Neutrinoless double beta decay of /sup 76/Ge. Status report of an experiment in the Frejus tunnel

As is well known the detection of the neutrinoless double beta decay of nuclei would indicate the violation of lepton number and - would provide significant information about the character, possible mass and chirality couplings of the neutrino. Several direct experiments aiming to detect such a rare event have been recently carried out or are being done. The authors present here the preliminary results of an attempt to search for the neutrinoless double beta decay of /sup 76/Ge, currently being done in the Frejus tunnel. The experiment (which uses a source and detector an assembly of 4 Germanium crystal) investigates the transition from the ground state 0/sup +/ of the /sup 76/Se, as well as to the excited state 2/sup +/ (559.1 KeV), of /sup 76/Se, looking for a gaussian shaped peak at E/sub 2β/=2040.7 KeV in the anti-coincidence spectrum, and at E=1481.6 KeV in the coincidence spectrum 2β/γ (E/sub γ/=559.1 KeV), respectively, by surrounding the 4-Ge detector with a 4π assembly of NaI scintillators. Transition to the 0/sub 2//sup +/ (1122.3 KeV) excited state is also investigated. The neutrinoless double beta decay is a low energy, very rare process which needs a proper matching between the helicities of the neutrinos emitted and absorbed, respectively, by two nucleons within a nucleus. In the neutrinoless 0/sup +/→0/sup +/ transitions that matching is provided by a neutrino mass term and/or by a right handed admixture in the neutrino coupling, whereas in the 0/sup +/→2/sup +/ transition, only the right handed mixture is involved. The 0/sup +/→2/sup +/ transition although is even more rare than the transition to the ground state, has the bonus of the deexcitation photon signal which help to clean the background (provided that the NaI crown be also extremely clean), and gives unambiguous information about the presence of the right handed couplings