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The neutrino is an elementary particle. It has spin 1/2 and so it is a fermion. Its mass is very small, as recent experiments (see Super-Kamiokande) have shown it to be different from zero. It only interacts through the weak interaction and feels neither the strong nor the electromagnetic interaction(but it feels gravity, but since it is extremely small, when gravity is already the weakest force, it hardly matters).

Because the neutrino only interacts weakly, when moving through ordinary matter its chance of interacting with it is very small. It would take a light year of lead to block half the neutrinos flowing through it. Neutrino detectors therefore typically contain hundreds of tons of a material constructed so that a few atoms per day would interact with the incoming neutrinos. In collapsing supernova, the densities at the core become high enough (1014 grams / cc) that the produced neutrinos can be detected.

There are three different kinds, or flavors, of neutrinos: the electron neutrino νe, the muon neutrino νμ and the tau neutrino ντ, named after their partner lepton in the Standard Model.

The neutrino was first postulated by Wolfgang Pauli to explain the continuous spectrum of the beta decay.

Massive neutrinos can oscillate between the three flavors, in a phenomenon known as neutrino oscillation (which provides a solution to the solar neutrino problem and the atmospheric neutrino problem at the same time).

Most of the energy of a collapsing supernova is radiated away on the form of neutrinos which are produced when protons and electrons in the core combine to form neutrons. This produces an inmense burst of neutrinos. The first experimental evidence came in the year 1987, when neutrinos coming from the supernova 1987a were detected.

Some years ago it was believed that massive neutrinos could account for the dark matter, though with the current knowledge of neutrino masses they don't contribute a significant fraction to it. Cosmological observations provide themselves limits on the properties of the neutrino.

Neutrino detectors

There are several types of neutrino detectors. Each type consists of a large amount of material in an underground cave designed to shield it from cosmic radiation.

See also solar neutrino problem, particle physics.