Our knowledge of the universe often progresses the most when something unexpected happens. The discovery of the neutrino was one of those occasions.
The proposal of the particle’s existence was prompted by the realisation, made by the late 1920s, that there was a problem with radioactive decay.
Observations of beta decay, in which an atomic nucleus decays with the emission of an electron, suggested that energy, momentum and angular momentum were not conserved. As these conservation laws were well tested and thought to be absolute, something else must be going on.
To resolve this oddity, Niels Bohr speculated that energy might be conserved only statistically, so that any individual decay might not necessarily obey the principle, but the collection of all of them would still conserve energy on average.
Wolfgang Pauli, on the other hand, suggested in 1930 that beta decay might involve another particle, one of neutral electrical charge and with very little mass, which carries away some of the energy and angular momentum. This is the neutrino.
Four years later, Enrico Fermi developed a complete theory of beta decay by putting together Pauli’s idea of the neutrino, Werner Heisenberg’s model of the atomic nucleus, and Paul Dirac’s theory and Carl D Anderson’s discovery of the electron’s antimatter counterpart, the positron. The neutrino was becoming an increasingly convincing answer to the question of why conservation laws appeared to be broken.
Further experimental evidence (specifically, beta particles’ energy spectra) favoured Pauli and Fermi’s solution over Bohr’s, lending further weight to the notion that the neutrino was a real particle. All that was left was to discover it.
- The 27th International Conference on Neutrino Physics and Astrophysics is taking place at Imperial College London from 4–9 July. The keynote speakers are Nobel Prize winners Professor Takaaki Kajita of the University of Tokyo and Professor Arthur B McDonald of Queen’s University, Canada. Follow the action from the conference on Twitter.