Mar. 15, 1999 MINNEAPOLIS / ST. PAUL--Physicists are preparing to go underground to solve one of nature's most baffling mysteries: whether the elusive subatomic particles known as neutrinos have mass. In search of a definitive answer, Fermi National Accelerator Laboratory in Chicago will send a beam of neutrinos underground to a University of Minnesota-run detection facility in Soudan Underground Mine State Park in northern Minnesota. Physicists will look for evidence that neutrinos leaving Fermilab change their character, or "flavor," before arriving at the mine a fraction of a second later; such a change would prove that neutrinos have mass.
The university has selected the Hugo, Minn., firm Lametti & Sons to excavate the new underground laboratory in Soudan that will house the Minnesota detector for the $146 million neutrino experiment, called MINOS (main injector neutrino oscillation search). The experiment, funded by the U.S. Department of Energy, the United Kingdom and the state of Minnesota, is expected to begin gathering data in 2002. Led by principal investigator Stanley Wojcicki (Voy-JIT-ski) of Stanford University, MINOS involves about 200 scientists from 20 institutions in five countries. MINOS is part of Fermilab's NUMI (Neutrinos at the Main Injector) project, headed by physicist Tom Fields.
Researchers hope that if the mass of neutrinos can be determined, so can their contribution to the total mass of the universe. Physicists estimate that about 80 to 90 percent of the mass in the universe is "dark matter": matter that can't be seen. Of this, neutrinos could account for as much as 10 percent. If so, their combined mass--and the gravity associated with objects that have mass--could have played a role in the formation of stars and galaxies throughout the universe. Further, knowing how much, if any, mass is tied up in neutrinos might help physicists develop a Theory of Everything to explain gravity, electromagnetism and the forces operating in the atomic nucleus, all in the same terms.
"Neutrinos are the lightest particles with mass, assuming they have mass," said University of Minnesota physicist Earl Peterson. "We want to know what the family ties between neutrinos are, just as we already know the family ties between quarks--the building blocks of protons and neutrons."
Previous studies of neutrinos coming from the upper atmosphere have hinted that the particles may have mass and change flavor while in motion. But studying the behavior of atmospheric neutrinos is difficult and fraught with uncertainties. Starting with a controlled and well understood population of neutrinos generated by a particle accelerator should make it easier to sort out what's going on, the researchers said.
"If the results from previous experiments turn out to be correct--if, indeed, neutrinos have mass--a new and very exciting area of scientific exploration will open up," said Wojcicki. "All of us are looking forward to being part of this adventure."
Two things for certain about neutrinos: They have no electric charge, and they are exceedingly small. Therefore, they usually pass through the densest matter without bumping into anything. This makes them very hard to detect. The University of Minnesota detection facility in the old Soudan iron mine in Tower, Minn., will await the beam of neutrinos with about 10 million pounds of steel plates--a huge, dense target to maximize the chance that neutrinos will hit an atomic nucleus.
"We'll probably run the beam of neutrinos nine months of the year for four years," said Peterson. "Each pulse will contain trillions of neutrinos. We might get a neutrino interaction, or hit, in about one in a thousand pulses. Each hit will produce a spatial pattern of electrical signals in detectors between the steel plates."
Neutrinos exist in three flavors: tau, muon and electron. They are produced naturally in the environment--for example, within the sun. Neutrinos are also produced when very energetic cosmic rays--nuclei of atoms streaming in from space--crash into atoms in the atmosphere. The collisions produce sprays of subatomic particles, which decay to leave two muon neutrinos for every electron neutrino. However, experiments detect too few muon neutrinos to correspond with that ratio. This deficit suggests that muon neutrinos change--or oscillate, as physicists put it--into other kinds of neutrinos as they travel from the upper atmosphere to detectors on the Earth.
Similarly, physicists in the MINOS experiment will be on the lookout for missing muon neutrinos. Fermilab will generate a beam of muon neutrinos and direct it through 445 miles of earth and rock to the Soudan mine in Tower, Minn. There, half a mile underground, the massive steel detector will determine whether the muon neutrinos all arrived, or whether some of them changed into other kinds en route. The detection must be performed underground to prevent interference from the millions of particles generated by cosmic rays.
The beam from Fermilab will send a pulse of neutrinos every 1.9 seconds. Each pulse will contain 300 trillion (300 million million) neutrinos. The distribution of neutrinos in the universe is about 300 per cubic centimeter.
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