Neutrinoless double-beta decay and neutrino mass spectrum

Implications of the neutrinoless double-beta (betabeta(0nu)) decay searches for the neutrino mass and mixing spectrum are discussed. We consider properties of the effective Majorana mass, m(ee), relevant for betabeta(0nu) decay. We find predictions or limits for m(ee) in the three neutrino schemes which explain the atmospheric and solar neutrino data. We show how combined analysis of results from betabeta(0nu)-decay searches, oscillation experiments as well as direct measurements of neutrino mass will allow to identify the spectrum. In this connection, several test equalities which relate m(ee) and the oscillation parameters in the context of certain neutrino spectra are suggested. Two issues are important for realization of the identification program: (i) high enough accuracy of determination of m(ee) which requires reliable knowledge of the nuclear matrix elements, and (ii) possibility to identify the mechanism of the betabeta(0nu) decay, in particular, to disentangle the decay due to exchange of the light Majorana neutrino and mechanisms related to exchange of heavy particles with m >> 1/r(nuclei).