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17 April 2014 12:48 pm ,
Vol. 344 ,
Officials last week revealed that the U.S. contribution to ITER could cost $3.9 billion by 2034—roughly four times the...
An experimental hepatitis B drug that looked safe in animal trials tragically killed five of 15 patients in 1993. Now,...
Using the two high-quality genomes that exist for Neandertals and Denisovans, researchers find clues to gene activity...
A new report from the Intergovernmental Panel on Climate Change (IPCC) concludes that humanity has done little to slow...
Astronomers have discovered an Earth-sized planet in the habitable zone of a red dwarf—a star cooler than the sun—500...
Three years ago, Jennifer Francis of Rutgers University proposed that a warming Arctic was altering the behavior of the...
- 17 April 2014 12:48 pm , Vol. 344 , #6181
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Physicists Edge Closer to Solving Neutrino Riddle
11 June 1999 8:00 pm
The first neutrinos have been spotted colliding with heavy water molecules in a giant tank at the bottom of an Ontario nickel mine. Announced on Wednesday, the events mark the inauguration of the Sudbury Neutrino Observatory (SNO), a new facility that physicists hope will finally solve the solar neutrino problem, which has been haunting the field for decades.
Generated by the sun's nuclear processes, nearly a billion solar neutrinos--ghostly subatomic particles that can easily pass through Earth without hitting anything--shower down on each square centimeter of the planet's surface every second. Although neutrinos come in three "flavors," electron, muon, and tau, existing detectors can only see the electron variety, and they only see half as many as theorists had predicted. To resolve this discrepancy, physicists have proposed that half of the solar electron neutrinos switch flavors, or "oscillate," on their way from the sun's center to Earth. SNO was built to test this theory.
The key is its ability to see several varieties of solar neutrinos at once. SNO contains 1000 tons of ultrapure heavy water, water in which the hydrogen atoms have been replaced with deuterons, whose nuclei have a proton and a neutron. When an electron neutrino collides with a heavy water molecule, it can split apart the neutron and the proton and eject an electron. Other neutrino flavors split the nuclei but don't scatter electrons. By counting both neutrons and electrons, SNO should be able to measure both the total number of incoming neutrinos and the fraction of electron neutrinos, says physicist and SNO spokesman David Wark of Oxford University, England. If SNO finds that the shortfall of electron neutrinos is made up in other flavors, it will provide strong support for oscillations.
"It is an extremely important experiment," agrees physicist Paul Langacker of the University of Pennsylvania in Philadelphia. "They will very likely ascertain definitively whether neutrino oscillations are taking place." Unfortunately, physicists will have to be patient: Neutrinos collide with matter so rarely that SNO will detect only some 20 neutrinos every day. As a result, says Wark, "It will be at least a year" before SNO has an answer.