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

In the type-1 seesaw mechanism, both the light Majorana neutrinos (nu(1), nu(2), nu(3)) and the heavy Majorana neutrinos (N-1,..., N-n) can mediate the neutrinoless double-beta (0 nu beta beta) decay. We point out that the contribution of nu(i) to this 0 nu beta beta process is also dependent on the masses M-k and the mixing parameters R-ek of N-k as a direct consequence of the exact seesaw relation, and the effective mass term of nu(i) is in most cases dominant over that of N-i. We obtain a new bound vertical bar Sigma(RekMk)-M-2 vertical bar < 0.23 eV (or < 0.85 eV as a more conservative limit) at the 2 sigma level, which is much stronger than vertical bar Sigma(RekMk-1)-M-2 vertical bar < 5 x 10(-8) GeV-1 used in some literature, from current experimental constraints on the 0 nu beta beta decay. Taking the minimal type-1 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. (C) 2009 Elsevier B.V. All rights reserved.