Beta and double-beta decays within an effective theory

We have developed an effective theory in terms of collective degrees of freedom to calculate the matrix elements for beta decays from odd-mass nuclei to excited states in spherical even-even nuclei. For the systems in which the effective theory consistently describes the data, we then also calculated the matrix element for the two-neutrino double-beta decay between the corresponding even-even nuclei. The systematic construction of the effective operators within the effective theory allows one to estimate theoretical uncertainties. The calculated two-neutrino double-beta decay matrix elements with associated theoretical uncertainties consistently describe experimental data where available, without the need for additional adjustments.