From gold rush to cosmic questions: a South Dakota mine and the search for the secrets of the universe

A mile beneath the Black Hills, scientists are building giant detectors to analyse neutrino particles – and possibly find the answer to existence

For more than a century, the mining town of Lead, in the Black Hills of South Dakota, attracted workers with a taste for gold. Now, a flagship experiment being built in the old mine is drawing scientists in search of more intriguing riches: answers to the deepest mysteries of the universe.

Construction of the Deep Underground Neutrino Experiment (Dune) in the former Homestake mine is due to start on Monday. Engineers will begin assembling four giant detectors inside caverns blasted from rock a mile underground.

Once switched on, the detectors will quietly spark and flicker as ghostly particles called neutrinos hurtle through them. Some will arrive from the sun and distant supernovae, others in an intense beam sent through 800 miles (1,300km) of rock from a government laboratory near Chicago, Illinois.

Observing the neutrinos could uncover entirely new science and shed light on cosmic spectacles, such as the death of stars and the birth of black holes. Scientists are particularly hopeful of solving one of the most enduring puzzles in nature: why everything around us is made from matter and not its “evil twin”, antimatter. In other words, why we, and why anything, is here at all.

“It’s a super exciting time to be working on this,” said Dr Kate Shaw, an experimental particle physicist on Dune from the University of Sussex. “Neutrinos do not make sense yet and they could open a door to new physics.”

Neutrinos are the most abundant matter particles in the universe, outnumbered only by photons, the massless particles of light that illuminate the universe. They are produced in natural nuclear reactions, such as those that power the sun, in radioactive decay and in particle accelerators.

Even by physicists’ standards, neutrinos are oddballs. The chargeless particles barely interact with other matter: about a trillion pass unnoticed through your body each second. Neutrinos come in three varieties, all with minuscule but different masses. Weirdly, they change from one variety to another, a process called oscillation, as they travel. “It’s a very strange behaviour,” said Shaw. “It’s like me throwing you an apple and it turns into a banana.”

Lead has a history with these peculiar particles. In the 1960s, the physicist Ray Davis followed miners down the shaft and built a detector from a 100,000-gallon tank filled with dry-cleaning fluid. Shielded so deep beneath the ground, the detector became the first to record neutrinos from the sun as they struck chlorine atoms in the fluid and turned them into argon.

The mine closed in 2001 and transitioned, thanks to visionary forethought, into a dedicated subterranean laboratory, the Sanford Underground Research Facility (Surf). It is already home to the Lux-Zeplin (LZ) experiment which last month reported the first potential detection of dark matter.

“We’ve gone from mining the Earth to helping scientists explore the universe,” said Robin Lucero, Lead’s city administrator, whose family has lived locally for generations. “We’re incredibly proud and lucky that Dune is happening here, and Surf, which has other experiments like LZ. Having world-class science brings new opportunities, new people and new possibilities for our local businesses and especially our young people.”

Unlike Davis’s apparatus, the Dune experiment is enormous. Made from more than 4,000 tonnes of steel, the detectors will stand up to five-storeys high and hold a combined 70,000 tonnes of liquid argon that must be chilled to -200C. Assembling the detectors should take nearly three years. Filling them with liquid argon, another year.

“This is a heavy duty engineering operation,” and a partnership with the skilled local workforce, said Prof David Newbold, of the UK’s Science and Technology Facilities Council and a Dune spokesperson. “It’s like building a ship underground.” The UK is the largest international partner in the US-led experiment and is making core detector components, critical computing to reconstruct events in the detectors, and the graphite target needed to generate neutrinos at Femilab, a particle physics facility in Illinois.

Once operational, Dune will study an intense beam of neutrinos sent from Fermilab through 800 miles of the Earth’s crust and mantle. Most neutrinos will shoot straight through, but every so often one will strike an argon atom, creating a flicker of light and a pulse of electric charge. Analysing these will reveal intimate details about the mysterious particles.

One big question surrounds the masses of different neutrinos. Which is lightest and which heaviest? The answer could reveal fundamental new forces of nature. And in an attempt to answer the matter v antimatter question, Dune will look for subtle differences between neutrinos and their antimatter twins, antineutrinos.

Beyond the beams of particles sent from Fermilab, Dune will also study neutrinos arriving from space, carrying with them fresh information on stellar explosions and the formation of black holes.

Earlier this month, the Nobel prize for physics honoured the Belgian researcher Francis Halzen, who used a cubic kilometre of Antarctica to detect high-energy neutrinos from deep space. But detection is far from the end of the story. “This really is going to be a golden era,” Newbold said. “There’s a lot more of that to come.”