Description of the two-neutrino beta beta decay of Mo-100 by pnMAVA

The microscopic anharmonic vibrator approach (MAVA) is a scheme where the one-and two-phonon states of an even-even nucleus are treated consistently by using a realistic microscopic nuclear Hamiltonian. This model has recently been extended to describe odd-odd nuclei by adding proton-neutron phonons in a scheme called the proton-neutron MAVA (pnMAVA). In this paper, we apply pnMAVA to compute the nuclear matrix elements corresponding to the two-neutrino double beta (2 nu beta beta) decay of Mo-100 to the ground state and the first excited 0(+) state of Ru-100 in a realistic single-particle space. We also compute the GT- and GT+ Gamow-Teller strength functions and compare them with the plain pnQRPA (proton-neutron QRPA) and available data. The redistribution of strength to four-quasiparticle degrees of freedom can be clearly seen in the GT+ function. The more striking effect is seen in the 2 nu beta beta matrix element corresponding to the ground-state transition where the incoherence of individual contributions is stronger for the pnMAVA than for the pnQRPA, and a 15% reduction in the magnitude of the matrix element is obtained for the pnMAVA. The 2 nu beta beta transition rate to the excited 0(+) state is zero in a pnQRPA calculation, whereas the pnMAVA result is not far from the measured decay rate.