Neutrinoless double beta decay
[7] The Italian physicist Ettore Majorana first introduced the concept of a particle being its own antiparticle in 1937.
In 1939, Wendell H. Furry proposed the idea of the Majorana nature of the neutrino, which was associated with beta decays.
[8] Furry stated the transition probability to even be higher for neutrinoless double beta decay.
[8][clarification needed] It was the first idea proposed to search for the violation of lepton number conservation.
[5] Weak beta decays normally produce one electron (or positron), emit an antineutrino (or neutrino) and increase (or decrease) the nucleus' proton number
There exist a number of elements that can decay into a nucleus of lower mass, but they cannot emit one electron only because the resulting nucleus is kinematically (that is, in terms of energy) not favorable (its energy would be higher).
[10] A number of isotopes have been observed already to show this two-neutrino double beta decay.
[3] This conventional double beta decay is allowed in the Standard Model of particle physics.
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.
[3] As Dirac particles, both the neutrinos produced by the decay of the W bosons would be emitted, and not absorbed after.
In the final state, the only remaining parts are the nucleus (with its changed proton number
the (squared) matrix element of this nuclear decay process (according to the Feynman diagram), and
[7] 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
[13] The calculation itself relies on sophisticated nuclear many-body theories and there exist different methods to do this.
Methods use Dirac wave functions, finite nuclear sizes and electron screening.
[7] 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).
[3] Nine different candidates of nuclei are being considered in experiments to confirm neutrinoless double beta-decay:
{\displaystyle \mathrm {^{48}Ca,^{76}Ge,^{82}Se,^{96}Zr,^{100}Mo,^{116}Cd,^{130}Te,^{136}Xe,^{150}Nd} }
Factors to be included and revised are natural abundance, reasonably priced enrichment, and a well understood and controlled experimental technique.
[3] Experimentally of interest and thus measured is the sum of the kinetic energies of the two emitted electrons.
From this, it can be deduced that neutrinoless double beta decay is an extremely rare process, if it occurs at all.
The so-called "Heidelberg-Moscow collaboration" (HDM; 1990–2003) of the German Max-Planck-Institut für Kernphysik and the Russian science center Kurchatov Institute in Moscow famously claimed to have found "evidence for neutrinoless double beta decay" (Heidelberg-Moscow controversy).
[3] To this day, no other experiment has ever confirmed or approved the result of the HDM group.
[7] Instead, recent results from the GERDA experiment for the lifetime limit clearly disfavor and reject the values of the HDM collaboration.
[23] Liquid argon was used for muon vetoing and as a shielding from background radiation.
[24] Phase II of the experiment started data-taking in 2015, and it used around 36 kg of germanium for the detectors.
[24] The exposure analyzed until July 2020 was 10.8 kg yr. Again, no signal was found and thus a new limit was set to
[25] The detector has stopped working and published its final results in December 2020.
[23] The experiment is located in New Mexico (US) and uses a time-projection chamber (TPC) for three-dimensional spatial and temporal resolution of the electron track depositions.
[19] When translated to effective Majorana mass, this is a limit of the same order as that obtained by GERDA I and II.
