text: If the nature of the neutrinos is Majorana, then they can be emitted and absorbed in the same process without showing up in the corresponding final state. As Dirac particles, both the neutrinos produced by the decay of the W bosons would be emitted, and not absorbed after.
  Neutrinoless double beta decay can only occur if
  * the neutrino particle is Majorana, and
  * there exists a right-handed component of the weak leptonic current or the neutrino can change its handedness between emission and absorption (between the two W vertices), which is possible for a non-zero neutrino mass (for at least one of the neutrino species).
  The simplest decay process is known as the light neutrino exchange. It features one neutrino emitted by one nucleon and absorbed by another nucleon (see figure to the right). In the final state, the only remaining parts are the nucleus (with its changed proton number ) and two electrons:
  The two electrons are emitted quasi-simultaneously.
  The two resulting electrons are then the only emitted particles in the final state and must carry approximately the difference of the sums of the binding energies of the two nuclei before and after the process as their kinetic energy. The heavy nuclei do not carry significant kinetic energy.
  In that case, the decay rate can be calculated with
  where  denotes the phase space factor,  the (squared) matrix element of this nuclear decay process (according to the Feynman diagram), and  the square of the effective Majorana mass.
  First, the effective Majorana mass can be obtained by
  where  are the Majorana neutrino masses (three neutrinos ) and  the elements of the neutrino mixing matrix  (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  (can only be done if the decay is actually generated by the neutrino masses).
  The nuclear matrix element (NME)  cannot be measured independently; it must, but also can, be calculated. The calculation itself relies on sophisticated nuclear many-body theories and there exist different methods to do this. The NME  differs also from nucleus to nucleus (i.e. chemical element to chemical element). Today, the calculation of the NME is a significant problem and it has been treated by different authors in different ways. One question is whether to treat the range of obtained values for  as the theoretical uncertainty and whether this is then to be understood as a statistical uncertainty. Different approaches are being chosen here. The obtained values for  often vary by factors of 2 up to about 5. Typical values lie in the range of from about 0.9 to 14, depending on the decaying nucleus/element.
  Lastly, the phase-space factor  must also be calculated. It depends on the total released kinetic energy (, i.e. "-value") and the atomic number . Methods use Dirac wave functions, finite nuclear sizes and electron screening. There exist high-precision results for  for various nuclei, ranging from about 0.23 (for ), and 0.90 () to about 24.14 ().
  It is believed that, if neutrinoless double beta decay is found under certain conditions (decay rate compatible with predictions based on experimental knowledge about neutrino masses and mixing), this would indeed "likely" point at Majorana neutrinos as the main mediator (and not other sources of new physics). There are 35 nuclei that can undergo neutrinoless double beta decay (according to the aforementioned decay conditions).