Retiring a detector

For over a century, the Iron Range of northern Minnesota provided the United States with a significant chunk of its iron ore. Many of the mines closed in the first half of the 20th century when it was no longer profitable to haul the ore up from deep underground. But near the small town of Tower, a mine, called Soudan, remains open to the public for guided tours. Donning hard hats and descending half a mile through 4.5-billion-year-old bedrock in a rattling mine cage, visitors can experience for themselves what the original conditions of the mine were like before it closed in 1962.

If, however, the visitors turn right instead of left when they leave the mine cage that brought them down to Level 27, they’ll come face to face with a bright red steel door. Beyond it, they’ll find a cavern as large as a football field and four stories tall. In the center sits a massive device made of alternating layers of steel and plastic. It’s a particle detector, called MINOS, short for Main Injector Neutrino Oscillation Search, and scientists are using it as part of their hunt to understand neutrinos, some of the most common yet elusive building blocks of matter in the universe.

“It’s actually like a James Bond villain’s laboratory in a mountain,” says Jim Essig, the park supervisor for Lake Vermilion-Soudan Underground Mine State Park. Aside from the mine tours, the state park also offers tours of the laboratory. Essig estimates that of the 32,000 visitors who go underground each year, about 15 percent choose to tour the laboratory instead of the mine. They include thousands of school kids, many of whose physics teachers work as tour guides during the summer. Essig thinks students have gotten excited about physics during their visits. “When I was a kid, physics was a dirty word. It was the last class you wanted to take. I think we’ve changed that. They get to see there’s a really cool side of physics.”

But that could all change soon. MINOS is set to run through the end of September 2016, but after that, its future is uncertain. What happens next at Soudan could determine the future of high energy physics research in the northern Midwest.

MINOS started taking data in 2003, when construction was completed. It’s the most recent detector in a partnership between Soudan and Fermi National Lab in Chicago that extends back to 1982. Currently, Fermilab fires a beam of subatomic particles called neutrinos more than 450 miles in a straight line — through solid rock — towards MINOS, as well as a closer detector located at Fermilab itself. This may seem like a strange thing to do, but it’s possible because of neutrinos’ bizarre behavior. They’re what are known as weakly interacting particles, which means that they almost always pass through objects without any effect. “You have tens of trillions of neutrinos passing through you every second from the sun,” says Bill Lee of Fermilab, who coordinates the MINOS project. But it’s extremely rare for them to actually interact with the matter they pass through. “Over your lifetime, you will probably have one solar neutrino interaction in your body.”

The Swiss physicist Wolfgang Pauli proposed the theoretical existence of the neutrino in 1930, and the ghost-like particle has enticed scientists ever since. Its name, meaning “little neutral one,” was coined by the Italian physicist Enrico Fermi to distinguish it from the larger and more well-known neutron. For three decades, it was assumed to simply be a particle with no electric charge and no mass. “We had a nice individual particle,” says Lee. “We thought we knew everything about them.”

But they had gotten it wrong. In the 1960s, American physicist Ray Davis discovered that physicists were only detecting a third of the expected number of solar neutrinos. From 1970 to 1994, he led the Homestake experiment, based in Homestake, South Dakota, to address this infamous solar neutrino problem. What had been a nice individual particle turned out to be three particles that could change into one another.

A single neutrino, as it travels, oscillates between three ‘flavor states,’ different varieties of neutrinos with different properties. “We’re getting into the oddness of quantum mechanics,” says Patricia Vahle at the College of William and Mary, who’s worked with MINOS since 2000. Vahle says that even things like the mass of a particle can be hard to determine at these small scales.

The confirmation of these oscillations was so important that this year’s Nobel Prize in Physics was given to two researchers running separate experiments, Arthur McDonald and Takaaki Kajita. They independently published similar results that proved neutrinos changed flavors in 1998, just as researchers were constructing MINOS.

“If they hadn’t made that announcement, MINOS may have provided the evidence for neutrino oscillations,” says Karol Lang from the University of Texas at Austin, who’s been associated with MINOS since its beginning. Although it came online a few years too late, it has still made major contributions to neutrino research in an area called mass splitting. Mass splitting refers to the difference in mass each flavor is most likely to have. For physicists, it still remains a hotly researched area. The specific problem that MINOS works on is the ‘mass hierarchy problem,’ since it’s still not clear which flavor is the most massive and which is least.

There are other areas of neutrino research, however, that have been gaining attention, and MINOS isn’t equipped to handle them. So at the end of its run in September 2016, it’s likely the detector will be dismantled and hauled up the same mine cage visitors use to take the mine tours. For now, what might happen to the space after that is an open question. “It’s a big conversation right now,” says Essig. “I would love to see another physics experiment there.”

And it might happen. Fermilab has another detector along the Soudan beamline, called NOvA, located up in Ash River on the Minnesota-Canada border. Researchers hope that this detector will help solve that mass hierarchy problem, says Bill Miller, the lab manager at MINOS for many years who is now managing NOvA. It could also help physicists explain why we live in a universe dominated by matter — not antimatter — something they currently have no good explanation for. Physicists think the universe has a lot of symmetry, so imbalances like this are always exciting. “When they aren’t symmetrical, it’s telling us things don’t work as expected,” says Vahle. Explaining this asymmetry may give physicists new insights into the fundamental nature of the universe.

But despite the hopes, Soudan may get passed over in favor of other options. Homestake Mine in South Dakota is the site for the next big underground laboratory, and Fermilab may decide to shoot its neutrinos in that direction in a few years. But the underground space is valuable and other research groups might find a use for it. Thanks to NOvA’s presence, the beam of neutrinos will pass through the cavern for the next five to ten years anyway, so it’s not unreasonable to think other physicists may want to use the space, Essig says. And even if that doesn’t happen, there are always other proposals, he adds. “‘What a great place to have a Hard Rock Cafe’ is one I hear a lot.”