ScienceDaily (Mar. 17, 2005) — Livermore scientists are working to solve a 50-year-old question: Can neutrinos – a particle that is relatively massless, has no electric charge yet is fundamental to the make-up of the universe – transform from one type to another?

Scientists are using two giant detectors, one at Fermi Lab and another in a historic iron mine in northern Minnesota, to work on the answer.

As part of the international team working on the Main Injector Neutrino Oscillation Search (MINOS) project, Lawrence Livermore National Laboratory researchers will use the detectors to explore the mysterious nature and properties of neutrinos. Namely, they will seek to discover how neutrinos "change flavors."

Neutrinos come in three "flavors:" electron, muon and tau. Each is related to a charged particle, which gives the corresponding neutrino its name. Neutrinos are extremely difficult to detect because they rarely interact with anything. Though they can easily pass through a planet, solid walls and even a human hand, they rarely leave a trace of their existence.

"The probability of a neutrino interacting with anything is very small," said LLNL's Peter Barnes, who along with Livermore's Doug Wright and Ed Hartouni, is working on the MINOS experiment. "If you want to detect any neutrinos, you need something big."

Barnes, Wright and Hartouni are hoping that something big is a 6,000-ton detector lying deep in the Soudan, Minn. mine. The neutrinos will be generated along the underground beam line at Fermi Lab, will pass through the near detector at Fermi, and will travel through the Earth to the detector in Minnesota. Neutrinos are more easily detected when they are generated at a high energy (such as those at Fermi Lab).

The MINOS scientists chose the distance to the far detector to maximize the oscillation probability, which gives them the best opportunity to directly study the neutrino "flavor change."

Fusion in the sun results in electron neutrinos and scientists have predicted that if they can measure the electron neutrinos coming from the sun, they can measure the core of the sun. However, early experiments showed that less than half the expected neutrinos were observed on Earth. The idea that the missing electron neutrinos may have transformed into another type or "flavor" came alive.

This conclusion indicates that neutrinos do have some mass, small as it may be, in order for them to oscillate. So a portion of the electron neutrinos emitted from the sun could have changed flavors to muon or tau neutrinos before reaching Earth, thus solving the missing neutrino problem.

But it still doesn't explain how or why this occurs, Barnes said. "Our goal is to understand the flavor oscillation properties of neutrinos," he said.

Studying the elusive neutrino will help scientists better understand particle physics, specifically how particles acquire mass, as well as its role in the formation of the universe and its relationship to dark matter.

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Livermore's portion of the project is funded by Laboratory Directed Research and Development and Physical Data Research Program dollars. The MINOS effort as a whole is funded by the Department of Energy's Office of Science, High Energy Physics division.

The MINOS far detector is located in a cavern half a mile underground in the Soudan Underground Laboratory, Minnesota. The 100-foot-long MINOS far 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, with the last one visible in the photo. The whole detector weighs 6,000 tons. Since August 2003, the far detector has collected data on cosmic rays and neutrinos. Now, scientists are using the detector to record man-made neutrinos. The MINOS collaboration expects to record about 1,000 neutrinos per year. (Image courtesy of Fermilab)

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