Low-energy limits on heavy Majorana neutrino masses from the neutrinoless double-beta decay and non-unitary neutrino mixing

In the type-I seesaw mechanism, both the light Majorana neutrinos (ν1,ν2,ν3) and the heavy Majorana neutrinos (N1,...,Nn) can mediate the neutrinoless double-beta (0νββ) decay. We point out that the contribution of νi to this 0νββ process is also dependent on the masses Mk and the mixing parameters Rek of Nk as a direct consequence of the exact seesaw relation, and the effective mass term of νi is in most cases dominant over that of Ni. We obtain a new bound |ΣRek2Mk|<0.23 eV (or <0.85 eV as a more conservative limit) at the 2σ level, which is much stronger than |ΣRek2Mk-1|<5x10-8 GeV-1 used in some literature, from current experimental constraints on the 0νββ decay. Taking the minimal type-I seesaw scenario for example, we illustrate the possibility of determining or constraining two heavy Majorana neutrino masses by using more accurate low-energy data on lepton number violation and non-unitarity of neutrino mixing.