Results from the Mainz Neutrino Mass Experiment and Perspectives of the next Generation Experiment KATRIN

Abstract

The most direct way to get information on neutrino masses is the investigation of weak decays and tritium is the best candidate due to its high specific activity and low endpoint energy. The relevance of this approach is based on the following arguments: • Neutrinos are not massless as we have convincing evidence for neutrino oscillations from experiments detecting atmospheric and solar neutrinos. The signals are interpreted as differences in the squares of neutrino masses from which a lower bound for the neutrino masses can be derived. • As these differences are rather small one can set limits for all neutrino masses from the lowest upper bound which is set by the Mainz group from tritium β decay: mv2 = −1.6 ± 2.5 ± 2.1 eV2/c4 corresponding to an upper limit of mv < 2.2 eV/c2 (95 % C.L.). Hence the sum of the three neutrino masses is less than about 7 eV/c2 The results are of interest for cosmology as neutrinos are part of the hot dark matter. The minimum total mass deduced from oscilation experiments is about equal to the sum of the stellar masses. The present upper bound from the direct mass measurements is about equal to the limit given by structure formation. The result of the two presently working experiments in Mainz and Troitsk and their systematic uncertainties will be presented. These experiment have almost reached their sensitivity limits. A next generation experiment KATRIN will be set up at Forschungszentrum Karlsruhe. Its aim is to push the sensitivity limit to mv < 0.3 eV/c2 (90 % C.L.). This would fully cover the cosmologically relevant parameter space and set the neutrino mass scale to discriminate between models with degenerated and hierachical neutrino mass scenarios.