Groundbreaking in the Land of 10,000 Neutrino Interactions
Neutrino hunters break ground underground for MINOS detector hall.
The long, straight road, empty of traffic, stretches through forests of pine, spruce, birch and poplar. Ahead, the silver skin of yet another lake gleams through the trees. Lost Lake, Clear Lake, Lake Vermilion. The road widens slightly for the town of Tower (pop. 561), where Zup’s supermarket (www.zups.com) advertises hot bologna (“A taste treat that’s hard to beat”) every Thursday. A local radio station promotes handsewn mukluks to keep the toes toasty in temperatures of 40 below—a reminder, on a warm summer afternoon, of the evanescence of the season. Northeastern Minnesota—the Iron Range, the self-proclaimed “End of the Road”—is a long way from the western suburbs of Chicago.
But not for a neutrino.
Neutrinos make the 450-mile trip from Fermilab’s Main Injector in Illinois to the Soudan Mine, just up the road from Tower, Minnesota, in about two milliseconds. And last month their destination got a lot closer with the groundbreaking, on July 20, for a deep cavern that will house the 5,000-ton steel MINOS detector that will record their journey.
Actually, it was an undergroundbreaking. Half a mile beneath the earth’s surface, where iron mining operations ceased in 1962, officials wielding gold-toned pickaxes took the first strokes to chip away, at least symbolically, at the mysteries surrounding the elusive neutrino.
“How appropriate it is,” said U.S. Representative James Oberstar (D-MN), in a videotaped statement shown at the groundbreaking, “in this place where miners from many nations unlocked nature’s hidden treasures and extracted the richest iron ore, that today scientists from many nations are now working this mine to wrest from the very same billion-year-old rock a new lode of riches as old as the origins of the universe itself, the baffling neutrino.”
Until recently, scientists believed that, unlike other fundamental particles of matter, neutrinos possessed neither mass nor electric charge. However, recent results from experiments in Japan, in Soudan itself, and elsewhere seem to point to a small mass for these particles. Because they are so numerous—each square meter of the atmosphere contains about 300 million—even a tiny mass for the neutrino would have big consequences for our understanding of the nature and distribution of mass in the universe.
For the MINOS experiment, Fermilab will direct an intense beam of one type of neutrinos, called muon neutrinos, from the newly-completed Main Injector accelerator to the Soudan detector, 450 miles away. Locating the detector half a mile below ground allows scientists to screen out cosmic rays that would otherwise flood the particle detector with unwanted signals. Beginning in early 2003, the MINOS (for “Main Injector Neutrino Oscillation Search”) collaboration will use the detector to determine whether some of the muon neutrinos in the beam have changed to another type, known as tau neutrinos.
Such a change, or oscillation, from one type to another, would constitute clear evidence for neutrino mass and would allow physicists to begin to calculate just how much mass the particles possess.
The MINOS experiment differs from earlier neutrino-mass experiments because it uses an accelerator-produced beam of neutrinos rather than naturally occurring neutrinos from reactions in the sun and from cosmic ray interactions in the atmosphere. Earlier experimenters detected fewer solar and cosmic-ray neutrinos than they expected, leading them to conclude that one type of neutrino had oscillated to another type and hence “disappeared” from detection. In contrast, the MINOS experiment is designed to detect not only the disappearance of muon neutrinos, but also their appearance as neutrinos of a different type, tau neutrinos. When the experiment begins operating, experimenters expect to observe upwards of 10,000 neutrino interactions each year.
Speakers at the undergroundbreaking included officials of the University of Minnesota, the Department of Energy, the Minnesota Department of Natural Resources, the MINOS collaboration and Fermilab.
“Just as we build telescopes to explore unknown reaches of the universe,” said Fermilab Director Mike Witherell, “we build accelerators to help us fill in this part of the universe, the neutrinos, that we don’t understand.”
Witherell invited guests at the groundbreaking to return in just over three years for “first light,” or its neutrino equivalent, at the detector.
“I remind you that this is northern Minnesota and we’ll be detecting the first neutrinos in the month of January,” quipped University of Minnesota physicist Earl Peterson, manager of the university’s Soudan Laboratory and master of ceremonies for the groundbreaking. “We’ll find out who really cares about this experiment.”
Christine Maziar, vice president for research and dean of the graduate school at the University of Minnesota, told the audience that groundbreaking day for MINOS had been ten years in the making.
“We are very eager to stop moving piles of paper and start moving rock,” she said, and added, “I hope that all of you will leave here today with the clear understanding that we Golden Gophers do it underground.”
Oberstar called the MINOS groundbreaking a “landmark event in the history of physics,” and noted that it would also have beneficial economic consequences for Northeastern Minnesota, creating new jobs and injecting some $14 million dollars into the local economy over the next few years.
That’ll buy a lot of mukluks—and, perhaps, a new understanding of the fundamental nature of the universe.