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Κυριακή, 6 Μαρτίου 2016

The Neutrino Event

Electron Neutrinos and Antineutrinos

The history of a particle that appeared to have no charge and no mass is an interesting one. The electron neutrino (a lepton) was first postulated in 1930 by Wolfgang Pauli to explain why the electrons in beta decay were not emitted with the full reaction energy of the nuclear transition.
The apparent violation of conservation of energy and momentum was most easily avoided by postulating another particle. Enrico Fermi called the particle a neutrino and developed a theory of beta decay based on it, but it was not experimentally observed until 1956. This elusive particle, with no charge and almost no mass, could penetrate vast thicknesses of material without interaction. The mean free path of a neutrino in water would be on the order of 10x the distance from the Earth to the Sun. In the standard Big Bang model, the neutrinos left over from the creation of the universe are the most abundant particles in the universe. This remnant neutrino density is put at 100 per cubic centimeter at an effective temperature of 2K (Simpson). The background temperature for neutrinos is lower than that for the microwave background (2.7K) because the neutrino transparency point came earlier. The sun emits vast numbers of neutrinos which can pass through the earth with little or no interaction. This leads to the statement "Solar neutrinos shine down on us during the day, and shine up on us during the night!" . Bahcall's modeling of the solar neutrino flux led to the prediction of about 5 x 106 neutrinos/cm2s.
A remarkable opportunity for observing neutrinos came with Supernova 1987A when the Japanese observing team detected neutrinos almost coincident with the discovery of the light from the supernova.
Neutrinos interact only by the weak interaction. Their interactions are usually represented in terms of Feynman diagrams.


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