“Here’s to low-grade ore and plenty of it,” mining magnate George Hearst reportedly said after he bought the Homestake Gold Mine in Lead, SD, in June 1877. Although the mine contained less than an ounce of the precious metal per ton, the sheer quantity of ore meant that the operation was profitable (1). From 1876 to 2001, miners dug thousands of feet in search of gold, making Homestake the largest and deepest gold mine in North America. Now, physicists and engineers are excavating gigantic caverns inside the defunct mine to look for something even more elusive: neutrinos.

New caverns being excavated inside the defunct Homestake Gold Mine in Lead, SD, will house DUNE. The world’s largest such detector, DUNE will start running in 2026. Image credit: Shutterstock/Tom Grundy.

The new caverns will house the Deep Underground Neutrino Experiment (DUNE), the world’s largest such detector. DUNE will start running in 2026, studying an intense beam of neutrinos coming from Fermilab, near Batavia, IL, about 1,300 kilometers away.

The experiment will focus on a profound question: what is the origin of matter? In the early universe, matter and antimatter should have formed in equal amounts and then annihilated each other to leave only radiation. But the observable universe with its galaxies, stars, and planets is made of matter, so clearly, not all of it was annihilated. “How did we get from a universe that was equal amounts of matter and antimatter to a universe where there’s a bit of matter left over?” asks neutrino physicist Deborah Harris of Fermilab. Neutrinos could be key to answering that question. Because of their special ability to transform from one type into another, they may reveal fundamental differences between matter and antimatter.

Subatomic Alchemy Homestake has history with the neutrino, offering up the first signs of this particle’s shifty nature. In the 1960s, Raymond Davis Jr., a chemist and physicist from the Brookhaven National Laboratory in Upton, NY, built a detector inside the mine using 100,000 gallons of a dry-cleaning solvent called tetrachloroethylene. He aimed to spot neutrinos streaming out from the sun. Neutrinos are subatomic particles that mostly pass right through matter, but very rarely a neutrino can smash into an atom head on. In Davis’s experiment, a neutrino colliding with a chlorine atom turned it into an atom of argon. Davis developed a technique to carefully count these argon atoms (2). To do so, he had to go deep underground. On Earth’s surface, there are too many confounding signals. For example, particles called muons, created when cosmic rays hit Earth’s atmosphere, can hit the detector and flip a chlorine atom into an argon atom. Muons are blocked by rock, so Davis built his experiment at level 4850 of the Homestake mine, giving him nearly 5,000 feet (around 1,500 meters) of rock overhead as protection. He did detect solar neutrinos—but only a third of the number predicted by a detailed model of the sun developed by late physicist John Bahcall of the Institute for Advanced Study in Princeton, NJ. Something was amiss. It would take nearly four decades and sustained experimental and theoretical work by Bahcall and others to solve what came to be called the solar neutrino problem. Davis’s detector was only sensitive to electron neutrinos, the type produced by nuclear reactions in the sun. According to the standard model of particle physics, neutrinos come in three types, known as flavors: electron, muon, and tau. To explain the missing neutrinos, researchers eventually hypothesized that electron neutrinos were morphing into other flavors on their journey from the sun to Earth, which Davis could not detect. In the early 2000s, other experiments detected muon and tau neutrinos, besides electron neutrinos, coming from the sun, adding up to the expected total number. This confirmed that some of the electron neutrinos were indeed changing into the other flavors on their journey, a process known as neutrino oscillation. Despite the theoretical possibility, seeing evidence of this process surprised physicists. “We didn’t think that neutrinos could behave in that way,” says Geralyn Zeller, deputy head of the neutrino division at Fermilab. In the standard model, neutrinos don’t have mass, and massless particles cannot oscillate. Neutrino oscillations prove that these particles have mass, giving a clear indication that there is physics beyond the standard model. ProtoDUNE, a scaled-down version of DUNE, is being built and tested at CERN near Geneva. Although only 1% the size of DUNE, ProtoDUNE is already the largest detector ever built that uses liquid argon to spot neutrinos. Image credit: CERN.

Beam Catcher DUNE will be a powerful probe of this new physics. The experiment begins in Fermilab, which will produce an intense neutrino beam. To do so, Fermilab is building an accelerator called the Proton Improvement Plan-II (PIP-II), which will be the first stage of a complex that will boost protons to energies of up to 120 billion electronvolts and smash them into a graphite and beryllium target (3). The collision will create a slew of charged particles, which will then decay. Among the decay products will be neutrinos and antineutrinos, which will speed straight through the ground. PIP-II can aim either a neutrino beam or an antineutrino beam toward DUNE. Before the beam reaches the “far detector” at Homestake, it will first encounter a “near detector” only about 600 meters away, designed to get a statistical sample of the number and energies of particles in the beam. “My hope is that neutrinos might be a wedge to continue breaking the standard model and hopefully reveal a bigger picture about how the world works.” —Joseph Formaggio The DUNE far detectors will use liquid argon. When a neutrino hits an argon atom, the collision will create a flash of light and a shower of charged particles that will knock off electrons from the argon atoms. The electrons will be attracted toward planes of electrified wires immersed in the liquid, allowing physicists to reconstruct the collision and, hence, the properties of the neutrino that smashed into the argon atom—specifically, its type and energy (4). This detector design is already being tested by using the ProtoDUNE detector at CERN, the European particle physics lab near Geneva, Switzerland. Holding 800 tons of liquid argon, the cube-like ProtoDUNE is about the size of a three-story house, and yet it is only about 1% of the size of the planned DUNE far detector. From 2026 onward, DUNE will study the neutrino beam coming from Fermilab, which will start out mostly made of muon neutrinos. The experiment will determine how many more electron neutrinos appear in the far detector relative to the number seen in the near detector and how many muon neutrinos disappear between the two detectors. The experiment will make similar measurements for antineutrinos.

Breaking Symmetries The aim is to cast light on two symmetries of particle physics. A symmetry exists when applying some transformation doesn’t change the laws of physics. In this case, the two transformations involve changing the charge of a particle to turn it into its antiparticle (C-symmetry) and turning the particle into its mirror image (P-symmetry). In the late 1950s, researchers discovered interactions involving the weak nuclear force that violated both C- and P-symmetries, individually. As a consequence, the Soviet physicist Lev Landau proposed that the true symmetry between matter and antimatter is C- and P-symmetries taken together, or CP-symmetry (5). This would mean that transforming all left-handed particles into right-handed antiparticles would make no difference to how physics works. If CP-symmetry is not violated, then the matter and antimatter that would have been produced in equal amounts in the early universe would have been annihilated, leaving behind only radiation, whereas theories with CP-violation can explain why, as the universe evolved, a little more matter survived than antimatter, and that makes up the matter we see. There’s strong evidence that elementary particles called quarks (which constitute protons and neutrons) do violate CP-symmetry. The first indication of this came in 1964, when Val Fitch and James Cronin at the Brookhaven National Laboratory discovered violations of CP-symmetry in the decay of K-mesons, which are composed of one type of quark and another type of antiquark. Fitch and Cronin were awarded the Nobel Prize in Physics in 1980 for their efforts. Still, the amount of violation is not enough to explain the excess of matter. So, physicists have begun looking for CP-violation in other classes of particles, such as leptons, the group which includes electrons and neutrinos, among others. This is where DUNE comes in. It’s going to see whether neutrinos and antineutrinos oscillate in the same manner. “If they don’t, that’s an indication that CP has been violated,” says Harris. The key will be to determine not only whether CP is violated by neutrinos but by how much. Part of the Sanford Underground Research Facility, DUNE will allow researchers to study an intense beam of neutrinos coming from Fermilab near Batavia, IL, about 1,300 kilometers (800 miles) away. Image credit: Fermilab.

Minecraft It will be no easy task, not least because of the challenging civil engineering required to build DUNE a mile underground. Engineers have to excavate vast caverns inside the Homestake mine to house DUNE’s argon tanks, cryogenics, and other instrumentation. The good news is that geologists in the team have studied the rock at level 4850 and found it to be mostly mica schist, an extremely hard metamorphic rock that is well suited for excavation. The engineers will drill holes into the rock and install explosives and blast them. “It’s a controlled engineered blast,” explains Douglas Pelletier, a project manager at Fermilab. The scale of the operation is daunting, requiring engineers to shift nearly 3,000 tons of rock per day when there is just one way in and one way out of the mine. “Every ton of rock has to come up that shaft, and every piece of equipment, every pound of steel or concrete and pipe and conduit that we have to build has to come down that same shaft,” says Pelletier. So before the excavation begins, the team will rehabilitate the mine’s infrastructure. Remarkably, an enormous hoist that was built in the 1930s to move a cage through the mine shaft still works. “It looks like something out of a Frankenstein movie,” says Pelletier. Today, this ancient machine takes about 15 minutes to lower scientists and engineers to level 4850. After upgrades to the hoist, the journey will take 5 minutes. The team must also replace the steel scaffolding inside the mine shaft and build a new cage. Once the caverns are excavated, the physicists will install four tanks that will contain a total of about 70,000 tons of pure liquid argon, cooled down to about 90 Kelvin (about –300 °F). Each tank will be 18 meters high, 19 meters wide, and 66 meters long—“about as tall and wide as a four-story building (or a brachiosaurus) and longer than a football field.” Harris points out that cooling down such vast amounts of argon to 90 Kelvin is not hard. “Humans have become very good at being able to contain very large amounts of cryogenic liquids,” she says. “The challenge is having that tank be clean enough inside that the argon you put in the tank stays very pure.” Achieving that will be no small feat deep inside a dusty mine. When finished, DUNE will be poised to say whether neutrinos violate CP-symmetry strongly enough to explain the existence of matter in our universe, says neutrino physicist Joseph Formaggio of Massachusetts Institute of Technology in Cambridge, MA, who is not associated with DUNE. “It’ll be a definitive measurement,” he adds. Neutrinos were the first particles that forced physicists to modify the standard model in nontrivial ways to accommodate their masses and oscillations, says Formaggio. “My hope is that neutrinos might be a wedge to continue breaking the standard model and hopefully reveal a bigger picture about how the world works.”