  We calculate nuclear matrix elements (NME) of neutrinoless double beta decay
in four different candidate nuclei (Ge-76, Se-82, Mo-100, Te-130) within the
quasiparticle random phase approximation (QRPA) and its uncertainties. We
assume (up to) four coexisting mechanisms for neutrinoless double beta decay,
mediated by light Majorana neutrino exchange, heavy Majorana neutrino exchange,
R-parity breaking supersymmetry, and squark-neutrino, interfering either
constructively or destructively with each other. We find that, unfortunately,
current NME uncertainties appear to prevent a robust determination of the
relative contribution of each mechanism to the decay amplitude, even assuming
accurate measurements of decay lifetimes. The near-degeneracy of the decay
mechanisms is analyzed with simple algebraic techniques, which do not involve
assumptions about the statistical distribution of errors. We discuss
implications of such degeneracy on prospective searches for absolute neutrino
masses.
