Two-neutrino double-beta decay of 76Ge in an anharmonic vibrator approach

We have calculated the nuclear matrix element of two-neutrino double-beta (2 nu beta beta) decay of Ge-76 by using the proton-neutron microscopic anharmonic vibrator approach (pnMAVA). In the pnMAVA the wavefunctions of the intermediate 1(+) states in As-76 have a one-phonon part containing the proton neutron QRPA (pnQRPA) phonons and a two-phonon part built by coupling the pnQRPA and charge-conserving QRPA phonons. We compare the measured GT- and GT+ Gamow-Teller strength functions with the measured ones. The two-phonon components of the pnMAVA wavefunctions cause the pnQRPA strength to redistribute over a finite energy range. This has only a small effect on the value 2 nu beta beta nuclear matrix element. This shows that at the spherical limit the pnQRPA is an appropriate tool to deal with the double-beta transitions in nuclear systems with only slight deviations from spherical symmetry.