Two years of SNO prove the case: Solar neutrinos really do change
Shown here under construction, the heart of the Sudbury Neutrino Observatory is a sphere 12 meters in diameter, surrounded by almost 10,000 photomultiplier tubes to catch the faint flashes of Cerenkov radiation that mark the passage of neutrinos through the heavy water filling the sphere.
May 13, 2002—The mystery of the missing solar neutrinos has been solved; case closed. Turns out the elementary 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.
Second-year results from SNO, a unique type of telescope located more than a mile underground in Canada, provides "unambiguous evidence" that most solar neutrinos undergo a metamorphosis during their 93-million-mile journey to Earth. These results, which were reported at the Joint American Physical Society/American Astronomical Society meetings in Albuquerque, New Mexico, on April 22, 2002, contradict the predictions of the Standard Model, the bedrock theory upon which rests our current understanding of the fundamental particles and forces of nature. However, they definitively answer a question that puzzled scientists for nearly three decades.
"For the first time, we are reporting on an important neutrino reaction in the SNO detector—a reaction in which all known neutrinos participate, regardless of their type," said Art McDonald of Queen's University, the director of the SNO project, which is a collaboration of close to 100 scientists at 11 universities and national laboratories in Canada, the United States, and the United Kingdom. "The successful observation of these neutrino signals has been a chief goal of the SNO collaboration and we are very pleased with the quality of the data obtained."
In June of 2001, the SNO collaboration reported results from its first year of operations that, in combination with experimental data from the Super-Kamiokande neutrino detector in Japan, indicated with greater than 99-percent certainty that neutrinos change type or, as physicists say, "flavor," on their way here from the sun. These new results close the door on any doubts.
Kevin Lesko, a physicist who leads the Neutrino Astrophysics Group for the Nuclear Science Division (NSD) of the Lawrence Berkeley National Laboratory (Berkeley Lab), explains that "because these results are derived from a single experiment, they do not involve the complications of combining other experiment's results. As a consequence, our evidence is so very strong that it is 99.999 percent certain that SNO has seen a neutrino flavor change."
Under 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 been able to measure far fewer solar neutrinos than they should have detected, based on what we know about thermonuclear reactions. SNO changed that by being the first neutrino telescope sensitive enough to simultaneously measure all three neutrino flavors.
Operating out of a nickel mine near Sudbury in the Canadian province of Ontario, SNO consists of a 58,000 pound stainless steel geodesic sphere, over 12 meters in diameter, suspended in a pool filled with 7,000 tons of purified water. Inside this sphere is an acrylic vessel filled with 1,000 metric tons of heavy water (deuterium oxide, or D2O). Attached to the sphere are 9,456 ultra-sensitive 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.
"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," said project leader McDonald. "The SNO team is really excited because these 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 mass, which 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 included physicists Bob Stokstad, Eric Norman, Yuen-dat Chan, and Alan Poon, plus postdocs 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 is hosted by Berkeley Lab, whose participation in SNO dates back to the project's earliest days in 1989.—by Lynn Yarris
Historical Neutrino Research: "The
Solar Neutrino Experiment," Brookhaven National
Laboratory's history of neutrino research dates to the
early 1970s, when scientist Ray Davis’s pioneering work
in a South Dakota gold mine sent the neutrino world
into an uproar by first documenting the missing electron
Funding: The Sudbury Neutrino Observatory is supported, in part, by the Department of Energy's Office of High Energy and Nuclear Physics within the Office of Science. A complete list of SNO participants and supporters is available on the SNO Institutions and Funding web page.
Author: Lynn Yarris, science writer and media coordinator, heads Berkeley Lab's efforts to assist science and technical news media in reporting on research. For more science news, see Berkeley Lab's Science Beat.
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