The DCBA/MTD Experiments for Neutrinoless Double Beta Decay Search

One of the most important problems in elementary particle physics is whether neutrino has Majorana nature or not. If neutrino has Majorana nature, Leptogenesis based on See-saw Mechanism is considered to be a hopeful theory in early universe. Neutrinoless double beta decay (0) experiment is only one practical method to investigate the neutrino Majorana nature. Since the half-life of 0 is theoretically expected so long as 10 26 y, the experimental detector has to have not only at least 1000 mol of decay source, but also excellent capabilities to eliminate backgrounds from natural radio activities including decay chains and cosmic rays. And also the excellent energy resolution is required at the Q-value of decay nucleus in order to distinguish 0 events from the tough backgrounds of conventional double beta decay (2). Drift Chamber Beta-ray Analyzer (DCBA) is an R&D for the future experiment temporarily called Magnetic Tracking Detector (MTD) which will realize conditions mentioned above. Both are momentum analyzers using the decay source of 100 Mo and 150 Nd and in progress at KEK. DCBA/MTD can identify -ray in the backgrounds of -, and cosmic rays. Since a module of MTD will accommodate 32 kg decay source, we can search for 0 events with several ten modules down to about 30 meV neutrino mass, corresponding to the half-life of 10 26 y.