Image: The Sudbury Neutrino Observatory. SNO is able to directly detect fluxes of neutrinos emanating from the sun.
Photo by Roy Kaltschmidt
The neutrino was predicted to have zero mass by Wolfgang Pauli in 1930. All of the evidence was consistent with that up to the establishment of the standard model in the late ‘60s. However, it was found by several groups in the early 1970s that the number of electron neutrinos arriving from the sun was 50-30% of the number predicted by the standard model. This discrepancy was known as the solar neutrino problem. It could be explained if neutrinos have mass, as conjectured by Bruno Pontecorvo in 1968. He compared neutrinos of different flavours to Kaons, known to undergo oscillations between one type and another. If neutrinos have mass not only can neutrinos of one helicity transform into neutrinos of opposite helicity, but those of one flavour (electron, muon, or tau) can transform into those of another. As a result the electron neutrino flux would be expected to decrease as a result of flavour oscillation.
However evidence for neutrino oscillation was slow in coming. The first indications that change of flavour was possible came in 1998, with observations of mesons produced by cosmic rays in the Earth's atmosphere by the Super-Kamiokande collaboration in Japan. More convincing, however, were measurements of the relative abundancies of solar electron-neutrinos and muon- and tao- neutrinos by the Sudbury Neutrino Observatory in Canada in 2001.
Neutrino oscillation observations can only provide information on the differences of the squares of the masses of the various flavours of neutrinos. Other methods are needed to obtain their absolute masses. One follows from big bang cosmology, which imposes a definite neutrino/photon ratio. The sum of the masses of the three flavours of neutrino is estimated in this way to be less than 0.3 ev, or approximately one millionth of the mass of the electron.