How are physics and technology connected?

"That would ponder cosmologists"

For more than twenty years, neutrinos existed only in theory - in the 1950s it was possible to detect these elementary particles for the first time. How much electron, muon and tau neutrinos weigh is not known exactly. In the summer of 2018, the Karlsruhe Tritium Neutrino Experiment - KATRIN for short - went into operation to precisely measure the mass of the elementary particles. Welt der Physik spoke to Christian Weinheimer from the University of Münster about the first results.

World of Physics: Why are physicists so interested in determining the mass of neutrinos as precisely as possible?

Christian Weinheimer: There are two main reasons why we are currently working intensively on determining the mass of neutrinos. This is motivated on the one hand by particle physics and on the other hand by cosmology. In particle physics, neutrinos are the last elementary particles whose mass cannot yet be precisely determined. In addition, the neutrino mass is extremely small. According to the current state of knowledge, such a small value seems rather unlikely and could be an indication that our models are not complete. Perhaps the neutrino masses hide new mechanisms for how particles get their mass. In order to be able to analyze this better, we would first have to know how big the mass of neutrinos actually is.

Christian Weinheimer

How is the world of the smallest, particle physics, connected with the very greatest, cosmology?

In addition to light particles, neutrinos are the most common type of particle in our universe. We are all constantly flowed through by a huge number of neutrinos, but they do not interact with us at all. So there are around a billion times more neutrinos in the universe than atoms. Because of this high number, they play a role in the formation of cosmic structures such as galaxy clusters. Since neutrinos are extremely light, this influence is small. But the details of cosmological structures depend on the neutrino mass. From this an upper limit for the neutrino mass can even be derived, which is even more precise than that which we can achieve with KATRIN. But such cosmological models always depend on a large number of partially unknown parameters. That means the other way round: With a direct mass determination like with KATRIN we can narrow down cosmological models better.

How is the experiment set up?

In our experiment we use tritium - an unstable isotope of hydrogen - and a large spectrometer. When a tritium atom decays it sends out an electron and an electron antineutrino. We cannot measure this neutrino, but we can determine the energy of the electron extremely precisely. The energy released during the decay is distributed between the two particles, whereby in the extreme case the electron absorbs the entire kinetic energy and the neutrino only receives its rest mass as energy. The idea behind KATRIN is to measure the energy spectrum of the electrons as precisely as possible in the area of ​​their maximum energy. This allows conclusions to be drawn about the rest mass of the neutrinos according to the Einstein equivalence of mass and energy.

How complex is the construction of the detectors with which you measure the energy of the electrons?

With such measurements it is crucial to be able to compare the results from several years. To do this, you have to control the experimental conditions extremely well - we first had to develop the necessary technology. A couple of examples: We have to keep the high voltage constant at one in a million. We need a vacuum that is as good as on the surface of the moon and is in the largest ultra-high vacuum container in the world. We have to keep the temperature, the pressure and the tritium content in the gaseous tritium source constant to one per thousand. And we have over 70 meters of superconducting magnets. The technical innovations that we have implemented here are the result of 18 years of research. We are pleased that many of these technologies are already being used in other areas. And of course we are very happy and proud that the whole apparatus works and that we have now been able to present the first results.

What have you already found out?

The upper limit that we can now specify with KATRIN after just one month of measurement is 1100 milli-electron volts. But we'll get around five times better over the next three years. This means that the neutrino masses can definitely be determined within narrow limits. Of course, the most interesting thing would be to find a specific mass. That would make cosmologists wonder.