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75 years of double beta decay: yesterday, today and tomorrow

In this report I will briefly review the motivation and history of double beta decay search since the first consideration of two neutrino process (2$β(2ν)$) by Maria Goeppert-Mayer in 1935. The first experiments on search for double beta decay in the late of 1940's and beginning of 1950's are considered. It is underlined that for the first time the 2$β(2ν)$ decay has been registered in geochemical experiment with $^{130}$Te in 1950. In direct (counter) experiment this type of decay for the first time has been registered in $^{82}$Se by Michael Moe's group in 1987. Now two neutrino double beta decay has been recorded for 10 nuclei ($^{48}$Ca, $^{76}$Ge, $^{82}$Se, $^{96}$Zr, $^{100}$Mo, $^{116}$Cd, $^{128}$Te, $^{130}$Te, $^{150}$Nd, $^{238}$U). In addition, the 2$β(2ν)$ decay of $^{100}$Mo and $^{150}$Nd to the 0$^+_1$ excited state of the daughter nucleus has been observed and the ECEC(2$ν$) process in $^{130}$Ba was observed too. As to neutrinoless double beta decay (2$β(0ν)$) this process has not yet been registered. In the review results of the most sensitive experiments (Heidelberg-Moscow, IGEX, CUORICINO, NEMO-3) are discussed and conservative upper limits on effective Majorana neutrino mass and the coupling constant of the Majoron to the neutrino are established as $\langle m_ν \rangle < 0.75$ eV and $\langle g_{ee} \rangle < 1.9 \cdot 10^{-4}$, respectively. The next-generation experiments, where the mass of the isotopes being studied will be as grand as 100 to 1000 kg, are discussed. These experiments will have started within a few years. In all probability, they will make it possible to reach the sensitivity to the neutrino mass at a level of 0.01 to 0.1 eV.