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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 is very low; the great majority flies through anything without effect. 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.

It comes in three varieties, the electron neutrino ν_e, the muon neutrino ν_μ, and the tau neutrino ν_τ. Theoretical physicists believe that there is a possibility that neutrinos can 'oscillate' between the three types; however, this is only possible if neutrinos have non-zero mass, which is not yet known. The question of neutrino mass also has cosmological significance. If the neutrino does have mass, then it could make up a significant fraction of the mass of the universe and help resolve the dark matter problem.

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 them from cosmic radiation.

• Chlorine detectors were the first used and consist of a tank filled with dry cleaning fluid. In these detectors a neutrino would convert a chlorine atom into one of argon. The fluid would periodically be purged with helium gas which would remove the argon. The helium would then be cooled to separate out the argon. These detectors had the failing in that it was impossible to determine the direction of the incoming neutron. It was the chlorine detector in Homestake, South Dakota, containing 520 tons of fluid, which first detected the deficit of neutrinos from the sun that led to the solar neutrino problem.
• Gallium detectors are similar to chlorine detectors but more sensitive to low energy neutrinos. A neutrino would convert gallium to germanium which could then be chemically detected. Again, this type of detector provides no information on the direction of the neutrino.
• Pure water detectors such as Super-Kamiokande contain a large area of pure water surrounded by sensitive light detectors known as photomultiplier tubes. In this detector, the neutrino transfers its energy to an electron which then travels faster than the speed of light in the medium (though slower than the speed of light in a vacuum). This generates an "optical shockwave" known as Cherenkov radiation which can be detected by the photomultiplier tubes. This detector has the advantage that the neutrino is recorded as soon as it enters the detector, and information about the direction of the neutrino can be gathered. It was this type of detector that recorded the neutrino burst from Supernova 1987a.
• Heavy water detectors use three types of reactions to detect the neutrino. The first is the same reaction as pure water detectors. The second involves the neutrino striking the deuterium atom releasing an electron. The third involves the neutrino breaking the deuterium atom into two. The results of these reactions can be detected by photomultiplier tubes. This type of detector is in operation in the Sudbury Neutrino observatory.

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See also solar neutrino problem, particle physics.