Neutrinos are the most ubiquitous particles — a hundred billion zip through your fingertip each second — yet they have no charge, almost no mass, and they barely interact with other matter.
A century ago, when the Italian physicist Wolfgang Pauli predicted their existence, it wasn’t even clear how to look for them. “I have done a terrible thing,” he famously said. “I have postulated a particle that cannot be detected.”
Fortunately, he spoke too soon. Neutrinos are in fact detectable, and physicists think they could explain fundamental facts about the nature of the universe that have eluded science for decades. But to measure them properly will require a colossal feat of engineering: the Deep Underground Neutrino Experiment, or DUNE for short.
What Is the Deep Underground Neutrino Experiment?
Shining anode plane assemblies dominate this panorama of the 35-ton-capacity prototype cryostat for LBNF/DUNE. (Image Courtesy of: Reidar Hahn)
The DUNE is actually two projects separated by 800 miles. On one end, near Chicago, a particle accelerator at Fermilab will generate an intense neutrino beam pointed precisely (through Earth’s crust) at the Black Hills of South Dakota. There, nearly a mile below the surface, a detector will record data on the incoming neutrinos.
Detector doesn’t do this behemoth justice, though. It will consist of four modules, each the size of a city block, erected in 7-story caverns at the Sanford Underground Research Center (800,000 tons of rock were excavated for the purpose). Each module will hold 38 million pounds of liquid argon, cooled to roughly negative 300 degrees Fahrenheit.
“It’s not just because we like to do hard things,” says Robert Wilson, a physics professor at Colorado State University and chair of the DUNE Institutional Board.
It’s because neutrinos are slippery little suckers, so infinitesimal and antisocial that they could travel through a lightyear of lead and not mingle with a single atom. Those massive tanks of super-dense argon raise the odds of neutrino interactions, and thus of detectable signals. As for the thick buffer of earth, it filters out atmospheric particles that would contaminate the data aboveground.
Read More: A Rare Type Of Energetic Neutrino Sent From Powerful Astronomical Objects
Explaining Why Matter Exists
All this effort could pay off in a big way if DUNE fulfills its mission: to reveal why the universe is how it is — specifically, why it’s filled with matter. Because, counterintuitively, that’s not what we’d expect to see under the Standard Model of particle physics, the theory that best describes the workings of the cosmos. The Big Bang should have produced equal numbers of particles and antiparticles, and they should have paired off in mutual annihilation, leaving behind only residual energy.
Instead, we see galaxies and stars and planets, all made from the sliver of matter that escaped destruction. By contrast, we see virtually no antimatter. So rather than the predicted balance, the universe must have begun with an asymmetry that slightly favored matter.
“There has to be something a little bit different about antiparticles and particles,” Wilson says, “such that all antiparticles got gobbled up.”
Why Study Neutrinos
This is where neutrinos come in. Unlike other particles, they don’t have a fixed identity; each one can shape-shift between three “flavors.” To show how dramatic the change is, Wilson put it in terms a fellow Colorado resident could understand: “It’s as if you started driving from Denver and someone saw you get in the car, and then by the time you got to Longmont someone else glanced in the car and it was your brother.”
The idea behind DUNE is to measure whether neutrinos and antineutrinos shift, or oscillate, at different rates. If they do, the mystery may be solved — if neutrinos oscillate faster, staying a step ahead of their doppelgängers, that could explain why more matter than antimatter emerged from the Big Bang. The two substances would then not be mirror images of each other, but fundamentally different entities with distinct properties.
When a particle and its antiparticle behave differently, physicists say that charge-parity has been violated. Such violation has already been observed in quarks (another building block of the universe), but that discrepancy wasn’t enough to account for the preponderance of matter over antimatter.
Neutrinos, the least understood of the fundamental particles, are perhaps the last place to look for another charge-parity violation that could tip the scales.
If DUNE doesn’t find it, Wilson says, “we will not know why the universe exists. We’ll have to put our thinking caps back on.”
Read More: Neutrinos Provide a New Way to Probe the Cosmos
The DUNE and Supernovas
Whatever we do or don’t learn about the origin of everything, DUNE has other goals. Most importantly, it could shed new light on how supernovas occur. Perhaps you’ve seen an image of one of these inconceivably powerful stellar explosions, a lone star outshining entire galaxies.
And yet what’s visible to us is only “a tiny, tiny fraction of the total energy emitted,” according to Wilson. About 99 percent is neutrinos, “and there are some on their way here.”
Supernova neutrinos have only reached Earth once in the 70 years we’ve observed them. It was in 1987, and though there must have been trillions upon trillions of them, the world’s three active detectors saw just two dozen interactions. Whenever the next burst comes along, DUNE’s detector — many times larger than those first-generation models — could catch hundreds or thousands.
“I’m just hoping they hold off,” Wilson says, laughing. On top of that, “if we’re super, super lucky,” he adds, we could even witness the birth of a black hole.
When an extremely massive star goes nova, it collapses under its own gravity and becomes so dense that nothing can escape — even neutrinos. So, from our vantage point, we’d see a steady stream of neutrinos (emitted by the initial supernova) that shuts off abruptly as soon as a black hole forms.
Racing Toward Discovery
If all goes according to plan, the argon detectors will be up and running — and sensitive to the coming hordes of supernova neutrinos — by the end of 2029, according to DUNE’s official timeline. Then the neutrino beam at Fermilab is scheduled to go live by 2031, allowing for oscillation measurements.
That said, the project has faced setbacks in recent years. And similar neutrino experiments are underway across the globe, most notably Japan’s Hyper-Kamiokande observatory, which is slated for completion two years earlier. With that head start, it could beat DUNE to some major discoveries.
Wilson notes, however, that DUNE and Hyper-K are just as complementary as they are competitive. One uses liquid argon for its detector, the other water, and the results will both support each other and provide different kinds of insight (as will the difference in distance between beam and detector: 800 miles versus 200).
These experiments could illuminate the earliest moments after the Big Bang, the mechanisms behind supernovas, and much more. On the flip side, they could also upend our decades-old understanding of the most abundant particle in the universe. Perhaps, for instance, there’s more to it than the trio of known flavors.
“We have this beautiful three-neutrino paradigm,” Wilson says. “But it’s dangerous to think that is the whole story, because neutrinos have made fools of us before.”
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Cody Cottier is a contributing writer at Discover who loves exploring big questions about the universe and our home planet, the nature of consciousness, the ethical implications of science and more. He holds a bachelor’s degree in journalism and media production from Washington State University.
