Early History

The neutrino is the least massive type of elementary particle known aside from the photon (generally accepted to have no mass at all, but nobody knows for sure). For quite a long time the neutrino was considered to be massless, but when push came to shove in interpreting various types of data this turned not to be the case. The findings on the neutrino having mass are so recent--or the period during which the opposite was assumed was so long--that the best model of what is known in particle physics right now would have to be called 'The Standard Model with Massive Neutrinos' rather than simply 'The Standard Model'.

The neutrino's storied history for us began with an open letter by Wolfgang Pauli to a conference in Tubingen on 4 December 1930. His letter was humorously addressed to "Dear Radioactive Ladies and Gentlemen" and in it the particle he proposed was to have a mass on the order of an electron, no charge, and be essentially undetectable. It was deemed necessary to account for some missing energy in radioactive beta decay. The model of the atomic nucleus that physicists had at the time was rather poor by today's standards, with what we now call neutrons not yet known, so this was the name he chose. After the 1932 discovery of the neutron by J. Chadwick--with its improvement in the model of the nucleus--Pauli's proposed particle received its current name, with the Italian diminutive suffix -ino. This name choice was made by Enrico Fermi, and he and another independently working physicist--F. Perrin--argued in late 1933 that the neutrino could be massless. As this was simpler, and physicists prefer whatever is simplest consistent with the data, it stuck.

The history of the neutrino is essentially at its beginnings a history of the weak nuclear interaction, so let's take a little look at that. Beta decay, one of three basic types of radioactive process, was discovered with Uranium in 1896 by Becquerel. What is seen in beta decay is the increase of an atomic nucleus's charge and the release of an electron. In 1914, Chadwick showed that the electron leaves with a variable amount of energy, and this was the first hint that something was missing. No doubt due to other concerns, this result was not duplicated until Ellis and Wooster did so in 1927. Then another physicist--Lise Meitner--laid the final groundwork for proposing a new particle by showing that gamma rays (photons) could not be carrying off the excess energy.

One suggestion by Niels Bohr that energy conservation (energy can only be transformed from one form to another, not created or destroyed) might be slightly false shows the aversion there was for stipulating a particle whose existence might be hard or impossible to directly detect, and when Pauli did propose it he is quoted as saying that he had "done a very bad thing" for a theoretical physicist to do. Nowadays, the place of theory in physics is such that there are numerous particles--even the whole category of supersymmetric partners of the known particles--that physicists heavily lean toward believing exist despite there so far being no detection of them, but at the time it was pretty bold for a well-known physicist to openly envision the neutrino.

After the discovery of the neutron, the situation in understanding the nucleus and its decay modes had improved greatly. So the first good effort at describing what was going on in beta decay--and how the weak nuclear force operated--was made by Fermi in 1934, but it would actually take another forty years to get the theory pretty much right, and that was still with massless neutrinos. This Entry would become far too technical if many details of this theoretical history were presented (with axial vector currents and scalar, pseudoscalar, and vector couplings), but some things are essential for a basic understanding.