Neutrino

A neutrino is a neutral particle with very low mass, possibly zero. It has spin 1/2 and so is a fermion. It does not interact with the strong force or the electromagnetic force, but does interact with the weak force (and with gravity if it turns out to have mass).

Because the neutrino only interacts with the weak nuclear force, when moving through matter its chance of actually reacting with it are very low; the great of majority flies through anything without effect. It would take a light year of lead to block half the neutrinos flowing through it.

It comes in three varieties, the electron neutrino ν_e, the muon neutrino ν_μ, and the tau neutrino ν_τ. The electron neutrino is by far the most common, the muon and tau neutrinos are much more massive and rare. Theoretical physicists believe that there is a possibility that neutrinos can 'oscillate' between the three types; however, this is only possible if the electron neutrino actually has non-zero mass, which is not yet known.

Neutrinos are detected by large bassins of heavy water, placed in deep caves to shield them from [cosmic radiation]?. These detectors have led to the solar neutrino problem: The Sun is sending out fewer neutrinos than theoretical models would predict. Neutrinos have also been detected from supernova 1987a.

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See also solar neutrino problem, particle physics.