Decay DancerWeak Nuclear Force / Beta Decay

A neutron, left alone, does not stay a neutron.

Inside a stable nucleus a neutron can last indefinitely. Set it free and on average it lasts a little under fifteen minutes before it becomes a proton, an electron, and an electron antineutrino. Underneath, one of its down quarks turns into an up quark by way of a \(W^{-}\) boson, a particle about eighty-five times heavier than the neutron itself, which exists here only as a fleeting intermediate and immediately becomes the electron and the antineutrino. Changing a quark's flavour this way is something only the weak force does.

The accounting is exact. A neutron outweighs a proton plus an electron by 0.782 MeV of rest energy, and the decay products share exactly that. Early measurements found the electrons coming out with a spread of energies below that maximum, as if energy were going missing. In 1930 Wolfgang Pauli proposed an unseen neutral particle carrying the difference. Enrico Fermi built a theory of beta decay around it in 1934 and gave it its name. Clyde Cowan and Frederick Reines detected it in 1956. The next year Chien-Shiung Wu's experiment with cobalt-60 showed that beta decay does not look the same in a mirror.

The strange part is the clock. Trap ultracold neutrons in a bottle and count how many survive: 878.4 ± 0.5 seconds. Measure a neutron beam instead and count the protons that come out: about nine seconds longer. Both methods are careful work, and they still disagree. Nobody yet knows why.

Decay Dancer is the neutron mid-turn: same mass number, new identity, something invisible leaving at the edge of the frame.