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

AbstraktAbstract

An improved formalism of the two-neutrino double-beta decay (2νββ-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 2νββ-decay rate and differential characteristics depend on additional phase-space factors weighted by the ratios of 2νββ-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 2νββ-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 2νββ-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 2νββ-decay predicts slightly suppressed 2νββ-decay background to the neutrinoless double-beta decay signal. Furthermore, an approach to determine the value of effective weak-coupling constant in nuclear medium gAeff is proposed.

An improved formalism of the two-neutrino double-beta decay (2νββ-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 2νββ-decay rate and differential characteristics depend on additional phase-space factors weighted by the ratios of 2νββ-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 2νββ-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 2νββ-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 2νββ-decay predicts slightly suppressed 2νββ-decay background to the neutrinoless double-beta decay signal. Furthermore, an approach to determine the value of effective weak-coupling constant in nuclear medium gAeff is proposed.