h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Experiments are now starting to show evidence for the mass of neutrino's. These experiments were set up to try and solve the solar neutrino problem. This is, the Sun emits electron neutrino's (but not tau or muon neutrino's) which we can detect here on Earth. Unfortunately we cannot detect enough to fit with theories. However, there is evidence that we are detecting more muon (or it could be tau) neutrino's than expected. An explanation of this is that neutrino's can change their type spontaneously, but to do this requires mass. You are right to say that this would be a very small amount and it would not account for all the Dark Matter, but it appears to be non-zero.

Wick

I was under the impression that neutrinos were first theorized when they found the total mass and energy produced after collisions and decay did not follow the law of conservation of mass and energy, so they felt something else must be given off, which were neutrinos. So they are not massless.

~Sirona

Neutrinos were introduced to explain beta decay, as this process did not appear to conserve energy or momentum, it was proposed a neutrino carries away a certain amount of energy and momentum. However a massless particle can still have finite energy and momentum. So neutrinos may still be massless.

But if neutrinos can change flavour between electron, muon, and tau types then they must have a small mass. The reason why is rather complicated - something like the mass and flavour wavefunctions getting out of phase. As there is growing evidense that flavour changing does happen, neutrinos are probably not massless.

see http://news.bbc.co.uk/hi/english/sci/tech/newsid_1394000/1394811.stm

I think you can read up on the latest results from SNO (Sudbury Neutrino Observatory), where recently Physics News Update from the American Institute of Physics reported that the people at SNO have the solar neutrino problem "solved". And yes, neutrinos do have mass and they do change into other types of neutrinos.

http://www.sno.phy.queensu.ca/