Three hints that neutrinos oscillate have been observed. This metamorphosis, known as neutrino oscillation, implies neutrino mass. This evidence stems from the apparent observation that a neutrino of one type or “flavor,” such as a muon neutrino, can metamorphose into a neutrino of another flavor, such as a tau neutrino. Recently, strong evidence that neutrinos do have nonzero masses has at last been found. In addition, because neutrinos are so abundant in the universe, even tiny neutrino masses can result in an important neutrino contribution to the mass density of the universe. Measured values of the masses would be a clue to the nature of the new physics. If the neutrino masses are tiny but nonvanishing, then they are probably caused by new physics beyond the realm of the highly successful standard model of the weak and electromagnetic interactions. The answer to this question has major consequences both for physics and for astrophysics. However, it has not been known whether they are completely massless or have small but nonzero masses. It has long been known that neutrinos are extremely light. Thus, it would be very nice to know something about them. Evidently, neutrinos are very common in the universe, and very commonly encountered by people. In addition, passing through every person every second are some 10 14 neutrinos from the Sun, another 10 3 neutrinos made in the Earth's atmosphere by cosmic rays, and additional neutrinos from other natural and human-made sources. Within a single human being, there are more than 10 7 neutrinos left over from the Big Bang. Neutrinos are among the most abundant particles in the universe.

Mass of neutrino