Neutrinoless double $\beta$ decay, neutrino mass hierarchy, and neutrino dark matter

Naoyuki Haba, Noboru Nakamura, Tomoharu Suzuki

Recently the evidence of the neutrinoless double $\beta$ ($0\nu \beta \beta$) decay has been announced. This means that neutrinos are Majorana particles and their mass hierarchy is forced to inver…