The neutrino 

The neutrino is a fundamental particle produced in massive numbers by nuclear reactions in stars (eg. the Sun). They are very difficult to detect since the vast majority of solar neutrinos pass completely through the Earth without interacting.

The neutrino is of historical significance in the development of particle physics because its existence was proposed as a wild solution to a major puzzle long before it was found.

Alpha particles acquire their energy from Coulomb repulsion in the electric field of the parent nucleus. All alpha particles from the same decay have the same energy. In sharp contrast beta particles leave the parent nucleus with a range of energies. The reason eluded physicists for many years - some even proposed the abandonment of the principle of the conservation of energy.

It was eventually found that the neutrino - a particle with no rest mass - was carrying away a varying portion of the energy in beta decay. Beta decay is a three body process, not a simple two body process like alpha decay.

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