Double beta decay transition mechanism
  After briefly reviewing $\beta \beta$ decay as a test of the neutrino mass, I examine the nuclear structure involved in this process. Simple formulas (\`{a} la Pad\'{e}) are designed for the transition amplitudes and the general behavior of $\beta \beta$ decay amplitudes in the quasiparticle random phase approximation are discussed. Results of a calculation for $^{48}Ca$, $^{76}Ge $, $^{82}Se$, $^{100}Mo$, $^{128}Te$ and $^{130}Te$ nuclei are presented, in which the particle-particle interaction strengths have been fixed by invoking the partial restoration of the isospin and Wigner SU(4) symmetries. An upper limit of $ <m_{\nu}>\approx 1$ eV is obtained for the effective neutrino mass. 
F. Krmpoti\'c