Probing low energy and mass scales

Well proved by a number of experiments carried out during the last ten years, the existence of neutrino oscillations has stimulated renewed interest in the study of neutrino properties. Neutrino oscillation results have shown that neutrinos mix and have finite masses; they have thus provided us with the first clear evidence of phenomena beyond the reach of Standard Model (SM). Unlike quark mixings, neutrino mixings are large, although the reason for this is not yet fully understood. On the other hand neutrino masses are much smaller than those of the charged leptons and their pattern (or absolute scale) is still unknown. Neutrino oscillations in fact only depend on the absolute value of the difference of the squares of the neutrino masses and two possible hierarchies (or orderings) are then implied by current available data: the normal (m1 < m2 ≪ m3) and the inverted hierarchy (m3 ≪ m1 < m2). These and other unsolved questions concerning neutrino properties are becoming subject of increasing interest and considered a unique tool to see what new Physics lies beyond SM predictions. This holds in particular for the Dirac/Majorana neutrino nature which represents one of the most important open questions in neutrino Physics. In the SM neutrinos are Dirac particles by construction (i.e. in order to conserve lepton number L). In the limit of vanishing masses however Lepton Number conservation can be equivalently stated in terms of neutrino helicity properties and the Majorana or Dirac descriptions for neutrino are equivalent (i.e. don’t change the physical content of the theory). For finite neutrino masses however the two descriptions are no more equivalent and can give rise to different Physical scenarios (e.g. mass generation mechanisms). Only experiments sensitive to m1 can aim at solving the mass hierarchy problem. This is the case for kinematic measurements of the β spectrum end-point, neutrinoless double beta decay (ββ[0ν] ) and cosmological measurements. Intense efforts are therefore being spended to fix (measure or bound) the absolute scale of neutrino masses (as contrasted to the ∆m measurements of neutrino oscillation experiments) with various observations and experiments, characterized by different systematics and sensitivities. Cosmological observations and laboratory experiments actually probe different quantities which can be however compared within specific models. In the case of 3 Majorana neutrinos and upon a good control of the systematics, cosmological observations and ββ[0ν] experiments have the largest success potential. In the same theoretical framework, complementary informations can come also from oscillation experiments if Ue3 is large enough. Kinematical measurements seem presently characterized by a lower sensitivity although they represent the only model independent measurement of the neutrino mass scale. On the other hand ββ[0ν] experiments