neutrino (thing)

Neutrino is the name given to three different but related elementary particles, all with small mass and zero charge. These are the electron neutrino (ν_e), the muon neutrino (ν_μ), and the tau neutrino (ν_τ), which in the Standard Model are paired with the electron, muon, and tau lepton, respectively, to form the three 'families' of leptons. Like all fundamental fermions, the neutrinos have spin 1/2.

Leptons are distinguished from the other elementary fermions, the quarks, by not interacting through the strong nuclear force. Neutrinos, through their lack of charge, also do not interact through the electromagnetic force, leaving only the weak nuclear force (and also gravity, but the effect of gravity on elementary particles is negligible). Since the weak nuclear force is, after all, weak, this means that neutrinos interact very weakly with matter. It is said that a neutrino could pass through a light-year of solid lead and have its trajectory and momentum unchanged.

Neutrinos were originally postulated by Wolfgang Pauli in relation to beta decay. When a nucleus undergoes beta decay, it spits out an electron (or a positron). Conservation of momentum would have that there be a characteristic direction and momentum for the emitted electron, but the electrons were observed to have a large spectrum of momenta and angles of emission. Hence, some momentum appeared to be 'disappearing'. Pauli postulated than an unobserved, massless particle would also be emitted to carry away the missing momentum. Enrico Fermi named this particle the 'neutrino'.

The particle postulated by Pauli and Fermi was the electron neutrino. When the muon and tau lepton were discovered, corresponding neutrinos were postulated and eventually observed. Each of these neutrinos has a corresponding antineutrino, and, in fact, the original 'neutrino' of beta decay is actually an antineutrino. For reasons explained at the W particle node, antineutrinos are produced along with negative charged leptons, and neutrinos are produced along with positively charged leptons (antileptons).

Neutrinos and antineutrinos have the interesting property of having only one helicity state; all antineutrinos have positive helicity meaning that their spins are parallel to their momentum, while all neutrinos have negative helicity, meaning that their spins are anti-parallel to their momentum. This is a consequence of parity violation in the weak interaction. It has been suggested that neutrinos are actually their own antiparticles and what are observed as antineutrinos are simply right-handed (positive helicity) neutrinos. This is an elegant theory, but it is by no means proven as of yet.

In the Standard Model, neutrinos are generally approximated as massless, but experiments, until recently, were unclear as to whether or not the neutrino has mass. Recent results from Super Kamiokande and the Sudbury Neutrino Observatory have shown convincing evidence for the phenomenon of neutrino oscillation, which requires that the neutrinos have mass. The actual masses have been constrained to be beneath 5 eV/c^2 (1/100,000 of an electron mass), but the nature of the oscillations makes assigning the masses complicated. Essentially, the oscillations predict the existence of three 'mass eigenstate' neutrinos, ν₁, ν₂, and ν₃, which combine to create the three 'flavour eigenstate' neutrinos ν_e, ν_μ, and ν_τ that each interact with their corresponding charged lepton. The mass eigenstates each have a definite mass, but the flavour eigenstates would then not have definite masses.

The Universe is suffused with neutrinos. Every square centimetre of the Earth's surface has billions of neutrinos pass through it every second, usually without any effect whatsoever. This makes them the closest thing to dark matter currently known in particle physics, although there are not nearly enough neutrinos to make them a viable dark matter candidate. Nevertheless, their role in modern particle physics is greatly expanded from their original postulation as an explanation for beta decay.