To conserve total lepton number, in nuclear beta decay, electron neutrinos appear together with only positrons (anti-electrons) or electron-antineutrinos, and electron antineutrinos with electrons or electron neutrinos. They are distinguished from the neutrinos by having opposite signs of lepton number and chirality. For each neutrino, there also exists a corresponding antiparticle, called an antineutrino, which also has half-integer spin and no electric charge. For example, an electron neutrino produced in a beta decay reaction may interact in a distant detector as a muon or tau neutrino. As a result, neutrinos oscillate between different flavors in flight. A neutrino created with a specific flavor is in an associated specific quantum superposition of all three mass states. Although neutrinos were long believed to be massless, it is now known that there are three discrete neutrino masses with different tiny values, but they do not correspond uniquely to the three flavors. Weak interactions create neutrinos in one of three leptonic flavors: electron neutrinos (νe), muon neutrinos (νμ), or tau neutrinos (ντ), in association with the corresponding charged lepton. Thus, neutrinos typically pass through normal matter unimpeded and undetected. The weak force has a very short range, gravity is extremely weak on the subatomic scale, and neutrinos, as leptons, do not participate in the strong interaction. The neutrino is so named because it is electrically neutral and because its rest mass is so small (-ino) that it was long thought to be zero. Although only differences of squares of the three mass values are known as of 2016, cosmological observations imply that the sum of the three masses must be less than one millionth that of the electron. The mass of the neutrino is much smaller than that of the other known elementary particles. If so, then despite their light weight, their abundance may in fact mean that neutrinos contribute significantly to the overall mass of the universe.A neutrino ( or ) (denoted by the Greek letter ν) is a fermion (an elementary particle with half-integer spin) that interacts only via the weak subatomic force and gravity. Recent analysis of neutrinos emanated by the Sun has suggested that each type of neutrino can spontaneously turn into one of the others in a process of neutrino oscillation, and for theoretical reasons this in turn would require that neutrinos have mass. Nevertheless, neutrinos can be detected, and three different types have been distinguished, each of which is associated with a particular lepton (the electron, the muon, and the taon) with which it is often paired in interactions involving the weak force. The Italian physicist Enrico Fermi then coined the term neutrino, which means little neutron in Italian.) Neutrinos are hard to detect because their mass, if they indeed have any, is extremely low, and they possess no electric charge a chunk of iron a few light-years thick would absorb only about half of the neutrinos that struck it. (He originally wanted to name the particle a neutron but didn't publish the suggestion, and a few years later the particle we now know as the neutron was discovered and named in print. Pauli suggested that the energy was carried away by a very small, electrically neutral particle that was not being detected. A certain amount of energy that was lost in these processes could not be accounted for. Pauli was studying certain radioactive decay processes called beta decay, processes now known to involve the decay of a neutron into a proton and an electron. A Closer Look Neutrinos were not observed until 1955, roughly a quarter of a century after the physicist Wolfgang Pauli first hypothesized their existence on theoretical grounds.
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