Introduction: Basics of Non-Baryonic  Neutrino Physics  Conclusion and outlook.

    
Dark Matter

1998-03-04    
 Dark Matter  Double beta decay  Interpretation of the data

Conclusion and outlook.

To summarise, in recent years the allowed $\beta\beta 2\nu$ decay has been observed in several nuclei. QRPA and shell model calculations reproduce fairly well the measured half lives. This gives some confidence that the matrix elements needed to interpret the experimental limits on $\beta\beta 0\nu$ decay can be calculated with reasonable reliability as well. Data from the most sensitive experiments on that decay lead to the limit eV for the effective neutrino mass. The Heidelberg-Moscow and IGEX experiments on ⁷⁶Ge are continuing. They may achieve sensitivities of order yr in ⁷⁶Ge, corresponding to eV. The 20 crystal bolometer array being built by the Milano group will presumably achieve comparable sensitivity, provided background problems can be solved. The NEMO III detector, for which NEMO II was a prototype, is being built and will be installed in the Fréjus lab. It is based on the same principles, but has a much larger source foil, and has a magnetic field for electron-positron identification. It is designed to study primarily ¹⁰⁰Mo, of which it can accommodate 10 kg. Because of the high sensitivity, but also the modest energy resolution (15 % FWHM) and the relatively short $\beta\beta 2\nu$ half life in ¹⁰⁰Mo, the major background source when looking for $\beta\beta 0\nu$ decay will be $\beta\beta 2\nu$ decay! After a few years of data taking, a sensitivity of order yr should be achieved corresponding to eV. The MUNU detector, a CF₄ gas TPC built for the study of neutrino-electron scattering at the Bugey nuclear reactor, could also be used to search for double beta decay. Installed underground, filled with 10 kg of enriched ¹³⁶Xe, it would achieve a sensitivity comparable to that of NEMO III. One can thus conclude that presently running or planned experiments will explore effective neutrino masses down to, say, 0.3 eV. Independently of what they find, it will be desirable to push further the sensitivity. To achieve this it is necessary to go to yet larger source masses. Larger detector sizes can be considered. Current double beta decay detectors, even considering NEMO III, are small by many standards. For instance, solar neutrino detectors are significantly larger. Also it does not seem impossible to produce enriched sources with masses as large as 100 kg. To take full advantage the background will need to be reduced in parallel. This seems possible with the experience gained in present generation double beta decay experiments and solar neutrino experiments. Also detector technology has brought advances leading to better event signature. Therefore a next generation experiment with a source of 100 kg of some enriched material appears feasible. It would have a sensitivity to $T_{1/2}^{0\nu}$ of order 10²⁶ yr, corresponding to 0.1 eV. Such a project can however only be carried out by a large collaboration. Several of the groups presently involved in various smaller experiments would have to join efforts. To go further, it seems impossible to envision enriched sources. This means that much larger detector masses, and still better event identification are necessary. One idea put forward is to use a liquid TPC filled with natural Xe, in which not only the electrons are detected, but also the positive ions. Much development needs however to be done before the construction of a full scale detector can be considered.