The neutrinoless double beta decay is a very old conjecture, but so far, the process has not been observed.
The two-neutrino double beta decay (2νββ) was first considered by Maria Göppert Mayer:
(A, Z) → (A, Z + 2) + e- + e- + -ve + -ve
Two years after Göppert Mayer, in 1937, Ettore Majorana proposed a new theory of neutrinos, with neutrinos being their own antiparticles. The only feasible way that physicists have come up to prove this far-reaching conjecture is the neutrinoless double beta decay (0νββ):
(A, Z) → (A, Z + 2) + 2 e-
Whereas the simultaneous emission of two electrons and two anti-neutrinos in the (2νββ) process conserves lepton number and is allowed within the Standard Model of particle physics, (0νββ) is forbidden because it violates lepton number by two units. An observation of (0νββ) would thus demonstrate lepton number violation in nature and would prove that neutrinos are Majorana and not Dirac particles. Apart from that, (0νββ) is also crucial for assessing the scale of the neutrino masses as well as the neutrino mass hierarchy by way of determining the half-live of the process with utmost sensitivity.
At the moment, several (0νββ) experiments are running deploying 76Ge, 82Se, 130Te and 136Xe, isptopes where (0νββ) is expected possible. The GERmanium Detector Array (Gerda) experiment at the Laboratori Nazionali del Gran Sasso searches for the 0νββ decay of 76Ge. High-purity germanium detectors isotopically enriched in 76Ge are operated bare and immersed in liquid argon in order to greatly reduce the environmental background with further suppressing measures deployed.
In the standard interpretation of (0νββ), the half-life of the decay is related to the Majorana mass of the particles. To achieve a sensitivity beyond the current level reached by Gerda and other experiments and to explore half-lives predicted by neutrino oscillation experiments, it is mandatory to reduce the backgrounds further by at least on order of magnitude.
The programm proposed in N01 therefore focuses on R&D to substantially enhance the performance of a future (0νββ) experiment using high-purity germanium technology or cryogenic detectors with the goal to fully cover the effective Majorana masses for the so-called inverse mass-ordering, a region which is so far inaccessible with state-of-the-art experiments.