SThey come from the depths of the universe, are generated inside the sun, arise in the cosmic radiation or emerge from the interior of the earth – neutrinos are by far the most common and at the same time the most mysterious elementary particles. Each cubic centimeter contains hundreds of them. Because they carry no charge and hardly react with normal matter, they are difficult to capture and measure. To this day, scientists do not know how large the mass of these particles is. This is one of the most important questions in modern particle physics, astrophysics and cosmology. Now researchers at the Karlsruhe Institute of Technology have come a step closer to the answer. They have been able to significantly reduce the mass range for neutrinos.
Neutrinos are released in radioactive beta decay, nuclear fusion, or other nuclear reactions. And yet for a long time they existed only in the minds of theorists. Wolfgang Pauli was the first to speculate regarding their existence in 1930, when he realized that without such a particle, two fundamental principles of physics would be violated in beta decay – the conservation of energy and angular momentum. To solve this difficulty, Pauli postulated a new particle. It is released together with the electron, has neither electrical charge nor mass itself, but can take the missing amount of energy with it. But it was not until 1956 that Frederick Reines and Clyde Cowan, with their legendary “poltergeist” experiment at a nuclear reactor in Los Alamos, were actually able to prove the “little neutrals” – the meaning of the name derived from Italian.
The question of whether neutrinos, of which three types are known, might not have a small mass, remained unanswered for a long time. At the end of the 1990s, scientists using the underground Super Kamiokande detector near the Japanese city of Kamioka found clear evidence for the first time that so-called muon neutrinos, which are produced in the atmosphere, can transform into another type of neutrino. Such neutrino oscillations, which have meanwhile also been observed in solar and artificially generated neutrinos, are only possible if these particles have a mass. However, it cannot be derived from the oscillations how large this is.
During measurement operation, the vacuum tank, which houses the electron spectrometer inside, is at a high voltage of 18,000 volts. For protection, the plant is surrounded by a cage.
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Image: Frank Röth
So it was just guesswork. After all, if the tiny creatures weigh something, then it might be that they are the ones behind the mysterious “dark matter” in the cosmos. But this idea has now largely been shattered. Insofar as dark matter is a matter of particles at all, they must not be too “hot”, i.e. rush through space too quickly, otherwise the wisps of matter in the early universe would not have been able to clump together under their own gravity in the way that they do one observes it. But apparently neutrinos weigh so little that their influence was too weak, even though they make up a small part of dark matter.
whizzing through space at practically the speed of light and would therefore be dark matter that is too hot.
Previous measurements have shown that the neutrio mass is in a range between two and 0.02 electron volts (eV). For comparison: the electron, the lightest charged particle, has around 511,000 electron volts, i.e. it is more than 200,000 times as heavy.
Telltale endpoint of the electron spectrum
The neutrino mass is of particular interest to particle physicists, because their so-called Standard Model does not envisage mass neutrinos. While electrons and quarks get their mass via the so-called Higgs mechanism, the cause of the neutrino mass is still unclear. This enigma is one of the reasons why the Standard Model cannot yet be the final word and new theoretical approaches are required.
Since the measuring sensitivity of earlier methods had been exhausted in order to further limit the range of the neutrino mass, plans were made in 2001 for a new spectrometer with a significantly higher resolution and sensitivity: the experiment “KATRIN” (“Karlsruhe Tritium Neutrino Experiment”), which started measuring operations in the Baden metropolis four years ago.
