Experimental Nuclear Matrix Elements for Two Neutrino Double Beta Decays and Single Beta Decays

Experimental nuclear matrix elements M-2 nu for two neutrino double beta decays (2 nu beta beta) are discussed in terms of experimental nuclear matrix elements for successive single-beta processes via Fermi-surface (low-lying) quasi-particle states (FSQP). They are reproduced approximately by the sum Sigma M-L(k) of the 2 nu beta beta matrix elements M-k through k-th intermediate quasi-particle states in the Fermi-surface. M-k is expressed by using the GT(J(pi) = 1(+)) matrix elements of M-i(k) and M-f(k) for the Successive single-beta transitions through the k-th intermediate state. The single-beta matrix elements are evaluated from experimental data of single-beta, EC and charge exchange reactions.