The beta-beta two neutrino decay in 48Ca
  A schematic study of the $\beta \beta 2\nu $-decay of $^{48}Ca$ is made in a shell-model approach. The emphasis is especially put on the role of the spin-orbit potential in relation with the contribution of other terms in the strong interaction. This is discussed with a particular attention to the behavior of these ones under the SU(4) symmetry. Different methods in calculating the transition amplitude are also looked at with the aim to determine their reliability and, eventually, why they don't work. Further aspects relative to the failure of the Operator Expansion Method to reproduce the results of more elaborate calculations are examined. 
M. T. Capilla, B. Desplanques and S. Noguera