Seminar 
Title: Majorana neutrino as Bogoliubov quasiparticle 
Speaker: Prof.Kazuo Fujikawa (University of Tokyo) 
Time: 10:00AM, May 25th, 2018(Friday) 
Place: Theoretical Physics Division, 319 

Abstract: The Lagrangian of the seesaw mechanism is C violating but the same Lagrangian when re-written in terms of Majorana neutrinos is manifestly C invariant. To resolve this puzzling feature, a relativistic analogue of Bogoliubov transformation, which preserves CP but explicitly breaks C and P separately, is introduced together with the notions of a Bogoliubov quasiparticle and an analogue of the energy gap in BCS theory. The idea of Majorana neutrino as Bogoliubov quasiparticle is then suggested. This scheme is compared with the conventional formulation of seesaw mechanism, and it is shown that the direct conversion of a Weyl fermion to a Majorana fermion is ill-defined. Motivated by our formulation, I briefly discuss the naturalness issue of seesaw mechanism. I also discuss the vacuum structure of the Majorana neutrino, which becomes heavy by absorbing the C-breaking, by comparing the Majorana vacuum with the vacuum of the Dirac neutrino following the analysis of Nambu and Jona-Lasinio.