Neutrino pair emission from excited atoms

We explore a possibility of measuring the absolute magnitude and the nature (Majorana vs Dirac) of neutrino masses, by using a novel process of neutrino pair emission from metastable excited atoms. Except lepton number nonconserving processes, the neutrino pair ($\nu \overline{\nu }$) emission is the unique process to directly distinguish the Majorana neutrino from the Dirac neutrino, using the interference effect of identical fermions. The small energy difference between atomic levels makes it easier to measure small neutrino masses as indicated by neutrino oscillation experiments. The crucial point is how to enhance the rate of pair emission without enhancing the radiative decay. We discuss two particular cases; (1) laser irradiated pair emission from metastable atoms, and (2) microwave irradiated emission from circular Rydberg states. A new mechanism of the parametric amplification to enhance the neutrino pair emission is pointed out when Rydberg atoms are irradiated by microwave, while the radiative process may be inhibited by the cavity QED effect. A great variety of measurable neutrino parameters and a variety of experimental methods make this investigation attractive.