Neutrinoless Double Beta Decay Searches

The oscillation experiments have demonstrated that neutrinos are massive particles and have provided most of the neutrino mixing matrix elements. However, two very important neutrino properties are still missing: their nature and the absolute scale of their masses. Neutrinoless double beta decay ( ββ (0 ν )) is presently the only practical tool for probing the character of neutrinos. In case neutrinos are Majorana particles ββ (0 ν ) can moreover provide fundamental informations on their absolute mass scale. The present status of experiments searching for ββ (0 ν ) is reviewed and the most relevant results discussed. The possibility to observe ββ (0 ν ) at a neutrino mass scale in the range 10-50 meV is attracting a lot of interest for ββ (0 ν ) searches. The achievement of the required experimental sensitivity is a real challenge faced by a number of new proposed projects. A review of the most relevant projects proposed for the future is given and the most relevant parameters contributing to the experimental sensitivity are finally outlined.