U.S. neutrino megaproject takes shape in abandoned gold mine

The United States’ most ambitious particle physics endeavor is moving closer to becoming a reality.

The Deep Underground Neutrino Experiment (DUNE) represents a monumental project in both financial and scientific terms. This multibillion-dollar Department of Energy initiative involves constructing a vast facility one mile beneath Lead, South Dakota. It will act as a receptor for elusive particles known as neutrinos, which will be sent from a laboratory in Illinois.

Scientists in the field of particle physics are optimistic that DUNE will address major unresolved questions within the framework of the Standard Model, the most comprehensive understanding of the universe. The experiment could potentially explore fundamental queries about the existence of matter.

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The construction of this crucial infrastructure is now underway. At a recent event at the Sanford Underground Research Facility in Lead, formerly the Homestake gold mine, project leaders and government officials signed the first steel beam destined for the underground site, marking the start of detector construction.

“As a South Dakotan, knowing that on this ground, our little piece of the planet, the fact that we are going to transform our understanding of matter is pretty incredible,” said Representative Dusty Johnson of South Dakota. DUNE is primarily financed by the Department of Energy, but it involves collaboration from 38 countries. The initial 10 million pounds of steel for the first vessel were provided by CERN, the European laboratory for particle physics.

“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. “It’s the moment when this becomes real.”

Neutrinos are nearly massless particles that move through matter with minimal interaction. A neutrino can pass through a block of lead a light-year long without hitting a single atom. They are also known for changing “flavors” as they travel; a neutrino produced in one form can be detected in another after a journey.

These unique characteristics make neutrinos the least understood entities in the Standard Model. Physicists are uncertain about the ordering of the three neutrino masses and hope that these peculiarities might provide insights into the profound question posed by the Standard Model: Why is there something rather than nothing?

The significance of neutrinos lies in their potential connection to fundamental processes that generate matter and antimatter in equal amounts. However, after the big bang, a tiny excess of matter over antimatter resulted in the formation of galaxies, stars, and life. The mysterious shape-shifting nature of neutrinos may have been pivotal in this cosmic imbalance.

Researchers have been probing neutrino “oscillation” for years by directing neutrinos from sources like particle colliders or nuclear reactors to distant detectors, measuring how many change flavor along the way.

DUNE seeks to elevate this method. Utilizing a particle accelerator at Fermilab in Batavia, Illinois, scientists aim to create the most potent beam of neutrinos ever, known as the Long-Baseline Neutrino Facility (LBNF). This beam will be directed downward and westward towards DUNE’s cavern in Lead, spanning a distance of 800 miles and filled with millions of pounds of liquid argon.

“Everything about DUNE is unprecedented: the most intense neutrino beam, the biggest liquid argon detectors, the longest distance neutrinos will travel,” Gollapinni says. “It’s truly amazing.”

To keep the argon in liquid form without freezing or boiling, it must remain at extreme cold temperatures, just shy of -300 degrees Fahrenheit. When neutrinos occasionally collide with argon atoms, they release electrons, which can be detected. Before reaching this stage, DUNE must construct two enormous steel containers for the argon. This marks the project’s current phase.

The task begins with transporting 10 million pounds of steel beams through a 20-foot-wide shaft, which only accounts for the first container. Project leaders compare it to building a ship inside a glass bottle, with the bottle’s neck being a mile long and the ship akin to a scaled-down aircraft carrier. The goal is to complete the first container within nine months.

Once both containers are assembled, they must be prepared to become the most advanced neutrino detectors ever. Before introducing argon, the containers will be equipped with extensive wire grids, comprising thousands of meticulously hand-strung wires currently under construction.

The project has already faced about five years of delay, and its cost has surged to nearly $5 billion. The aim is to have the first detector operational by early 2030. Even under optimal conditions, determining the neutrino mass order may not occur until 2034, with insights into the matter-antimatter imbalance potentially emerging towards the decade’s end.

This timeline poses a challenge as the U.S. is part of a global effort to uncover the last particle in the best model of reality. Japan’s Hyper-Kamiokande (Hyper-K) neutrino experiment is set to gather data in 2028. Hyper-K might observe the matter-antimatter asymmetry before DUNE, depending on its schedule and the feasibility of its approach.

Meanwhile, China’s Jiangmen Underground Neutrino Observatory (JUNO) released its first results recently. JUNO, an independent, scaled-down version of DUNE, is located about 90 miles from Hong Kong. It uses a different type of liquid detector to analyze neutrino beams from nuclear reactors. China’s project has already achieved precision in measuring the gap between the two smallest neutrino masses, crucial for determining their order. JUNO aims to surpass DUNE in this aspect but isn’t designed to address matter’s abundance alone.

“I don’t think people are spending every day thinking, ‘We’ve got to be first,’” says Edward Blucher, a DUNE physicist at the University of Chicago. “In 20 years, we’re going to know much more about this kind of science, and it’s going to be a result of things that were measured with Hyper-K and JUNO and DUNE.”

“All of us are acutely aware that a huge investment has been made in this project and that we have to execute it successfully,” Blucher concludes. “It’s very important for this experiment itself, but I think it’s very important for the future of particle physics in the U.S., too.”