1927: Certain materials had been observed to emit electrons (beta decay). Since both the atom and the nucleus have discrete energy levels, it is hard to see how electrons produced in transition could have a continuous spectrum (see 1930 for an answer.)
1930: Wolfgang Pauli suggests the neutrino to explain the continuous electron spectrum for beta decay.
1933: Enrico Fermi puts forth a theory of beta decay that introduces the weak interaction. This is the first theory to explicitly use neutrinos and particle flavor changes.
1956: Frederick Reines and Clyde Cowan detect antineutrinos
1957: Julian Schwinger, Sidney Bludman, and Sheldon Glashow, in separate papers, suggest that all weak interactions are mediated by charged heavy bosons, later called W+ and W-. Actually, it was Yukawa who first discussed boson exchange twenty years earlier, but he proposed the pion as the mediator of the weak force.
1962: Experiments verify that there are two distinct types of neutrinos (electron and muon neutrinos). This was earlier inferred from theoretical considerations.
1962: Leon Lederman shows that the electron neutrino is distinct from the muon neutrino
1967: Steven Weinberg puts forth his electroweak model of leptons
1971: Gerard 't Hooft shows that the Glashow-Salam-Weinberg electroweak model can be renormalized
Quotes from forgotten sources.
The weak nuclear force causes instability in some nuclei. This is the force that causes radioactive decay. Electric change produces electromagnetic forces and color produces strong force. Leptons have no color, so they do not participate in the strong interactions; neutrinos have no change, so they experience no electromagnetic forces, but all of them join in the weak interactions. There are two kinds of weak interactions: charged mediated by the W's and neutral mediated by the Z.
The "electroweak" interactions are a unification of the electromagnetic and nuclear weak interactions, and are described by the Weinberg-Salam theory (sometimes called "quantum flavordynamics"; also called the Glashow-Weinberg-Salam theory)
"anti-neutrino + proton -> neutron + positron. The target was water with CdCl_2 dissolved in it. The positron was detected by its slowing down and annihilating with an electron producing two 0.5 MeV gamma rays in opposite directions. The pair of gamma rays was detected in time coincidence in liquid scintillator above and below the water by photomultiplier tubes detecting the scintillation light. The neutron was also slowed by the water and captured by the cadmium microseconds after the positron capture. In the capture several gamma rays were emitted which were also detected in the scintillator as a delayed coincidence after the positron's annihilation gamma ray detection. The detector contained 200 liters of water in two tanks with up to 40 kg of dissolved CdCl_2. The water tanks were sandwiched between three scintillator layers which contained 110 5" photomultipliers each, and the whole experiment measured only about 2 meters in each direction."
The search was like listening for a gnat's whisper in a hurricane. -- Frederick Reines, discoverer of the neutrino
Reactions of interest
Source of neutrinos
Necessary extensions to QFD
Neutrinos have no charge, interact very little with other particles, and are probably massless. The sun produces huge numbers of neutrinos every second, but detectors on earth haven't seen them in the numbers solar physicists expected. Obviously, we have a problem here.
Is it the sun? That's not even the right question. The sun does what it does no matter what physicists say or don't say. The real question is, are there problems with our current models of the sun? Maybe, but probably not. Models of the sun have otherwise been quite successful. Why should they fail on this one account? Is it the detectors? Probably not. Neutrinos may be governed by the weak force, but this doesn't make them undetectable. When used to measure the neutrino output from other sources they too have been quite successful. If the problem isn't with solar models or neutrino detectors it must lie in the space between the sun and the detectors. Something's happening to the solar neutrinos on their 150 million kilometer journey.
The short answer is it's a flavor thing. When speaking of subatomic particles, it might be correct to say "the electron", but it is not at all correct to say "the neutrino". The word neutrino refers to a small group of three different, but similar particles: the electron neutrino, the muon neutrino, and the tau neutrino.
Neutrinos are exactly massless in the Standard Model, but recent experimental observations show neutrinos oscillating between different flavors. This implies that neutrinos are very, very light, but not massless. In addition, the mixing ratios hint at the possibility of a fourth neutrino; the sterile neutrino, so called as it does not appear to interact with any other particle. Maybe this really isn't a weak link — more of an unresolved problem.
Current experiment: The MINOS (Main Injector Neutrino Oscillation Search) experiment at Fermilab. Neutrinos are created at a known rate at the laboratory in Batavia, Illinois (a western suburb of Chicago) and directed through the earth to a detector in an abandoned iron mine in Soudan, Minnesota (near the Canadian border). The idea is this: measure the number of neutrinos produced at Fermilab and the number detected at the iron mine in Soudan. The important calculation is the number of neutrinos that were "lost" along the way. Standard Model predictions at the time predict that none should be lost, but earlier experiments hint at the possibility of neutrino oscillations. If the number detected in Minnesota is significantly different from the number generated in Illinois, then the Standard Model as it is currently interpreted is in need of modification.
According to the Standard Model, the W and Z bosons should be massless, but experimental evidence shows them to be quite heavy. A way around this is to introduce another variety of intermediate vector boson called the higgs particle, higgs boson, or (my personal favorite) higgson. When W and Z particles interact with the higgson, they acquire mass.
Four methods for producing a higgs particle.