US neutrino megaproject takes shape in abandoned gold mine

The most ambitious particle physics project ever carried out in the United States is one step closer to reality.

The Deep Underground Neutrino Experiment (DUNE) will be a giant in both budgetary and basic science terms: a cavernous, multibillion-dollar Department of Energy facility a mile beneath the town of Lead, SD, that will serve as a catching gauntlet for the ghostly particles, called neutrinostransmitted from a laboratory in Illinois.

Particle physicists hope DUNE will finally solve the problem the biggest open questions in their most coherent picture of the universe, the Standard Model. It might even answer humanity’s oldest question: why we (or any other question) exist.

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Now, this existential catcher’s mitt is finally being built. At an event yesterday at the Sanford Underground Research Center in Lead, formerly the Homestake gold mine, project leaders and government supporters gathered to sign the first steel beam to be sent underground, beginning construction of the facility’s detectors.

“As a South Dakotan, knowing that on this earth, our little piece of the planet, the fact that we are going to transform our understanding of matter is pretty incredible,” said Rep. Dusty Johnson of South Dakota. DUNE is funded primarily by the Department of Energy. But it is an international collaboration involving 38 countries: the 10 million pounds of steel for the first ship were provided by CERN, the European particle physics laboratory.

“DUNE has been the dream of many in the physics community for more than two decades,” says Sowjanya Gollapinni, co-spokesperson of the DUNE collaboration. “This is the moment where it becomes real.”

THE neutrino is an almost weightless particle that sails through matter like a fantasy. No other known particle is so timid in its interactions: a neutrino can pass through a light-year block of lead without touching a single atom. He is also a shapeshifter; produce one of three neutrino “flavors” in a beam heading west from New York, and by the time your friend in Los Angeles measures it, that neutrino will probably have a different flavor.

These mind-blowing properties explain why the neutrino remains the least understood of all the characters in the Standard Model. Physicists can’t even say how the three neutrino masses are ordered, let alone determine their exact values. They hope that the particle’s quirks might hide an answer to an almost philosophical question raised by the Standard Model: Why is there something rather than nothing?

The connection between neutrinos and such heavy matter is that fundamentally, every fundamental process of generating matter also produces antimatter in equal amounts. Yet the result of the Big Bang was somehow a tiny slice of more matter than antimatter: all the galaxies, dust and living things in the universe belong to this tiny excess. Many physicists suspect that the strange shape-changing behavior of neutrinos may have played a key role in this cosmic conundrum.

Scientists have studied neutrino “oscillation” for decades by transmitting neutrinos from sources (such as particle colliders or nuclear reactors) to distant detectors. Then they measure how many neutrinos changed flavor during transit.

DUNE intends to push this approach to its limits. Physicists will use a particle accelerator at Fermilab in Batavia, Illinois, to produce the most intense neutrino beam ever, a companion to DUNE officially dubbed the Long Baseline Neutrino Facility (LBNF). The LBNF will be pointed down and west from Fermilab, aimed directly at the core of the DUNE cavern beneath Lead, 800 miles away, which will be filled with tens of millions of pounds of liquid argon.

“Everything about DUNE is unprecedented: the most intense neutrino beam, the largest liquid argon detectors, the longest distance traveled by neutrinos,” says Gollapinni. “It’s really amazing.”

To remain liquid without freezing or boiling, all that argon must be kept within a narrow range of extreme cold, just a few degrees from about -300 degrees Fahrenheit. Jostling argon atoms release electrons when, all too rarely, they are hit by passing neutrinos, creating a signal that physicists can detect. But before that happens, DUNE staff must build two massive steel containers for the argon. This is the phase of the project that begins now.

The first step involves driving 10 million pounds of steel beams underground through a 20-foot-wide shaft, and that only covers the first container. Project managers liken the task to building a ship inside a glass bottle, except the bottle’s neck is a mile long and the ship is an aircraft carrier on one-tenth scale. They hope to have the first container completed in about nine months.

But even once the two containers are assembled, they will still need to prepare them to become the most elaborate and sensitive neutrino detectors ever built. Before any argon can be delivered, the containers must be interwoven with hundreds of massive grids, each made of thousands of fine, hand-stretched wires that are currently under construction.

The project vast ambitions have already accumulated about five years of delay and, in total, the cost to taxpayers has climbed to almost $5 billion. The current goal is to have the first detector in operation by early 2030. This could mean that, even in the best case scenario, DUNE will not determine the mass order of neutrinos until 2034 – and no answer to the question of matter-antimatter imbalance will come before the end of this decade.

That’s a long time to wait, given that the United States is not the only contender in what is truly a global race to elucidate the last particle of physicists’ best model of reality. Hyper-Kamiokande (Hyper-K) from Japan The neutrino experiment is on track to begin collecting data in 2028. Hyper-K could measure matter-antimatter asymmetry before DUNE, but that will depend on how well the Japanese project can stay on schedule and whether the as-yet-unknown answer is within reach of that competing project’s more modest approach.

Meanwhile, China Jiangmen Underground Neutrino Observatory (JUNO) has published its first results at the end of last year. JUNO is essentially a scaled-down, entirely independent version of DUNE, an underground facility about 90 miles west of Hong Kong that places a smaller, different liquid detector in the path of neutrino beams from two nuclear reactors. The Chinese project has already provided state-of-the-art precision for the gap between the two smallest neutrino masses, a key element in determining order. JUNO hopes to beat DUNE to this answer, but is not designed to deal with excess matter on its own.

“I don’t think people spend every day thinking, ‘We must be first,'” says Edward Blucher, a DUNE physicist at the University of Chicago. “In 20 years, we will know a lot more about this type of science, and it will be the result of things that were measured with Hyper-K, JUNO and DUNE.”

“We are all fully aware that a huge investment has been made in this project and that we must see it through,” concludes Blucher. “It’s very important for this experiment itself, but I think it’s also very important for the future of particle physics in the United States.”