Development and application of nuclear structure models to calculate double beta decay matrix elements

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To get a more accurate description of the nuclear matrix elements relevant for double beta decay, new nuclear structure methods are to be developed and applied. We are planning to develop and apply for the first time the continuum version of the QRPA (continuum-QRPA) to describe the double beta decay amplitudes. The development of such a continuum-QRPA approach is important also for other physical applications such as muon capture, muon-electron conversion, neutrino-nucleus reactions etc. Analysis based on the concept of the dynamically broken isospin and Wigner spin-isospin symmetries will shed light on the origin of the sensitive parameter dependence of the double beta decay amplitudes and will help in constraining the model parameters involved in the calculations. To estimate the effect of the many-particle configurations and thereby to bridge the gap between the QRPA and modern shell model, many-phonon states will be included into the diagonalization procedure. Reformulation of the QRPA directly in the intermediate odd-odd nuclei will help to avoid using the artificial overlap factor in calculations of the double beta decay amplitudes. Different mechanisms for the double beta decay (light and heavy neutrino exchange, right currents, etc.) are also to be analyzed.