There are three known flavors of neutrinos: the electron neutrino ν_e, the muon neutrino ν_μ and the tau neutrino ν_τ, named after their partner leptons in the Standard Model. Experiments probing energy scales thousands of times larger have not revealed the existence of any additional neutrinos therefore it is widely believed that there are only three. Also the anecdotal correspondance between the six quarks in the Standard Model and the six leptons, including three neutrinos, provides a hint that serves as further proof to some that there can be only three neutrinos. Nevertheless, proof that there are only three neutrinos remains an elusive goal of particle physics.

In a phenomenon known as neutrino oscillation neutrinos are observed to take a well defined mass only after a corresponding measurement. In fact, before measurement the flavor of the neutrino is considered to exist as a superposition of the multiple, generation 1 through generation 3 states. Whereas conventional wisdom implies that there are separate neutrino and antineutrino states, observation implies otherwise. Although, it cannot presently be ruled out that neutrinos have zero mass, the phenomenon of neutrino oscillation provides a compelling solution to the solar neutrino problem and seems to be experimentally validated. A neutrino with zero mass would not exhibit observable oscillation and therefore would not provide an explaination to the solar neutrino problem.

The basic Standard Model of particle physics assumes that the neutrino is massless, although adding massive neutrinos to the basic framework is not difficult, and recent experiments suggest that the neutrino has a small although non-zero mass.

The strongest upper limits on the mass of the neutrino come from cosmology. The Big Bang model predicts that there is a fixed ratio between the number of neutrinos and the number of photons in the cosmic microwave background. If the total mass of all three types of neutrinos exceeded 50 electron volts (per neutrino), there would be so much mass in the universe that it would collapse. This limit can be circumvented by assuming that the neutrino is unstable; however, there are limits within the Standard Model that make this difficult.

However, it is now widely believed that the mass of the neutrino is non-zero. When one extends the Standard Model to include neutrino masses, one finds that massive neutrinos can change type whereas massless neutrinos cannot. This phenomenon, known as neutrino oscillation, explains why there are many fewer electron neutrinos observed from the sun and the upper atmosphere than expected, and has also been directly observed. One form of quantum gravity theory, due to Burkhard Heim, made neutrino mass predictions in the 1980s which are still consistent with experiment: ( 0.00381 ev, 0.00537 Mev, 0.010752 Mev) compared to current upper limit of (< 2.5 ev, .< 0.17 Mev, < 18.2 Mev) respectively.

• Human generated.

  Nuclear power stations are the major source of human generated neutrinos. An average plant may generate over 50,000 neutrinos per second. Particle accelerators are another source.

• The Earth.

  Neutrinos are produced as a result of natural background radiation.

• Atmospheric neutrinos.

  Atmospheric neutrinos result from the interaction of cosmic rays with atoms within the Earth's atmosphere, creating showers of particles including neutrinos.

• Solar neutrinos.

  Solar neutrinos originate from the nuclear fusion powering the Sun and other stars.

• Cosmological phenomena.

  Neutrinos are an important product of supernovas. Most of the energy produced in supernovas is radiated away in the form of an immense burst of neutrinos, which are produced when protons and electrons in the core combine to form neutrons. The first experimental evidence of this phenomenon came in the year 1987, when neutrinos coming from the supernova 1987a were detected. In such events, the densities at the core becomes so high (10¹⁴ g/cm³) that interaction between the produced neutrinos and surrounding stellar matter becomes significant. It is thought that neutrinos would also be produced from other events such as the collision of neutron stars.

  Because neutrinos interact so little with matter, it is thought that a supernova's neutrino emissions carry information about the innermost regions of the explosion. Much of the visible light comes from the decay of radioactive elements produced by the supernova shock wave, and even light from the explosion itself is scattered by dense and turbulent gases. Neutrinos, on the other hand, pass through these gases, providing information about the supernova core (where the densities were large enough to influence the neutrino signal). Furthermore, the neutrino burst is expected to reach Earth before any electromagnetic waves, including visible light, gamma rays or radio waves. The exact time delay is unknown, but for a Type II supernova, astronomers expect the neutrino flood to be released seconds after the stellar core collapse, while the first electromagnetic signal may be hours or days later. The SNEWS (http://snews.bnl.gov) project uses a network of neutrino detectors to monitor the sky for candidate supernova events; it is hoped that the neutrino signal will provide a useful advance warning of an exploding star.

• Cosmic background radiation.

  It is thought that the cosmic background radiation left over from the Big Bang includes a background of low energy neutrinos. In the 1980s it was proposed that these may be the explanation for the dark matter thought to exist in the universe. Neutrinos have one important advantage over most other dark matter candidates: we know they exist. However, they also have serious problems. From particle experiments, it is known that neutrinos tend to be hot, i.e. move at speeds close to the speed of light—hence this scenario was also known as hot dark matter. The problem is that being hot and fast moving, the neutrinos would tend to spread out evenly in the universe. This would tend to cause matter to be smeared out and prevent the large galactic structures that we see.