Philip Ball in Prospect:
Neutrinos were always trouble. These elusive little fundamental particles were first proposed in 1930 by the physicist Wolfgang Pauli to explain where some of the energy and momentum went during the process of radioactive beta decay of atomic nuclei. The Italian Enrico Fermi took up the idea, and helped to coin the name, in a 1934 paper that got rejected by Nature as too speculative and was published in an Italian journal to such scant interest that Fermi became an experimentalist instead. (Eight years later he created the first ever nuclear reactor in Chicago.)
At this stage neutrinos were just a hypothetical convenience to make the nuclear sums come out right. They seemed so bland as to verge on the pointless: as they have no electrical charge and seemed at first perhaps to have no mass, ordinary matter barely “feels” them at all. They weren’t detected until 1956 (in work that belatedly won the 1995 Nobel prize), because they are so damned hard to see.
Then in the 1960s, experiments using neutrino detectors—buried deep underground to shield them from false signals made by cosmic rays—showed that these particles weren’t being produced by the nuclear reactions in the Sun at anything like the predicted rate. More head-scratching ensued, until scientists figured that neutrinos must be able to switch in flight between three different varieties (“flavours”). Another Nobel prize (2015) followed for that discovery, not least because it resolved another long-standing issue: neutrinos must after all have some mass, because only then are these “oscillations” between flavours possible.