Neutrino Pair Emission from Excited Atoms
  We explore a possibility of measuring the magnitude and the Majorana nature of neutrino masses, by using a novel process of neutrino pair emission from excited atoms. Except lepton number non-conserving processes, the neutrino pair ($\nu \bar{\nu}$) emission is the unique process to distinguish the Majorana neutrino from the Dirac neutrino. 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 have in mind 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 are conceivable. 
M. Yoshimura