where m i are the Majorana neutrino masses (three neutrinos ν ν i ) and U e i the elements of the neutrino mixing matrix U (see PMNS matrix). Contemporary experiments to find neutrinoless double beta decays (see section on experiments) aim at both the proof of the Majorana nature of neutrinos and the measurement of this effective Majorana mass ⟨ ⟨ m β β β β ⟩ ⟩ (can only be done if the decay is actually generated by the neutrino masses).