In a list of basic questions in astroparticle physics, neutrinos play a special role. What are the fundamental properties of these particles? How do they affect the cosmic evolution? Neutrinoless Double Beta Decay is an essential clue to address these issues. This rare nuclear process violates the lepton number conservation and is not allowed in the Standard Model. A clear signal of this rare transition would establish that neutrinos are the only fermions that coincide with their own antiparticles (Majorana particles): they would represent a new type of matter. In addition, Neutrinoless Double Beta Decay may determine a parameter known as the effective neutrino mass, which is directly related to the presently unknown absolute neutrino mass scale. LUMINEU has the ambition to revolutionate the strategy of the research for this rare nuclear transition.
The present limits on the effective neutrino mass are around 0.2 eV (EXO-200, KamLAND-Zen experiments), of the same order as the value declared in a controversial claim by a part of the Heidelberg-Moscow collaboration. There are three possible ranges for the effective neutrino mass. Two of them (corresponding to the degenerate and the inverted hierarchy scenario, respectively) are accessible in principle with present methods. The European next-stage searches (CUORE, GERDA and Super-NEMO), the Canadian SNO+ together with the aforementioned American EXO-200 and Japanese KamLAND-ZEN will prove or discard degenerate scenarios with sensitivities around 50-100 meV. In order to cover the inverted hierarchy region, on needs to go down to 20 meV. This requires new detectors and major steps forward in the background control. LUMINEU aims at developing a technology capable performing this further step, resolving all the technical issues related to a large-scale search with this potential.