Searching for Majorana neutrinos with double beta decay and with beta beams

The leading questions in today neutrino physics concern the transformation properties of neutrinos under charge conjugation, the lepton number conservation hypothesis and the values of neutrino masses. The answers can be obtained if the neutrinoless double beta decay, 0ν2β, is observed and its rate is measured. The mere observation of the process proves the Majorana nature of neutrinos as well as the violation of the lepton number conservation, while the rate is a sensitive measure of the neutrino mass. The search for this effect at present concerns mainly the double β- decay. This paper describes the recent proposal to search for the neutrinoless radiative double electron capture as an attractive alternative to the double β- decay. It is shown, moreover, that the same information which is expected from the 0ν2β studies can in principle be also obtained from experiments using intense νe beams produced by radioactive ions decaying in flight (the ''beta beams''). (orig.)