electron neutrino

The electron neutrino, symbol ν_e, is an elementary particle, one of the six leptons. It is a neutral particle with a very small mass, less than 5 eV/c^2 (0.001% of the electron mass). As a lepton, it has spin 1/2, and as the name implies it is the counterpart of the electron, the two particles forming a 'doublet'. Neutrinos are stable particles and only interact with other matter via the weak nuclear force and gravity. Thus, they rarely interact and are therefore difficult to detect.

Electron neutrinos are most commonly produced from the decay of a positive W particle, along with a positron. Its antiparticle, the electron antineutrino, is produced from the decay of a negative W particle along with an electron. W particles are produced by most weak decays, including beta decay and muon decay, as well as in nuclear fusion reactions. Due to maximal parity violation in the weak interaction, electron neutrinos always have negative helicity, meaning that their spin is oriented antiparallel to their momentum. The opposite is true for electron antineutrinos and some speculate that this is actually the only distinction between neutrino and antineutrino.

Neutrino production in nuclear fusion is very important in the study of neutrino physics. Many nuclear fusion reactions produce neutrinos, including several that occur routinely in the core of the Sun. These neutrinos are called solar neutrinos and are studied intensely, particularly at the Sudbury Neutrino Observatory (SNO). Solar neutrinos were key to the discovery of neutrino oscillation, which is the other way that electron neutrinos can be produced.

Neutrinos, as they propagate, have been found to have their 'flavour' oscillate; i.e. they change between between being electron neutrinos, being muon neutrinos, and being tau neutrinos. Thus electron neutrinos can be observed from processes that produce the other flavours of neutrinos, such as in the so-called 'atmospheric neutrinos'. Atmospheric neutrinos are muon neutrinos produced along with a muon when a cosmic ray strikes the Earth's atmosphere. Atmospheric neutrinos are studied at Super Kamiokande (Super-K).

When electron neutrinos interact with other matter, they interact in one of two ways. The first way is called 'charged current', where the neutrino transforms into an electron in the interaction, and the second is 'neutral current', where no particle transformations occur, only scattering. They can interact with both nucleons and electrons, interaction with nucleons being the more important for neutrino detection.

For example, SNO uses deuterium as its detector medium. A charged current interaction between an electron neutrino and the neutron in deuterium transforms the neutrino into an electron and the neutron into a proton, separating the deuteron into two independent protons. A neutral current interaction does not transform any particles but it generally imparts enough energy to the deuteron to split it into a neutron and a proton. Since the neutral current interaction can occur with any neutrino, SNO can use these two reactions to discover what fraction of the incident neutrinos are electron neutrinos.

The electron neutrino is at once both one of the most important elementary particles in particle physics and one of the most elusive. Although most of its basic properties are well understood, one of the major outstanding problems in neutrino physics is determining its mass and the masses of the other neutrinos.