Nuclear responses for double-beta decays by hadron, photon, and neutrino probes and MOON experiment

Neutrino-less double-beta decays (0 nu beta beta) with the mass sensitivities of the solar and atmospheric nu masses are of great interest for studying the Majorana nature of neutrinos and the absolute mass spectrum as suggested by recent nu oscillation experiments. Here nuclear responses (nuclear matrix elements) for 0 nu beta beta are crucial. They are well studied experimentally by using charge-exchange, photo-nuclear and neutrino reactions. MOON(Mo Observatory Of Neutrinos) is a high sensitivity 0 nu beta beta experiment with the mass sensitivity of an order of 30 meV. Experimental studies of the nuclear responses and the present status of MOON are briefly discussed.