Neutrinos Properties Beyond the Standard Model

One of the biggest drawbacks of the Standard Model (SM) is that the masslessness of neutrinos is not dictated by an underlying grand principle, such as that of gauge invariance in the case of the photon: the SM simply postulates that neutrinos are massless and, as a result, all their properties are trivial, e.g. magnetic and transitionmoments are zero, etc. Massless neutrinos would be exceptional particles, since no other such fermions exist. If massive, neutrinos would present another puzzle, of why are their masses so much smaller than those of the charged fermions. The fact that neutrinos are the only electrically neutral elementary fermions may hold the key to the answer, namely neutrinos could be Majorana fermions, the most fundamental ones. In this case the suppression of their mass could be associated to lepton number conservation, as actually happens in many extensions of the SM. From the observational point of view nonzero neutrino masses now seem required in order to account for the data on solar and atmospheric neutrinos, as well as the (hot) dark matter in the universe. Detecting neutrino masses is one of the most outstanding chal-