The experimental challenge

The weak interaction manifests –as the name suggests- in tiny interactions rates of neutrinos with matter. An often quoted illustration is the fact that our sun emits per second billiards of neutrinos per second and cm² towards the earth. Only a handful of them interact with the earth crust and core such that they will be stuck in it. The rest of it gets away.  The neutrino properties of interacting weak are on the other hand quite a burden for experimentalists. Already in the very early beginnings of neutrino experiments, physicists were aware that it will need unusual large detectors and strong neutrino sources to observe them. First attempts were even made during nuclear bomb explosions, but the breakthrough being achieved finally in 1956 by Cowan and Reines by using neutrinos from a nuclear reactor. This first neutrino detector had an active mass of 200 kg.

On the other hand the tiny interaction probabilities imply that neutrinos can easily carry energy away from very high matter density regions. This is the reason, why neutrinos play a key role in stellar fusions, supernova processes and most important in the matter formation after the big bang. Over decades, the research community has made successfully efforts to detect such neutrinos e.g from the sun or to detect neutrinos from a stellar death, and to learn out of it.

Nowadays, detectors which detect neutrino directly have masses of several kilotons and even Megaton-detectors are in planning, construction or operation. The largest neutrino detector currently is the IceCube neutrino observatory near the South Pole. It is a detector employed into the arctic ice in a depth of 1400 m and a total volume of 1 km³.  IceCube is searching for extremely high energy neutrinos that come from supernova explosions, gamma-ray bursts, black holes, and other extra-galactic events. It is the solely weak interaction of the neutrinos, which is required that they can make their trip from their origin to the earth over astronomical distances. On the other hand one has to pay for that with very large detectors. There are only a few exceptions like beta-decay experiments, where one can directly learn on neutrino properties without observing the neutrinos itself in a detector. Nevertheless, the experimental requirements remain challenging high, as such experiments look for very subtle effects. For example the KATRIN experiment looks for tiny spectral distortions of the energy spectrum of beta-electrons, which are produced at the same time as neutrinos in a beta-decay, in a region where only 10^-11 of all decays occur. Or the GERDA experiment, it searches for decay modes of ⁷⁶Ge with life times up to 10²⁶ years.

Neutrino physics has developed slowly but steadily from an ad hoc “invention” to rescue energy and momentum conservation in radioactive beta-decays in 1930 by Wolfgang Pauli to a multi-disciplinary research field. Whereas physicists for a long time thought that such neutrinos never can be detected, there is nowadays a global research community spread over all continents to use and explore neutrinos to reveal secrets of nature. And it is the mass of the neutrinos, they look especially for.