To see the smallest, most elusive particle in the universe, you need only drive five hours north of the Twin Cities to Soudan, an Iron Range hamlet of around 500 souls located 20 miles from Ely. Follow a winding road up the bluff overlooking the town to the site of an old iron mine, then take a clanking elevator down 2,343 feet to a tunnel full of bats and smelling of incipient mildew, walk through a big green door that looks a bit like the gates of Oz, and into a high-ceilinged cavern roughly the size and shape of an airplane hangar. There, past a bank of fluorescent-lit offices where strange, smart men do inscrutable things with computers, past a huge, gaudy mural of the solar system painted on the cave wall, you will come upon the MINOS Far Detector, a gigantic assemblage of steel and wiring resembling a house-sized loaf of bread.
Campbell Laird
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MINOS, which stands for Main Injector Neutrino Oscillation Search, has two parts. The first, the far detector, is constructed from 486 one-inch-thick octagonal steel plates. Sandwiched between these are half-inch-thick layers of plastic connected by nests of cable to thousands of phototubes. The detector weighs some 6,000 tons, cost around $40 million to build, and contains as much steel as a typical battleship.
The second part of the MINOS experiment is 450 miles away, at Fermi National Accelerator Laboratory in Batavia, Illinois. Sometime early in 2005, Fermi Lab's particle accelerator, the most powerful on Earth, will begin shooting trillions of protons northward in timed bursts. Traveling through a 5,000-foot-long tunnel beneath Fermi Lab's campus, the protons will be winnowed into still smaller particles, which will then fan out toward northern Minnesota. Passing through hundreds of miles of rock beneath the Midwest, these subatomic rockets will reach their destination in about two-thousandths of a second. If MINOS works as planned and if luck holds, one or two of them will rustle the particles of that steel bread-loaf as they whistle past toward the edge of the galaxy.
"We're expecting one interaction every few hours," explains Earl Peterson, manager of the MINOS lab in Soudan. Peterson is a physicist at the University of Minnesota--chief among the 35 institutions from six countries collaborating on the experiment--and he looks very much the part, with a slightly scraggly white beard and a blue cardigan worn through at the elbows. A miasma of cigarette smoke follows Peterson as he shows me around the underground lab where he's spent a good part of the past 20 years designing increasingly sophisticated and ambitious physics experiments. On this particular day in late July, he is overseeing the completion of the far detector, which, as it turns out, has come together well ahead of schedule and long before Peterson's colleagues at Fermi Lab will be ready to begin their subatomic fusillade. "The beam diverges like a flashlight," Peterson explains. "Traveling through 450 miles of rock, you will lose maybe one-one-thousandth of a percent of the neutrinos."
Peterson walks over to a computer station on a catwalk near the loaf's flank. "You don't have a pacemaker, do you?" he asks, noting that the detector produces a magnetic field 10 times that of the Earth's in order to corral wayward neutrinos. He picks up a sheet of paper printed with a spray of colored dots--a simple graphic representation of the wake a neutrino might leave as it slices through the detector's sandwich layers.
The thing about neutrinos is that they're small. Infinitesimally small. So small that they can slip through galaxies unperturbed, passing through stars and planets and human beings without ever touching anything. So small, in fact, that until relatively recently scientists believed that they didn't have any mass at all. Blow your typical hydrogen atom up to 1,000 feet in diameter, and an electron would be about the size of this period. (An ant, by way of comparison, would be 20 million miles long.) The smallest neutrino is 10,000 times smaller still. Blow that hydrogen atom up to the size of the Earth, and the neutrino would be the size of a dust mote. That, as you would expect, makes them an especially elusive quarry for physicists: Imagine trying to fire a baseball at a catcher's mitt 100,000 miles away, and you get some idea of the scale of difficulty involved.
The problem of detecting neutrinos is further complicated by their chameleonic nature. The particles come in three flavors, the electron, tau-, and muon-neutrino. (A 1995 experiment at Los Alamos National Laboratory in New Mexico suggested the possibility of a fourth, a large "sterile" neutrino, but Peterson, like most physicists, remains skeptical of its existence.) As they follow their lonely trajectories through the heavens, neutrinos oscillate from one flavor to another, vibrating like the pitch of a musical instrument from, say, C-sharp to B-flat. Imagine that the neutrinos fired from Fermi Lab are SUVs on a highway; by the time they finish their commute to Soudan, they may have modulated into compact cars or semi-trucks or some mix of the two. If you were to set up a roadblock only for SUVs, the compacts and trucks would simply slip past, and you'd never be the wiser.
