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A Brief History of the Neutrino

Author/Source: B. Katzenstein
ETH Zürich Picture Library

In 1930 Wolfgang Pauli postulated the existence of a new particle. One had investigated α and γ decays in detail and observed the discrete energies of the particles emitted in those processes. But while researching the β decay it became apparent, that the electrons produced there showed a continuous energy spectrum. This came as a surprise since at this time one expected the β decay to be a two-body decay, like the α and γ decay, which have a discrete energy spectrum. Additionally, regarding only the particles known to be involved in the decay, the process seemed to violate angular momentum conservation.
Pauli offered a solution to those problems by proposing an additional particle, which was emitted during the decay but not detected, explaining the electron's continuous energy distribution and fixing angular momentum conservation. He called it the "neutron", which was later changed to "neutrino".
The problem was that this newly postulated particle seemed to escape all detection. Pauli himself stated, "I have done a terrible thing. I invented a particle that cannot be detected." The theory of the β decay including the neutrino worked well, but it took about another 25 years until an experiment was able to finally confirm the neutrino's existence: Reines and Cowan managed to detect the positrons produced in a water tank with dissolved CdCl2 when a neutrino reacted with an atomic core of the solution in a so-called inverse β decay.
But that by far was not the end of this particle's investigation. Many experiments followed with the goal to understand the properties and reactions of this particle so very hard to detect. Some peculiar and exciting features were discovered and today neutrinos are still not completely understood.

The Standard Model classifies neutrinos as massless particles with a spin of 1/2, so-called fermions. As the only particles listed there they exclusively carry a weak charge. As a result, of the three fundamental interactions appearing in the Standard Model, they are not affected by neither the electromagnetic nor the strong interaction. Only through the weak interaction they are able to interact with their environment. This is what makes them so difficult to detect.
The Standard Model distinguishes six different versions of the neutrino: The electron-, muon- and tau-neutrino as well as the according antiparticles. Those different neutrino types are named after the respective electromagnetically charged lepton they interact with in charged current weak interactions. For example in a reaction with an electron and a W-boson, the exchange particle of charged current weak interactions, there will always also be an electron-neutrino (νe) or its antiparticle involved, but never a muon- (νμ) or tau-neutrino (ντ). Therefore those different appearances of the neutrino, electron-, muon- and tau-neutrino, are called the neutrino's eigenstates of the weak interaction, since it can be predicted unambiguously which of those neutrinos will be produced in a weak current interaction with a certain charged lepton.

Those different types of neutrinos one observes when they interact, are not the mass eigenstates of the particles, though. That means, that neither the electron- or muon- nor tau-neutrino have a defined mass, which stays constant over time. Instead each of those eigenstates of the weak interaction can be expressed as a linear combination of the three existing neutrino mass eigenstates ν1, ν2 and ν3, with the same applying to the according antiparticles, for example:

The prefactors Uαi are the elements of the Pontecorvo-Maki-Nakagawa-Sakata-matrix, analogous to the Cabibbo-Kobayashi-Maskawa-matrix for the mixing of quarks.
Those mass eigenstates now show a well defined behavior over time t, dictated by their fixed masses mi and momentum pi:

In case the masses of the different mass eigenstates have different values, their according time developments also differ from each other. That in turn means for example, that a particle originally being an electron-neutrino, for which it is well defined to what parts it contains ν1, ν2 and ν3, after a while, with different time developments of those components, changes its composition. When this neutrino then reacts in a weak interaction, it takes the form of a weak interaction eigenstate again, which, because of the changed composition, has not necessarily to be the one of an electron-neutrino any more, but can also be a muon- or tau-neutrino.

This change of the neutrino flavor is called oscillation. It was first confirmed by the Super-Kamiokande experiment in 1998 and later confirmed by SNO. It was able to explain the long-lasting issue, that much less solar electron-neutrinos, than predicted by solar models, reach earth, called solar neutrino problem. The theory of this oscillation, however, states that it only occurs in case the mass eigenstates posses different masses and accordingly different time developments. Its observation hence means, that at least two of those neutrino masses have to be unequal to zero, in contrast to the massless neutrinos predicted by the Standard Model. And even if those neutrino masses are smaller than currently measurable, this means that neutrinos show physics beyond the Standard Model.


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