A Novel Nuclear Model for Double Beta Decay
  The possibility of applying the Quasiparticle Tamm-Dancoff Approximation (QTDA) to describe the nuclear double beta decay is explored. Several serious inconveniences found in the Quasiparticle Random Phase Approximation (QRPA), such as: i) the extreme sensitivity of the $2\nu\beta \beta$ decay amplitudes ${\cal M}_{2{\nu}}$ on the residual interaction in the particle-particle channel, ii) the ambiguity in treating the intermediate states, and iii) the need for performing a second charge-conserving QRPA to describe the $\beta\beta$-decays to the excited final states, are not present in the QTDA. Also, the QTDA allows for explicit evaluation of energy distributions of the double-charge-exchange transition strengths and of their sum rules, and can be straightforwardly applied to single- and double-closed shell nuclei. As an example, the $^{48}$Ca$\go^{48}$Ti decay is discussed within the $1fp$-shell in the particle-hole limit of the QTDA. The general [$(1,1)$-Pad\'e-approximant-like] behavior of the $2\nu\beta \beta$-decay amplitude in the plain QRPA as well as within its different variations is briefly reviewed. 
Franjo Krmpoti\'c