Elementary particle with extremely low mass that interacts only via the weak force and gravity

Neutrinos are created by various radioactive decays; the following list is not exhaustive, but includes some of those processes:

Research is intense in the hunt to elucidate the essential nature of neutrinos, with aspirations of finding:

As well as specific sources, a general background level of neutrinos is expected to pervade the universe, theorized to occur due to two main sources.

It is very hard to uniquely identify neutrino interactions among the natural background of radioactivity. For this reason, in early experiments a special reaction channel was chosen to facilitate the identification: the interaction of an antineutrino with one of the hydrogen nuclei in the water molecules. A hydrogen nucleus is a single proton, so simultaneous nuclear interactions, which would occur within a heavier nucleus, don't need to be considered for the detection experiment. Within a cubic metre of water placed right outside a nuclear reactor, only relatively few such interactions can be recorded, but the setup is now used for measuring the reactor's plutonium production rate.

There are several active research areas involving the neutrino. Some are concerned with testing predictions of neutrino behavior. Other research is focused on measurement of unknown properties of neutrinos; there is special interest in experiments that determine their masses and rates of CP violation, which cannot be predicted from current theory.

Despite their tiny masses, neutrinos are so numerous that their gravitational force can influence other matter in the universe.

Another hypothesis concerns "neutrinoless double-beta decay", which, if it exists, would violate lepton number conservation. Searches for this mechanism are underway but have not yet found evidence for it. If they were to, then what are now called antineutrinos could not be true antiparticles.

Providing for neutrino mass can be done in two ways, and some proposals use both:

Neutrinos' low mass and neutral charge mean they interact exceedingly weakly with other particles and fields. This feature of weak interaction interests scientists because it means neutrinos can be used to probe environments that other radiation (such as light or radio waves) cannot penetrate.