Weak magnetism in two neutrino double beta decay

We have extended the formalism for the two-neutrino double beta decay by including the weak-magnetism term, as well as other second-forbidden corrections. The weak magnetism diminishes the calculated half-lives in ∼10%, independently of the nuclear structure. Numerical computations were performed within the pn-QRPA, for 76Ge, 82Se, 100Mo, 128Te and 130Te nuclei. Not one of the second-forbidden corrections modifies significantly the spectrum shapes. The total reduction in the calculated half lives varies from 6% up to 32%, and strongly depend on the nuclear interaction in the particle-particle S=1,T=0 channel. We conclude that the higher order effects in the weak Hamiltonian cannot be observed in the two-neutrino double beta experiments.