Neutrino is the name given to three different but related
elementary particles, all with small mass and zero charge. These are
the electron neutrino (νe), the muon neutrino
(νμ), and the tau neutrino (ντ),
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
νe, νμ, and ντ 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.
This writeup is copyright 2004 D.G. Roberge and is released under the Creative Commons Attribution-NoDerivs-NonCommercial licence. Details can be found at http://creativecommons.org/licenses/by-nd-nc/2.0/ .