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What exactly are neutrinos and why are they important?

The existence of neutrinos was postulated to explain what was observed when a nucleus undergoes beta decay via electron emission (a neutron in the nucleus changes into a proton, and the nucleus emits an electron). Observation seemed to indicate that, in general, neither momentum nor total energy was conserved in such a reaction. This was, understandably, disturbing. In 1931 the physicist Pauli proposed that another, unseen particle was also being emitted in the reaction, accounting for the "missing" momentum and mass-energy. Fermi named this particle neutrino" meaning "little neutral one". The name reflected the assumption that it had no electric charge (so that the law of charge conservation would remain intact); additionally, it was postulated to have zero spin (keeping the law of conservation of angular momentum) and to interact only weakly with matter (to explain why it was virtually undetectable). Eventually the neutrino was detected, in the early 1950's, through nuclear reactors (which produce huge numbers of neutrinos). Currently it is believed that there are three kinds of neutrinos (plus their antiparticles, making a total of six) -- the electron- neutrino, the muon-neutrino, and the tau-neutrino. Neutrinos are considered to be the force-carriers associated with the fundamental force called the weak force. The weak force is very important -- it contributes to the production of elements of higher atomic number, without which life as we know it would be impossible.

Ronald Winther

And they are important in astronomy for several reasons: supernovas emit huge quantities of neutrinos (according to theory). Supernova 1989A, which was nearby in one of the Magellanic clouds, was detected by several neutrino detectors that had been designed and built to detect neutrinos from the sun: the best theories of how the sun works predict three times the number of neutrinos than are actually detected! Neutrinos may play a major role in the dark or missing matter problem of cosmology.

John Hawley

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