Prediction of Neutrino

During the 1920's physicists came to accept the view that matter is built of only two kinds of elementary particles, electrons and protons, which they often called “negative and positive electrons.” A neutral atom of mass number A and atomic number Z was supposed to contain A protons, all in the nucleus, and A negative electrons, A-Z in the nucleus and the rest making up the external electronshells of the atom. Their belief that both protons and negative electrons were to be found in the nucleus arose from the observations that protons could be knocked out of light elements by alpha‐particle bombardment, while electrons emerged spontaneously (mostly from very heavy nuclei) in radioactive beta decay. Any other elementary constituent of the atom would have been considered superfluous, and to imagine that another might exist was abhorrent to the prevailing natural philosophy.

In order to save the law of conservation of energy, the Swiss physicist Wolfgang Pauli postulated the existence of an electrically-neutral, low mass (at most 1/100 the mass of the proton) particle that would be emitted along with the beta particle. This hypothetical third body could then take away whatever energy was not given to the beta particle; solving that most vexing of issues. Pauli first proposed this hypothesis in a humorous letter to his colleagues Lise Meitner and Hans Geiger.

Pauli was reluctant to publish a paper on this unusual hypothesis, but he penned a letter to a group of prominent nuclear physicists in Tuebingen, Germany, asking for input regarding means of detecting such a particle experimentally.

“I have done something very bad today by proposing a particle that cannot be detected; it is something no theorist should ever do,” he wrote, describing his idea as “a desperate remedy.”

Enrico Fermi, the great Italian physicist, was immediately convinced. Building on the discussions held at the Solvay Conference on October 1933 (devoted to James Chadwick’s discovery of the neutron), he proposed the theory of beta decay based on a hypothesis that an electron-neutrino pair is spontaneously produced by a nucleus in the same way that photons can spontaneously be emitted by excited atoms.

At the Solvay, Pauli said, speaking about “his” particle: “... their mass cannot be very much more than the electron mass. In order to distinguish them from heavy neutrons, mister Fermi has proposed to name them “neutrinos”. It is possible that the proper mass of neutrinos be zero… It seems to me plausible that neutrinos have a spin ½… We know nothing about the interaction of neutrinos with the other particles of matter and with photons: the hypothesis that they have a magnetic moment seems to me not funded at all.“

Francis Perrin concluded that for such a particle to exist it must either have zero mass or be lightweight even by comparison with the electron. Fermi’s theory was the precursor of today theory of ‘weak interaction’. The Fermi’s theory of a point-like interaction of four particles (for instance, an initial neutron becoming a proton, an electron and an antineutrino), was accepted as a canon, an article of faith without proof during a quarter of century.

Consequences

The neutrino remained a hypothetical particle until evidence for its existence was brought forward by Reines and Cowan in 1956.

In 1955, the American physicist Murray Gell Mann introduced the notion of a weak force active not only in radioactive processes, but in other processes whose effects involving several types of neutrinos could be seen in a host of subatomic phenomena.

Since its ‘invention’ by Pauli and the subsequent experimental evidence found in its favour, the neutrino and its antiparticle have played a vital role in the fundamentals of particle physics. Possessing no electrical charge, neutrinos are affected solely by the weak interaction, which allows for closer examination of that force without the ‘background noise’ provided by the other nuclear or atomic forces.

Besides the crucial role it plays in the physics of the infinitely small, the neutrino plays an important role in astrophysics. It could be for instance a key to understand the ‘hidden mass of the Universe’.

REFERENCES

Physics Today. Available in: https://physicstoday.scitation.org/doi/pdf/10.1063/1.2995181. Access in: 23/09/2018.

American Physics Society. Available in: https://www.aps.org/publications/capitolhillquarterly/201110/physicshistory.cfm. Access in: 23/09/2018.

History of the Neutrino. Available in: https://neutrino-history.in2p3.fr/prehistory-and-birth-of-the-neutrino/. Access in: 23/09/2018.