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UNIT 3

Conservation of Momentum and the Discovery of the Neutrino

The Missing Momentum in P Decay

In physics we always seek to interpret the observalional facts regarding natural phenomena in the simplest possible terms. The various conservation laws provide us wilh the means toward this end. We know that in every physical process certain quanlities do not change - mass (actually, mass-energy), linear momentum, and angular momentum. These conservation laws have been enormously useful in providing the essential clues in solving many fundamental problems in physics.

One of the successes of the conservation principles in recent years was the discovery of an elusive elementary particie, called the neutrino. In radioaclive (3 decay the atomie nucleus emits an electron and in the process is transformed into a new atomie species that differs in atomie number by one unit.

Electron

In actual p decay it is found that the momenta of the electron and the recoil nucleus are not co-linear. There is some "missing" linear momentum which is attributed to the emission of an additional "particie", the neutrino.

Nucleus B

Pe

A

P₂

"Missing" momentum (neutrino, v)

An additional problem arose concerning angular momentum. In radioactive p decay it was found that the spin of nucleus A did not equal the (vector) sum of the spins of nucleus B and the electron. It therefore appeared that angular momentum was not conserved.

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