Absolute Neutrino Masses: Physics beyond Sm, Double Beta Decay and Cosmic Rays

Solar and atmospheric neutrino oscillations have established solid evidence for non-vanishing neutrino masses and determined mass squared differences δm and the mixing matrix U with increasing accuracy. The puzzle of the absolute mass scale for neutrinos, though, is still unsolved . In fact it is a true experimental challenge to determine an absolute neutrino mass below 1 eV. Three approaches have the potential to accomplish the task, id sunt larger versions of the tritium end-point distortion measurements, limits from the evaluation of the large scale structure in the universe, and next-generation neutrinoless double beta decay (0νββ) experiments. In addition there is a fourth possibility: the extreme-energy cosmic-ray experiments in the context of the recently emphasized Z-burst model. This article is organized as follows: in section 2 the specific role of the neutrino among the elementary fermions of the SM is reviewed, as are the two most popular mechanisms for neutrino mass generation. Also, the link of absolute neutrino masses to the physics underlying the standard model is discussed. Section 3 deals with direct determinations of the absolute neutrinos mass via tritium beta decay and cosmology. In section 4 0νββ is discussed, which may test very small values of neutrino masses, when information obtained from oscillation studies is input. Section 5 finally deals with the connection of the sub-eV neutrino mass scale and the ZeV energy scale of extreme