Rare meson decays with three pairs of quasidegenerate heavy neutrinos

We consider a low-scale seesaw scenario where the masses of its heavy Majorana neutrinos are arranged in a pattern of three quasidegenerate pairs, in the range of O(1-6 GeV). Since they can violate lepton number by two units, they contribute to rare decays of Ds and Bc mesons, providing the conditions for maximal CP violation. We find that new phenomenology is possible depending on how many of on-shell pairs mediate these decays. In particular, we present new constraints on muon-heavy neutrino mixing parameters.