Word: neutrinos
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Dates: during 1990-1999
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...universe may have just gotten a lot heavier. A team of American and Japanese physicists announced Friday that the neutrino -- a pervasive but elusive subatomic particle long considered to be weightless -- may have mass after all, which could solve the mystery of why the universe doesn't seem as heavy as science says it should...
...working on a neutrino experiment at Harvard and in a high energy physics lab in Geneva, Switzerland, said that he is also sticking with physics in graduate school...
This is how existing neutrino detectors work in Japan, Italy, Russia and the U.S. What makes S.N.O. different is its exclusive use of heavy water, abundantly available in Canada because it is stockpiled for use in a type of nuclear reactor Canadians favor. Says Barry Robertson, S.N.O.'s associate director: "It's the heavy water that makes this project worth the trouble." An extra neutron in the nucleus doesn't make the water's appearance, chemistry or taste any different from ordinary water used in other detectors. It does, however, change its nuclear structure enough to make this observatory sensitive...
This property has Bahcall and other physicists speaking in superlatives about S.N.O., because it will allow the device to solve one of the enduring mysteries of astrophysics. Known as the solar neutrino problem, it was discovered back in the 1960s. According to calculations originally made by Bahcall, the nuclear fusion reactions at the sun's core should be generating about 200 trillion trillion trillion electron neutrinos every second. But when physicists set out to find them, they were shocked to see evidence of only about a third that number. Among the possible explanations: perhaps scientists didn't understand nuclear physics...
...physicists eventually became intrigued with a third idea. Perhaps some electron neutrinos were switching identities, changing by a process called oscillation into muon or tau neutrinos (the two other varieties) en route to Earth. If so, existing detectors could never see them. And while some of the fine print in the laws of physics says that a massless neutrino can't change its stripes, a neutrino with even a tiny bit of mass might. If neutrinos have mass, they can change; conversely, if they can change, they must have mass, despite what textbooks have been saying for decades...