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The Case of the Purloined Identity
    Sherlock Holmes would have been proud. The mystery of the missing solar neutrinos has been solved and the solution was elementary. Turns out the ghostlike particles with no electric charge and little mass, which are emitted out of thermonuclear reactions in the core of the sun, weren't missing after all. They were merely changing their identity in transit from the sun to the earth.
Kevin Lesko, a physicist who leads the Neutrino Astrophysics Group for Berkeley Lab's Nuclear Science Division, poses with a single photomultiplier tube from the Sudbury Neutrino Observatory (SNO), a unique telescope located more than a mile underground to study the elusive neutrino.  

Berkeley Lab researchers were part of an international team of scientific sleuths that cracked the case through the use of the Sudbury Neutrino Observatory (SNO), a unique type of telescope located more than a mile underground in Canada. Two years of investigation yielded "unambiguous evidence" that most solar neutrinos undergo a metamorphosis during their 93-million-mile journey to Earth, changing from one type or, as physicists say, "flavor," to another.

These results contradict the predictions of the Standard Model, the bedrock theory upon which rests the current scientific understanding of the fundamental particles and forces of nature, but they definitively answer a question that has puzzled scientists for nearly three decades.

"Because these results are derived from a single experiment, they do not involve the complications of combining other experiments' results," said Kevin Lesko, a physicist who leads the Neutrino Astrophysics Group for the Berkeley Lab's Nuclear Science Division (NSD). "As a consequence, our evidence is so very strong that it is 99.999 percent certain that SNO has seen a flavor change in neutrinos."

In June of 2001, the SNO collaborators reported results from their first year of investigation that, in combination with experimental data from a neutrino detector in Japan, the Super-Kamiokande, strongly pointed to neutrinos switching their flavor on their way here from the sun. This second year's worth of data should erase any lingering doubts.

According to the Standard Model, neutrinos come in three flavors--electron, muon and tau neutrinos. Electron neutrinos are by far the most common and are produced within the core of the sun (and in supernovae) at the rate of more than two hundred trillion trillion every second. For more than 30 years, experimenters have measured far fewer solar neutrinos than they should have detected based on what is known about thermonuclear reactions. However, the rules of neutrino observation have been substantially upgraded with the arrival of SNO, which is the first neutrino telescope sensitive enough to simultaneously measure all three neutrino flavors.

"Two years of investigation yielded "unambiguous evidence" that most solar neutrinos undergo a metamorphosis during their 93-million-mile journey to Earth."

Operating out of a nickel mine near Sudbury, in the Canadian province of Ontario, SNO consists of an 18-meter-diameter, 58,000-pound stainless steel geodesic sphere suspended in a pool filled with 7,000 tons of purified water. Inside this sphere is an acrylic vessel filled with 1,000 tons of heavy water (deuterium oxide or D2O). Attached to the sphere are 9,456 ultrasensitive light sensors called photomultiplier tubes. When neutrinos passing through the heavy water interact with deuterium nuclei, flashes of light, called Cerenkov radiation, are emitted. The photomultiplier tubes detect these light flashes and convert them into electronic signals that scientists can analyze.

In a press release announcing the results from the second year of investigation, Art McDonald of Queen's University, director of the SNO research team, a collaboration of close to 100 scientists at 11 universities and national laboratories in Canada, the United States, and the United Kingdom, declared the case of the missing solar neutrinos to be closed.

"While the number of electron neutrinos we detected is only about one-third the number expected, the total number of the three types of neutrinos we observe is in excellent agreement with calculations of the nuclear reactions powering the sun," McDonald said. "The SNO team is very pleased with the quality of the data obtained and really excited because our measurements enable neutrino properties to be defined with much greater certainty in fundamental theories of elementary particles."

The fact that neutrinos are changing flavor during their journey means they have must have mass, even though it is very small. This again runs contrary to Standard Model predictions.

Says Lesko, "The Standard Model needs to be expanded to embrace neutrino mass and mixing. This is a challenge, and one that will take some time."

In addition to Lesko, other members of the Neutrino Astrophysics Group at Berkeley Lab who contributed to the latest SNO results include physicists Bob Stokstad, Eric Norman, Yuen-dat Chan, and Alan Poon, plus post-docs Colin Okada and Xin Chen, graduate students Alysia Marino of UC Berkeley and Sarah Rosendahl from Sweden, and undergraduate students Kathy Opachich of UC Berkeley and Noah Oblath from Cornell University. To run their data analysis, the SNO collaboration made extensive use of the supercomputing facilities at the National Energy Research Scientific Computing Center (NERSC), which is hosted by Berkeley Lab.

-- Lynn Yarris

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