
“Neutrinos are the particles that we understand the least,” says theoretical physicist André de Gouvêa. “And they are important for understanding a lot of natural phenomena.” A professor at Northwestern University, de Gouvêa has spent much of his career studying neutrinos and developing models to explain how they fit into our understanding of the rest of the universe.
Neutrinos are born when the nucleus of an atom changes in some way. That change could come about when atoms in a radioactive material break down. But it could also happen when atomic nuclei join together (fusion) or split apart (fission), two of the most energetically intense events known to humankind.
In a trailer-like clean room in the depths of the mine, Gratta’s team monitors the breakdown of a radioactive form of the element xenon. Radioactive material is inherently unstable. As the xenon decays, the nuclei within its atoms may release other particles. Gratta and others want to see whether neutrinos emerge and then annihilate each other.
Certain theories predict such a find. But if the interaction exists, it will be tough to spot. “You are dealing with very rare processes,” Gratta explains.
The salt mines offer an ideal location for their project. On Earth’s surface, we are constantly bombarded by subatomic particles that would be difficult to disentangle from an experiment’s data set. Deep underground, the layers of salt and earth create a shield that blocks most unwanted phenomena from the scientists’ detector.
At the moment, the experiment — called EXO-200 — is in a race, of sorts, with several other particle physics experiments that have the same end goal. “We were the first of these experiments to turn on,” Gratta says. “And hopefully, by this summer, we will be ahead.”
The stakes for their search are high. If neutrinos truly can cancel each other out, they could be the key to explaining one of the mysteries of how we came to be. Physicists recognize that when two particles of equal mass but opposite charge meet, they can collide and leave nothing but energy behind. When such an event occurs, it means that one particle was the other’s antiparticle. Antiparticles together make antimatter, and the existence of antimatter raises some uncomfortable questions. Nearly all particles of matter have antimatter counterparts, and because nature tends to favor balance, many scientists suspect that matter and antimatter were created equally at the dawn of time. But if that were the case, one might expect the two would simply destroy each other. Instead, we exist in a universe dominated by matter.
If neutrinos can behave as their own antiparticles, that could help scientists decipher how matter particles came to outnumber their antimatter counterparts. Thus far, the search has come up empty. But the EXO-200 team is not discouraged. Regardless of whether they see this strange antimatter process, Gratta, Kravitz and their colleagues have already begun providing insight into an enigmatic particle. “Neutrinos have a number of peculiarities,” Gratta says. They have surprised scientists many times before and are bound to surprise them again.
WHAT’S INSIDE: The EXO-200 experiment’s detector is composed of two parts — a central copper drum (bottom), which will be filled with xenon, and a large cooling system to keep the element in its liquid form. The detector enables Gratta and his team to study neutrinos. (Photos: The EXO-200 Collaboration)
Shape-shifters
Every second, trillions of neutrinos pass through you, but over a lifetime, only one or two may actually hit another particle in your body. They are small enough that they rarely bump into other particles, and because of their neutrality, they do not respond to the forces of electricity and magnetism. As a result, they can travel uninterrupted for long distances, and they are extremely difficult to catch. Stanford emeritus professor of physics Stan Wojcicki, another neutrino aficionado, explains that these particles have other curious properties. For example, the neutrino can come in any one of three types, or “flavors,” called electron, muon and tau. “As they travel across the atmosphere, they morph into another flavor,” Wojcicki says. From the late ’90s into the 2000s, Wojcicki studied those transformations.
Shape-shifting is not unheard of among fundamental particles, but it was unexpected in neutrinos. Such changes are possible only through a quantum mechanical process that requires the particles involved to have mass. Yet, “the current theory of particle physics as a whole predicts that the mass of the neutrino should be zero,” de Gouvêa says. If scientists knew the precise mass, theoretical physicists could rework the existing models to build new, more comprehensive theories.
The EXO-200 experiment could be well-positioned to resolve that quandary. If scientists succeed in their quest to see neutrinos behave as their own antimatter particles, Gratta explains, they will be “automatically measuring the mass of the particles involved.” That’s because physicists have worked out a relationship between the rate at which materials decay and the mass of the neutrinos present. Heavier neutrinos, for example, would be involved in more frequent decays than lighter neutrinos.
More broadly, the EXO-200 experiment is testing two competing theories from the early 20th century about the nature of neutrinos. In one, proposed by a theoretical physicist named Paul Dirac, all particles can appear in one of four states that relate to their charge and how they move. But because neutrinos are neutral, another theorist, Ettore Majorana, suggested that each neutrino type might have just two variants. If so, he predicted that the two variants would have opposite qualities and therefore the same particle could cancel itself out — implying that the neutrino is its own antiparticle. (In July, physicists at Stanford and UC-Irvine demonstrated that they could create situations in which particle-like bundles of energy behave as both particle and antiparticle, adding extra heft to Majorana’s ideas.) “So, that’s what we’re after,” Gratta says. “We’re trying to detect whether neutrinos are Majorana particles.”
Questions about the mass and variety of neutrinos are not merely academic. By better understanding their traits and behavior, scientists can also advance the study of the phenomena that produce them. Neutrinos from the sun, for example, are the product of fiery processes happening at that star’s core; by detecting those neutrinos, scientists have proof of a reaction they cannot otherwise observe (see sidebar, “Where Do Neutrinos Come From?”).
Ghosts in the Machine
Tucked within a corner of the salt mine, far from the waste storage, are metal containers set up by EXO-200 physicists to house their bright white, ultrasterilized “clean room.” The contrast between dark, gritty tunnel and hospital-esque experimental space is stark.
As soon as they arrive, the scientists crowd into the clean room, three at a time, to change from miner gear into special Tyvek coveralls called bunny suits. This uniform prevents dirt, oil, hair and other detritus from dirtying their instruments. A large lead wall between the physicists and the detector itself forms an added radiation barrier. Without it, even a scientist’s sack lunch could significantly alter the detector’s readings.
The EXO-200 detector was assembled at Stanford and moved into the mines in 2007, thanks to a collaborative effort among scientists that now spans 25 institutions in seven countries. EXO is an acronym for Enriched Xenon Observatory, and at its heart is a 200-kilogram (440-pound) tank filled with xenon. Radioactive xenon decays so slowly that the detector may only pick up data from a few events each year. To boost these numbers, the xenon is “enriched” to increase the odds of observing interesting decays. That means the scientists remove isotopes they won’t need while preserving the ones they expect to produce a neutrinoless decay.
The task of neutrino hunting has been all about probability from the start. In 1930, the Austrian theoretical physicist Wolfgang Pauli first proposed the neutrino’s existence. At the time, he and his colleagues were puzzled. Several experiments had shown that radioactive elements decay and release electrons with much less energy than expected, given the materials involved. That finding might be explained, Pauli suggested, if another particle — incredibly small and without any charge — was also emerging from the decay.
He figured the odds were so slim that anyone would be able to find this tiny neutral particle that he bet a case of Champagne against it. He is said to have remarked: “I have committed the cardinal sin of a theorist. I made a prediction which can never be tested.”
Nonetheless, in the 1950s, American physicists Fred Reines and Clyde Cowan, based at the Los Alamos laboratory in New Mexico, took up the challenge. In honor of the strange ghostlike ability of their quarry, they named the experiment Project Poltergeist.
Reines and Cowan built a giant detector for its time, 1 cubic meter in size. Because the neutrino only rarely touches other particles, the reasoning went — and still goes — that physicists need to monitor a very large amount of material for a very long time to increase the likelihood of spotting a neutrino in action. In 1956, Reines and Cowan confirmed that they had seen a neutrino interact with protons in tanks of cadmium chloride.
Typically, “a neutrino comes into a detector and most of the time you don’t see it,” Stan Wojcicki says. “But very, very seldom, it will satisfy your curiosity and interact.” When that happens, depending on the materials involved, light, heat or even sound can be produced and measured.
Researchers have built many kinds of neutrino detectors in the past half century — uncovering, in the process, the three flavors and the fact that neutrinos transform from one form into another. Today, there are essentially two types of detector. One catches particles that journey into the experiment from disparate sources, such as stars and power plants. Scientists on the Super-Kamiokande experiment in Japan, for example, observe neutrinos from the sun and atmosphere as they interact with particles within a detector that contains 50,000 tons of water and is situated beneath a mountain.
The other class of detectors, which includes EXO-200, references the experiments that first inspired Pauli; they produce neutrinos within the detector using radioactive material and then measure the energy of particles such as electrons created during the decay. Using that information, the EXO-200 team can then determine whether neutrinos are present — or, as many hope, absent. Seeing a neutrinoless decay would indicate “a totally new process,” says Kravitz. “It implies that there are other particles out there that we don’t know about.”
The Next Generation
To date, there is still no confirmation that neutrinos serve as their own antiparticles. But Gratta’s team has made some interesting finds. In 2011, the researchers observed a rare pattern of decay in which a nucleus from an atom of xenon broke down to release two electrons and two neutrinos. The discovery is evidence of one of the slowest decay processes ever studied; it would ultimately take sextillions of years for their total sample to break down in this manner, longer than our universe has existed to date.
In addition, the researchers have advanced the quest to ascertain the neutrino’s mass. Based on the languid rates of decay they have seen, the EXO-200 physicists can conclude that the neutrino is at least 3.5 million times lighter than an electron.
To learn more, the scientists say it’s time for a new detector. EXO-200 will only continue to collect data for another year and a half, at which point the researchers believe they will have learned as much as they’re able to with that equipment. The proposed successor, dubbed NEXO, would contain a tank 25 times larger than the one in EXO-200’s detector. Able to accommodate about five tons of liquid xenon, NEXO would give the researchers significantly more decay data to study, accelerating the rate at which they can learn about these particles and enabling them to capitalize on what they’ve learned so far.
In the meantime, the physicists have gained a certain fondness for their salt mine. “In many ways, it’s much more pleasant than other mines that I’ve been to,” Gratta says. Whereas some sites leave scientists mired in mud, he notes, “salt is very healthy.”
Admittedly, it presents its challenges. When asked to provide a photograph from the tunnels, Gratta explains that salt dust and flash photography don’t mix — the images are speckled with bright white dots. “They look like stars,” he observes. The visual is a curious reminder of all the motes and particles we otherwise never see.
Daisy Yuhas is a science journalist based in Austin, Texas.
