Signal of neutrinoless double beta decay, neutrino spectrum and oscillation scenarios

The lower and upper bounds on the neutrinoless double beta (0/ν 2/β) decay rate are obtained, as functions of the parameters of neutrino oscillations and of the lightest neutrino mass. The constraints on these parameters from the search for the 0/ν 2/β transition, as well as from the interpretation of solar and atmospheric neutrino data in terms of oscillations, can be conveniently represented in one unitarity triangle. This representation helps to clarify the cases when the 0/ν 2/β rate is small; the crucial dependence on the scenarios assumed for solar neutrino oscillations and on the neutrino spectrum is emphasized. We consider hierarchical and non-hierarchical neutrino spectra, and discuss their interest in view of future searches of the 0/ν 2/β decay. (author)