Double Beta Decays and Neutrino Masses

Neutrino-less double beta decays (0νββ), which violate the lepton number conservation law by Δ L = 2, are of great interest for studying the fundamental properties of neutrinos beyond the standard electroweak theory. High-sensitivity 0 νββ studies with mass sensitivities of the solar and atmospheric ν-masses are crucial for studying the Majorana nature of ν's, the ν mass spectrum, the absolute ν-mass scale, the Majorana CP phases and other fundamental properties of neutrinos and weak interactions. Actually, high-sensitivity experiments of 0νββ are the unique and practical method for studying all these fundamental properties of neutrinos in the foreseeable future. On the basis of the recent ν oscillation studies, the effective mass sensitivity required for observing the 0νββ rate is of the order of the atmospheric ν mass scale of δ m A ∼50 meV in the case of the inverted mass hierarchy and of the order of the solar ν mass scale of δ m S ∼8 meV in the case of the normal hierarchy. The present ββ detectors with sensitivities of 150–300 meV are effective in the case of the quasi-degenerate ν mass spectrum. Future detectors with higher sensitivities of the orders of δ m A –δ m S , using different nuclei and methods (calorimetric, spectroscopic), are indispensable for establishing 0νββ. Theoretical and experimental studies for evaluating nuclear matrix elements M 0ν within 20–30% are important for extracting the sensible ν mass from the 0νββ rate. Charge exchange reactions by means of nuclear, electron and ν probes provide useful data for M 0ν . International collaboration for experimental and theoretical ββ works are encouraged to perform next-generation ββ experiments. High-sensitivity ββ detectors can be used for studying rare nuclear processes such as solar ν'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 0 νββ studies for the Majorana ν mass.