Scientists Try to Pin Down Elusive Neutrinos

By Bijal P. Trivedi
National Geographic Today
May 21, 2002

Deep in a 19th-century iron mine in a Minnesota state park, in a football-field-size cavern, physicists are building a 6,000-ton steel trap for neutrinos, sub-atomic particles so elusive scientists don't even know if they have any mass.

Neutrinos rarely interact with other matter but are essential in the running of the universe. The sun, for instance, couldn't shine without them.

"There are 300 neutrinos in every cubic centimeter of the entire universe," says Stanley Wojcicki, a Stanford University physics professor with MINOS (Main Injector Neutrino Oscillation Search), an international consortium determined to pin down the neutrinos, which have been around since the universe began. More are generated each time a star collapses.

The U.S. $160 million project is designed to encourage the ghost-like particles to stop by long enough for scientists to take a closer look at just what they are, how they behave and what it all means in understanding the nature of the universe. Several hundred researchers from 30 institutions in the United States, Great Britain, Russia, China, and Greece are involved.

Scientists want to know if the neutrinos change form as they travel, which would suggest they have mass after all. The findings may challenge prevailing theories about how it all began and overturn fundamental laws of physics, predicts Janet Conrad, a neutrino physicist at Columbia University.

"If a neutrino has mass, that tells you that the Standard Model is wrong. And that is exciting," she says. Since the 1960s, physicists have used the Standard Model to explain the how the universe works. In that model, neutrino mass should be zero.

To encounter enough of the shy particles for the study, MINOS will send an intense beam of neutrinos from an accelerator at the Fermilab near Chicago to a detection chamber in Minnesota, one-half mile below the Soudan Underground Mine State Park, not far from the Canadian border.

The neutrinos will be shot 450 miles (735 kilometers) through the Earth, making the trip in a fraction of a second, in a series of experiments beginning in 2005. The Soudan laboratory, one of a handful of underground physics laboratories around the world, has also been used in researching proton decay as a way of unraveling the mysteries of the universe.

Accelerating the stream and trapping the neutrinos in an underground physics lab will increase the chances of meeting up with the particles, but it will take trillions of neutrinos to obtain even a few thousand interaction "events," as the scientists call them.

Most neutrinos will pass through all of the nearly 500, 12-ton steel plates in the trap without ever bumping to any of the steel atoms. A few, however, will collide with atomic nuclei in the plates, generating a shower of electrically charged particles to trigger a flash of light.

Wolfgang Pauli, who won the Nobel Prize in physics in 1945, suggested the existence of neutrinos in 1930 as an explanation for an apparent nonconservation of energy when radioactive particles decayed, but it wasn't until 1956 that anyone ever detected one. Subsequent research identified three neutrino species—electron, muon, and tau.

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