Double beta decays and neutrino masses

Neutrino-less double beta decays (Ov beta beta), which violate the lepton number conservation law by Delta L = 2, are of great interest for studying the fundamental properties of neutrinos beyond the standard electroweak theory. High-sensitivity Ov beta beta studies with mass sensitivities of the solar and atmospheric Vmasses are crucial for studying the Majorana nature of v's, the v mass spectrum, the absolute v-mass scale, the Majorana CP phases and other fundamental properties of neutrinos and weak interactions. Actually, high-sensitivity experiments of Ov beta beta are the unique and practical method for studying all these fundamental properties of neutrinos in the foreseeable future. On the basis of the recent v oscillation studies, the effective mass sensitivity required for observing the Ov beta beta rate is of the order of the atmospheric v mass scale of delta m(A) similar to 50 meV in the case of the inverted mass hierarchy and of the order of the solar v mass scale of Sins similar to 8 meV in the case of the normal hierarchy. The present beta beta detectors with sensitivities of 150 - 300 meV are effective in the case of the quasi-degenerate v mass spectrum. Future detectors with higher sensitivities of the orders Of delta m(A)-delta m(S), using different nuclei and methods (calorimetric, spectroscopic), are indispensable for establishing Ov beta beta. Theoretical and experimental studies for evaluating nuclear matrix elements M-Ov within 20-30% are important for extracting the sensible v mass from the Ov beta beta rate. Charge exchange reactions by means of nuclear, electron and v probes provide useful data for M-Ov. International collaboration for experimental and theoretical beta beta works are encouraged to perform next-generation Pp experiments. High-sensitivity beta beta detectors can be used for studying rare nuclear processes such as solar v's, dark matter, charge nonconservation, and nucleon decays. This report is a brief review of double beta decays and neutrinos with emphasis on high-sensitivity Ov beta beta studies for the Majorana v mass.