Open house at MINOS, in an old iron ore mine in Soudan, Minn. In the center is one of the 486 massive octagonal steel sheets that make up the 100-foot-long neutrino detector.

Pitt Researcher Begins
Worldwide Collaboration
To Study Neutrinos

How is it possible to weigh particles formerly thought to have no mass? With a 6,000-ton machine located half a mile underground, of course.

University of Pittsburgh physicist Donna Naples will collaborate with more than 200 scientists worldwide for the Main Injector Neutrino Oscillations Search (MINOS) at the U.S. Department of Energy's (DOE) Fermi National Accelerator Laboratory. Data from the massive detector will be used to explore the phenomenon of neutrino mass.

The three known neutrinos are elementary particles, and they rarely interact with matter, despite their abundance-10 million per cubic foot. Experimental results obtained over the last five years have confirmed that the evasive particles have mass and switch back and forth among their three different identities, but most of their properties still remain unknown.

"Nonzero neutrino mass has important implications for cosmology. Neutrinos, which are stable and abundant particles, had a role in the dynamics of the early universe," said Naples, assistant professor in the Department of Physics and Astronomy. "The catch is, to see a few neutrinos interact, we need a very large detector with lots of protons and neutrons as targets for the neutrinos to hit.

"The MINOS experiment will make a more precise determination of neutrinos' mass. A beam of one type of neutrinos—muon neutrinos—with well-known energy will be produced at Fermilab," said Naples, who is one of 32 principal investigators. "The neutrino beam will travel through the earth, some 730 kilometers, and emerge at our detector in Soudan, Minn., where we will observe what type of neutrinos are present in the beam. If neutrinos have oscillated, we will observe a different type—tau neutrinos—at the Soudan location."

In July, after four years of mining and construction, workers finished building the first of two detectors of the ambitious MINOS particle physics experiment, which will take place deep in a historic iron mine in northern Minnesota. The looming 100-foot-long detector consists of 486 massive octagonal planes, lined up like the slices of a loaf of bread. Each plane consists of a sheet of steel about 25-feet high and one-inch thick, covered on one side with a layer of scintillating plastic, which converts particle collisions into digital data signals. To construct the detector, technicians transported all detector components in small sections through a tiny antique elevator cage that once transported miners underground.

"It was like building a ship in a bottle," said MINOS spokesperson Stanley Wojcicki, professor in Stanford University's Department of Physics. "We needed to bring all the material underground and assemble it right there. The last step was to install a magnetic coil and energize it. MINOS is the only large-scale neutrino experiment underground that can separate neutrino and antineutrino interactions, allowing us to look for differences in their behavior."

At present, the new detector is recording cosmic ray showers penetrating the earth. The data will provide first tests of matter-antimatter symmetry in neutrino processes. In early 2005, when the construction of a neutrino beamline at Fermilab is complete, the experiment will enter its next phase. More than a trillion man-made neutrinos per year will pass through the MINOS detector in Soudan. Because neutrinos rarely interact with their surroundings, only about 1,500 of them will collide with an atomic nucleus inside the detector. The rest will traverse the detector without leaving a track.

Funding for the MINOS experiment has come from the DOE's Office of Science, the British Particle Physics and Astronomy Research Council, the National Science Foundation, the State of Minnesota, and the University of Minnesota. Scientists from Brazil, France, Greece, Russia, the United Kingdom, and the United States are involved in the project. • CK