Construction for America's next grand particle physics experiment began this summer. The Deep Underground Neutrino Experiment, or DUNE, will study some seriously ghostly subatomic particles. The subterranean experiment will entail shooting a powerful beam of neutrinos through Earth's mantle – reaching a maximum depth of 30 miles (48 kilometers) – and perhaps unlocking some of our universe's greatest mysteries in the process.
The experiment, managed and funded by an international collaboration, will span 800 miles (1,300 kilometers), beginning at Fermi National Accelerator Laboratory (Fermilab) in Batavia, Illinois, and ending over a mile underground beneath an abandoned gold mine in Lead, South Dakota. When complete, DUNE will become a part of the Long-Baseline Neutrino Facility (LBNF), a dual-site facility that will start at Fermilab in Illinois and end at the Sanford Underground Research Facility (SURF) in South Dakota.
Going Deeper Underground
Eight hundred miles (1,287 kilometers) of rock is inconsequential to neutrinos. These strange subatomic particles are fermions that have very low mass and zero charge. They travel at close to the speed of light (as they are the lowest mass particles known to exist) and are extremely weakly interacting with normal matter. They flood our universe and travel through everything in their paths, whether it's us or miles of rock.
How do scientists even know these things exist if they're so ghostly? This is where building-sized cryogenic detectors come in. DUNE will maintain two underground detectors, one will be near the Fermilab source (known as the "near detector"), and the other will reside in a huge facility at SURF (the "far detector"). After an upgrade to Fermilab's facilities, the world's highest-intensity neutrino beam ever produced will be directed through the near detector and intersect with the far detector – composed of four massive, cryogenically cooled tanks of liquid argon. How massive? Each tank will be six stories high and a football field long, and will contain 18,739 tons (17,000 metric tons) of super-cooled liquid argon.
What's with the argon? Well, neutrinos are weakly interacting, but they do very occasionally make a direct hit with the atomic nuclei held in matter. So, by aiming a very intense beam of neutrinos at sufficiently large tanks of ultrapure argon, a very small proportion of the ghostly particles will, by pure chance, hit the argon atoms. When collisions occur, ultrasensitive detectors inside the tanks will note a flash (known as scintillation) and then the interaction can be studied. But as these detectors are so sensitive and the interactions are very small, neutrino detectors are generally buried deep underground to shield them against interference by cosmic rays and other radiation that would wreak havoc if they were exposed on the surface.
These weak interactions could open our eyes to new physics and will boost our understanding of one of the least understood particles in quantum physics.
Getting to Know Neutrinos
Scientists like neutrinos for lots of reasons. Here's one: They provide a direct link between us and our sun's core. During nuclear fusion processes, neutrinos and high-energy photons are produced. The photons are absorbed when they collide with the dense solar plasma and then re-emitted at a lower energy (a process that repeats itself for up to a million years before the energy from the solar core finally gets emitted as light that we see), but neutrinos will shoot straight from the sun's core, through the dense plasma and reach Earth in a matter of minutes. So, if physicists want to know about the fusion environment in the center of our sun right now, they will turn to solar neutrinos.
But there's a mysterious twist to solar neutrinos.
As best we know, neutrinos come in three "flavors" – the electron neutrino, muon neutrino and tau neutrino – and their antiparticles. As neutrinos travel, they "oscillate" between the three flavors, like a chameleon would change color in response to the color of its surroundings.
The sun is only able to generate electron neutrinos in its core, however, so when physicists set out to detect these tiny apparitions using the first ultra-sensitive detectors in the 1960s, they detected far fewer neutrinos than theory predicted. In Nobel Prize-winning work , physicists finally found the reason. It turns out that the electron neutrinos produced by the sun's fusion naturally oscillate between the neutrino flavors – electron, muon and tau. Since the detectors could only observe electron neutrinos, the muon and tau neutrinos went undetected. There wasn't an anomalous deficiency of solar electron neutrinos – they had simply switched flavor when they reached the detector.
Which brings us back to DUNE. We need a controlled experiment on Earth like DUNE to understand these flavor changes. During the experiment, the flavor of the neutrinos being produced by Fermilab's particle accelerator will be measured as soon as they are sent to the converted gold mine in South Dakota. The received neutrinos at SURF can then be compared with the ones that were sent, and a new understanding about the quantum nature of neutrinos may be forged. Scientists will precisely measure the masses of these neutrinos. They may even uncover other neutrinos beyond the known three flavors.
But Wait, There's More. A Lot More
DUNE will go way beyond studying neutrino oscillations. It could help us understand the not-so-small mystery of how our universe even exists. This may sound like a philosophical quandary, but the fact that our universe is composed of mostly matter and not antimatter is one of the biggest questions looming over modern science.
During the Big Bang, some 13.8 billion years ago, matter and antimatter should have been created in equal parts. Of course, we all know what happens when matter and antimatter meet – it explodes, or annihilates, leaving nothing but energy behind. So, if the Big Bang did produce equal parts matter and antimatter, there'd be nothing here.
The fact that we ARE here means the universe produced slightly more matter than antimatter, so when all that annihilation happened at the birth of the universe, matter won out and antimatter became an extreme rarity. This means that some basic physical laws were broken at the Big Bang, a conundrum that physicists call a violation of charge-parity symmetry – or a "CP violation." Particle accelerators like the Large Hadron Collider can test why nature favors matter over antimatter, and DUNE will do this, too, by experimenting with neutrinos and their antimatter partner, the antineutrino.
The neutrino beam at Fermilab's production facility is expected to be operational by 2026, and construction of the final DUNE detector is expected to be complete by 2027. Hopes are high that we could be on the verge of another Higgs-like discovery.