Neutrinos
Neutrinos are elementary particles denoted by the Greek letter ν. Travelling at or close to the speed of light, lacking electric charge and able to pass through ordinary matter almost undisturbed, they are extremely difficult to detect. Neutrinos have a minuscule, but non-zero, mass that is too small to be measured at this time.
Neutrinos are created as a result of certain types of radioactive decay or nuclear reactions such as those in the sun, in nuclear reactors, or when cosmic rays hit atoms. There are three types, or "flavors", of neutrinos: electron neutrinos, muon neutrinos and tau neutrinos; each type also has an antimatter partner, called an antineutrino. Electron neutrinos or antineutrinos are generated whenever neutrons change into protons or vice versa, the two forms of beta decay.
Most neutrinos which pass through the Earth emanate from the sun, and more than 50 trillion solar electron neutrinos pass through the human body every second.
Types of neutrinos
There are three known types of neutrinos: electron neutrino νe, muon neutrino νμ and tau neutrino νÏ, named after their partner leptons in the Standard Model .The current best measurement of the number of neutrino types comes from observing the decay of the Z boson. This particle can decay into any light neutrino and its antineutrino, and the more types of light neutrinos available, the shorter the lifetime of the Z boson. Measurements of the Z lifetime have shown that the number of light neutrino types (where "light" means having mass less than half the Z mass) is 3. The correspondence between the six quarks in the Standard Model and the six leptons, among them the three neutrinos, suggests to physicsts' intuition that there should be exactly three types of neutrino. However, actual proof that there are only three kinds of neutrinos remains an elusive goal of particle physics.
The possibility of sterile neutrinos — neutrinos which do not participate in the weak interaction but which could be created through flavour oscillation is unaffected by these Z-boson-based measurements, and the existence of such particles is in fact supported by experimental data from the LSND experiment. However, the currently running MiniBooNE experiment contradicted this controversial result in 2007.