Improved description of the 2νββ-decay and a possibility to determine the effective axial-vector coupling constant

An improved formalism of the two-neutrino double-beta decay (2 nu beta beta-decay) rate is presented, which takes into account the dependence of energy denominators on lepton energies via the Taylor expansion. Until now, only the leading term in this expansion has been considered. The revised 2 nu beta beta-decayrate and differential characteristics depend on additional phase-space factors weighted by the ratios of 2 nu beta beta-decay nuclear matrix elements with different powers of the energy denominator. For nuclei of experimental interest all phase-space factors are calculated by using exact Dirac wave functions with finite nuclear size and electron screening. For isotopes with measured 2 nu beta beta-decay half-life the involved nuclear matrix elements are determined within the quasiparticle random-phase approximation with partial isospin restoration. The importance of correction terms to the 2 nu beta beta-decay rate due to Taylor expansion is established and the modification of shape of single and summed electron energy distributions is discussed. It is found that the improved calculation of the 2 nu beta beta-decay predicts slightly suppressed 2 nu beta beta-decay background to the neutrinoless double-beta decay signal. Furthermore, an approach to determine the value of effective weak-coupling constant in nuclear medium g(A)(eff) is proposed.