Hypothesis

Wolfgang Pauli can be seen as the somewhat reluctant father of the neutrino, being the first one to predict its existence but not being very happy to do so! Researchers had noticed that nuclear beta decay seemed to break the laws of conservation of energy and linear and angular momentum. The energy of the beta radiation followed a continuous spectrum. This was surprising since other types of radioactive decay known at that time (alpha decays) emitted radiation with discrete energy levels.

Pauli proceeded to propose a "desperate remedy" to explain that puzzling situation and to save the conservation laws. Unable to attend a physics meeting in December 1930, he sent a letter to the other "radioactive ladies and gentlemen" presenting what his simple but unheard-of way of thinking had led him to propose. He suggested that the missing energy was being carried away by a third particle emitted in the decay. It seemed that Pauli himself was not very comfortable with it. In the first place, it took him some time to go public with his idea at all, and even then he apologized for predicting a particle that he thought would be impossible to detect since it had no electric charge and therefore was not very likely to interact with matter. Also, the particle was believed to have virtually zero mass and be spin half. Pauli called that particle the neutron and it would be emitted together with the electron in beta decay so that the total energy and momentum would be conserved.

Two years later, James Chadwick discovered what we now call the neutron, but it was evident that this particle was far too massive to be the neutron Pauli had predicted. However, his elusive particle played an important role in the first theory of nuclear beta decay formulated by Enrico Fermi in 1933. Since the name neutron was now taken, Fermi gave it the name neutrino by which it goes today, meaning "the little neutral one". Despite Fermi's work in the 1930s, the neutrino at first did not draw many physicists' attention; in fact, it was considered to be a somewhat "arrogant" particle, which simply ignored matter and whose one and only task was to carry off energy in beta decay.

On 16 January 1934, Enrico Fermi submitted for publication his famous paper on the first viable theory of the weak force, which is responsible for beta decay. He explained the beta decay of a nucleus in terms of the creation and immediate emission of an electron simultaneously with an invisible neutrino. However, his paper was rejected by the journal Nature, because "it contained speculations too remote from reality to be of interest to the reader".

Discovery

Pauli himself was so thoroughly convinced that the neutrino could not be detected that he actually bet a case of champagne that nobody would ever detect the neutrino. Luckily, he was proven wrong when the first Nobel-Prize winning detection of neutrinos in 1956 by Fred Reines and Clyde Cowan of Los Alamos cost him a case of champagne. After all, he seemed quite pleased with himself, sending back a telegram reading: "Thanks for the message. Everything comes to him who knows how to wait."

Standard Model

The standard model provides a powerful picture of particle physics altogether and can be regarded as the basic theory of all matter. It is based on three "generations" of fundamental spin 1/2 particles called quarks and leptons. The neutrino is a lepton and comes in three different "flavours": the electron neutrino in the first generation, the muon neutrino in the second generation and the tau neutrino in the third generation. In the standard model, they are massless and there is no known generation-mixing flavour-changing mechanism provided in the leptonic sector. In the quark sector, however, cross-generational flavour-mixing is included and was observed in detectors at the point of interaction. Fundamental particles interact with each other by exchanging gauge bosons with spin 1, which are also called force carriers. Photons mediate the electromagnetic force, the W⁺, W^- and Z⁰ particles carry the weak force and eight gluons carry the strong force. Quarks interact via all three of these forces, whereas charged leptons interact via the electromagnetic and the weak force, and neutral leptons interact only via the weak force. The force of gravity is not included in the model, because at the energies and masses relevant to the standard model it is insignificant. The standard model provides a theory of fundamental constituents with which one can describe the structure and behaviour of all forms of matter.

Solar Neutrinos

In the late 1960s, Ray Davis of the Brookhaven National Laboratory, USA, began to think about experiments using neutrinos as potential probes of the Sun's interior. The very first solar neutrino experiment was then carried out by him in the late 1960s using a technique from radiochemistry. The heart of the experiment was a large number of suitable target nuclei. Suitable nuclei would be those which very occasionally underwent a nuclear reaction triggered by an incoming solar neutrino from boron-8 decay. The target consisted of an isotope of chlorine and came in the form of 600,000 litres of cleaning fluid. Various laboratory experiments had provided sound knowledge on nuclear properties of argon-37 and chlorine-37 from which theoretical calculations concerning the electron neutrino capture cross section of chlorine-37 could be made and hence it was possible to infer the solar electron neutrino flux. Right from the beginning, it was acknowledged that the observed flux of boron-8 decay reaction neutrinos was substantially less than that predicted by the solar model, assuming that nothing happened to the electron neutrinos after they were created inside the sun. The observed discrepancy became known as the "solar neutrino problem", which over the following decades became a great source of confusion and speculation among particle and astrophysicists. Due to the scepticism prevailing among many particle physicists, much time from 1968 onwards was spent investigating the robustness of the solar model predictions of neutrino fluxes, instead of looking for insufficiencies in the standard model. It took several more sophisticated experiments including and to finally prove in 1990 that the problem did not lie with the solar model by showing that the low-energy neutrinos were missing too. At this time, the "solar neutrino problem" had long turned into a major field of physics research, since it was becoming more than obvious that new physics had been found that needed explanation.