  |
 |
 |
| |
|
|
In the early 1960s, experiments at accelerators proved that there were two types of neutrinos (the electron-neutrino and the muon-neutrino). Leon Lederman, Melvin Schwartz and Jack Steinberger received the Nobel prize for this discovery. Since the discovery of the tau lepton we now believe that there must also be a tau-neutrino (even though it hasn't been discovered yet) and there are a number of experiments searching for it (NOMAD and CHORUS at CERN and E-872 at Fermilab).
All attempts to measure the mass of neutrinos have failed so far. The most sensitive are those of tritium beta decay which attempt to measure distortions at the end of the beta spectrum. So far, all we know is that the mass of the electron neutrino is less than 15 eV (remember that the mass of an electron is 510,999 eV). The mass of muon neutrinos should be able to be inferred from the decay of muons, and all we know is that the mass is less than 0.17 MeV (mega electron volts or 1 million electron volts). Tau neutrinos have not been observed but we know they have to have a mass of less than 24 MeV, from the 5 pion decay of the the tau measured at ALEPH in CERN. All of these negative results show the difficulty involved in performing neutrino experiments, but also the opportunities still available to make discoveries in the field.
An aternate way of measuring the mass of a neutrino is by the observation of neutrino-less double-beta decay. Beta decay is a radioactive process in which a nucleus emits an electron (beta particle) and a neutrino and transforms into another nucleus with one extra proton. Because it is a three-body decay (the nucleus, the electron and the neutrino), the energy of the electron is not fixed, but forms a continuum up to a maximum energy given by the total available energy between the original and the final nuclei. This is a common process, but it is possible that in some cases the energy state of the final nucleus would be higher than the original state. In this case, beta decay is forbidden so the much rarer double-beta decay can occur. The nucleus emits two electrons (hence the name double-beta) and two neutrinos and the nucleus transforms into one with two extra protons. These decays are very rare (half-lifes about 10E21 years and above), but they have been observed in some double-beta decay experiments for some compounds like germanium-76, molybdenum-100 and a few others. The energy spectrum is also a continuum. However, if the neutrino were its own anti-particle (these types of particles are called Majorana particles) the two neutrinos would not appear in the decay, so the two electrons would take the total energy available and form a "spike" at the end of the double-beta spectrum. Since this spike has never been observed, experiments like the Moscow-Heidelberg experiment can give an upper limit of 0.5 eV for the mass of the electron-neutrino only in the case that neutrinos were Majorana particles.
Neutrino experiments have also played a role in the understanding of weak interactions. Weak interactions can be mediated by one of three vector bosons, the charged W particles (positive and negative) and the neutral Z particle. Interactions in which the exchange of a charged W occur are called charged current interactions and those in which a neutral Z particle are exchanged are called neutral current interactions. The first observation of neutral current interactions was made in 1973 at the Gargamelle bubble chamber that was recording neutrino interactions at the CERN neutrino beam. This was the first (indirect) evidence of the existence of the W and Z particles which were explicitely discovered 10 years later also at CERN by the UA1 and UA2 experiments (Carlo Rubbia and Simon van der Meer obtained Nobel prizes for this discovery).
Neutrinos serve as a sensitive probe of the standard model. Throughout the 1970s and 1980s, a number of neutrino experiments, mainly at CERN and Fermilab, were able to obtain a multitude of high energy neutrino interactions in which they measured fundamental parameters of the theory (like the measurement of the weak mixing angle, one of the parameters in the theory that is depends on the ratio of the masses of the Z and W particles). They were also able to probe the internal structure of nucleons and determine properties of the quarks that are inside them (for example, the most sensitive measurement of the mass of the charm quark comes from neutrino experiments).
There is also a lot of interest in neutrino oscillations because this seems to be the most sensitive avenue to study whether neutrinos have mass or not. Neutrinos have also been detected from astronomical sources, like solar neutrinos, neutrinos from cosmic rays and neutrinos from the supernova explosion 1987A. Some of these topics are pursued in the neutrino oscillations page.
Neutrinos provide a wealth of knowledge into how the sub-atomic world really works and still continue to surprise us. Due to the many open questions that remain about neutrinos and their properties, a large "neutrino industry", both theoretical and experimental, operates around the world. A link to this Neutrino Industry web site can provide further links to other neutrino experiments that might be of interest.