The Present State of Neutrino Masses
Uploaded by ohm07 on Jun 25, 2001
One of the current questions in physics is whether or not neutrinos have mass and what this mass is. Neutrinos are subatomic particles that have no electrical charge and interact only via the weak nuclear force. They are products of radioactive decay processes, and thus are produced abundantly in our Sun, our atmosphere, and in other astrophysical sources such as supernovae and active galactic nuclei. Millions and millions of them are crossing through the Earth every second, but only very few of them will interact with the Earth. In practice you can say they are invisible. But fortunately we can detect them by building a very large detector and waiting long enough.
There are several reasons to search for a possible non-zero neutrino mass. Fermion masses in general are one of the major mysteries/problems of the standard model. Observation or nonobservation of the neutrino masses could introduce a useful new perspective on the subject. Nonzero neutrino masses are predicted in most extensions of the standard model. They therefore constitute a powerful probe of new physics. Also, there may be a hot dark matter component to the universe. If so, neutrinos would be (one of) the most important things in the universe. The observed spectral distortion and deficit of solar neutrinos is most easily accounted for by the oscillations/conversions of a massive neutrino.
The largest neutrino detector is the Super-Kamiokande and is located in the Kamioka Mine, about 200 km north of Tokyo. It is water cerenkov detector, which means it is a large (40 meters diameter by 40 meters tall) tank of ultra-pure water viewed by thousands of sensitive phototubes. Super-Kamiokande will address some of the most important open questions in physics today, such as: why does the Sun appear to produce only half as many neutrinos as theory would predict? Do neutrinos have mass? Do protons decay, as predicted by Grand Unification Theory?
One source of neutrinos are nuclear reactions. Inside our Sun nuclear reactions are occurring on a gigantic scale. Lots of neutrinos are produced. There are enough of them, that when they reach the Earth they can still be detected. Since physicists can calculate how many of them should be seen, there is a big problem because we see too few, roughly two times too few. This is so called the solar neutrino problem.
There can be several solutions to the puzzle. One is that we do not...