Of all elementary particles, neutrinos are the most mysterious ones. As early as 1930, Wolfgang Pauli had postulated the existence of these electrically neutral and only weakly interacting "phantom particles" so that he would not have to abandon the idea of energy and angular momentum continuity in beta decay. Although detecting a neutrino was first thought to be impossible, Clyde Cowan and Frederick Reines eventually succeeded in 1956.

Later, researchers realized that electron neutrinos (and their antiparticles) were not the sole existing neutrinos, but that there were two further kinds: muon neutrinos and tau neutrinos. Surprisingly, neutrinos of one specific kind can turn into another kind. This is only possible if the three kinds of neutrinos have different masses. Since neutrinos occur frequently in the universe, the values of the neutrino masses have a considerable influence on cosmic development. Dr. Jochen Bonn and Dr. Christian Weinheimer of Ernst Otten's working group at Mainz University succeeded in determining an exact limit to the mass of the electron neutrino. They were awarded the 2001 Helmholtz Prize for their work. It was the first time the Prize was awarded jointly by the Helmholtz Fund and the Stifterverband für die Deutsche Wissenschaft.

To determine an upper limit to the mass of an electron neutrino, the two physicists and their colleagues from Mainz analyzed the beta decay of tritium. The decay of a tritium nucleus creates a helium-3 nucleus, an electron and an electron antineutrino. Since the daughter nucleus has a much greater mass than the electron and the neutrino, it takes up a considerable impulse, but hardly absorbs any energy.

The energy released during the decay is distributed randomly between the electron and the neutrino. At the endpoint of the energy spectrum of the electron, at 18.6 keV, where its energy is at its highest, the influence of the neutrino mass m_ν becomes noticeable in a subtle way. For one thing, the endpoint of the spectrum towards low energies is shifted by m_νc². For another, the probability of an electron having a certain energy is reduced by a value proportional to m_ν². If we assume, for example, m_ν = 1 eV/c², then only a minute part of 2 × 10^–13 of the electrons created have an energy which is so close to the endpoint of the energy spectrum that the influence of m_ν on the spectrum becomes detectable.

The researchers measured the energy spectrum of such electrons that were as close to the maximum energy as possible. With the aid of two superconducting coils, they generated a magnetic field whose field lines were kept together at the coils and dispersed in the space between the two coils. One of the coils contained a tritium sample in the form of a frozen film; the other contained an electron detector. The electrons generated by the decaying tritium moved along the field lines into the area where the field lines were dispersed and where they came upon an electric potential limit of variable height generated by ring electrodes. Only such electrons were able to overcome that limit and get to the detector which had a sufficiently high energy. In this way, the researchers discovered a lower limit to the maximum energy of the electrons. From this lower limit, they derived an upper limit for the neutrino mass: m_ν < 2.8 eV/c². In the meantime, both the researcher group from Mainz and one from Troitsk, Russia, have succeeded in improving this value to 2.05 eV/c². Significant progress is expected from the Karlsruhe Tritium Neutrino (KATRIN) experiment at the Karlsruhe Institute of Technology (KIT), which is currently being set up by a joint group of international scientists including Christian Weinheimer and Jochen Bonn (until his death). This experiment is expected to help reduce the upper limit for m_ν down to 0.2 eV/c² or find the neutrino mass.