Outlook and recommendations  Neutrino Masses  Search for Heavy Neutrinos

Conclusion and Prospect

Groups investigating tritium decay for neutrino mass experiments have undertaken high efforts to understand their spectra. In the next future one may expect solutions for irregularities in the spectral shape in order to be able to set a reliable limit on the mass of electron neutrino below ca. 5 eV. New developments, like the investigation of cryogenic bolometers might allow in future the use of alternative sources as well as new detection methods. Recently, first experimental results on the beta decay of ¹⁸⁷Re achieved with a cryogenic microcalorimeter were presented by the group of Genova. A recent measurement on the muon neutrino mass shows improved limits. Furthermore, this result seems to be free now from obsolete pion mass values. However, it might be very difficult to improve existing limits substantially with existing detection techniques. The same appears for experiments looking for the tau neutrino mass. A new upper limit of 24 MeV has been established. Future experiments at LEP-II and LHC might still improve this constraint. The search for heavy neutrinos already covers a mass range from a few 100 eV up to 100 GeV and is sensitive to even very small mixing amplitudes. Up to now no evidence for a heavy neutrino was found. Future experiments at accelerators are able to extend the mass scale above the Z-mass up to 150 GeV.