beta decay

Beta decay is the original face of the weak force — the radioactivity that first hinted that something more than electromagnetism and the strong force was at work. In beta decay an unstable atomic nucleus quietly rebalances itself by turning a neutron into a proton (or vice versa) and spitting out a fast electron, the historic 'beta particle'. It is one of the three classic kinds of radioactivity, alongside alpha and gamma.

Underneath, beta decay is the weak force changing a quark's flavour. A neutron is made of two down quarks and an up quark; a proton of two ups and a down. When one of the neutron's down quarks emits a W boson and becomes an up quark, the neutron becomes a proton. The W then turns into an electron and an electron antineutrino, which fly out of the nucleus. That elusive antineutrino is the unsung third product: it carries off energy invisibly, which is why the emitted electrons come out with a spread of energies rather than one fixed value.

Beta decay is historically priceless. The fact that the electron's energy varied, instead of being sharp, was so baffling that Wolfgang Pauli proposed an invisible particle to balance the books — the neutrino — decades before one was detected. Beta decay is also where parity violation was discovered, overturning the assumption that nature treats left and right alike. And it remains practical: it powers some medical scanners, drives certain nuclear reactor processes, and is central to dating and detecting radioactivity.