Hidden by their tiny mass, neutrinos may carry a profound secret with far-reaching consequences for particle physics and cosmology. Given zero electric charge and no colour, they may be Majorana particles; fermions that are their own anti-particles. Double beta decay offers a unique probe for this hypothesis. Finding no neutrinos in its final state would prove lepton number non-conservation and identify neutrinos as Majorana particles. The experimental signature is the emission of two electrons, sharing the full available decay energy; hence, a mono-energetic peak at the end of a standard-model allowed continuum. Current experiments are aiming for neutrinoless double beta decay half-lives beyond 10²⁶ yr.

[Extract from C. Wiesinger PhD Thesis]