Double beta decays and neutrino nuclear responses

Neutrino-less double beta decays ( 0 ν β β ), which violate the lepton number conservation law by Δ L = 2 , are unique and realistic probes for studying the Majorana nature of neutrinos, the ν mass spectrum and the absolute mass scale, the lepton sector CP and others beyond the standard electro-weak theory. Neutrino nuclear responses (nuclear matrix elements M 0 ν ) are crucial to extract neutrino properties of the particle physics interests from experimental 0 ν β β rates. Subjects discussed include 0 ν β β processes and Majorana neutrinos, the present and future 0 ν β β experiments, and neutrino nuclear responses ( M 0 ν ) and charge exchange reactions.