Absence of Majorana-Weyl fermions in d=4 and the theory of Majorana fermions

Abstract

It is customary to identify $\psi_{+}=\nu_{R} + C\overline{\nu_{R}}^{T}$ with a Majorana fermion on the basis of chirality changing charge conjugation $\tilde{C}: \nu_{R}\rightarrow C\overline{\nu_{R}}^{T}$ and parity $\tilde{P}: \nu_{R}\rightarrow i\gamma^{0}\nu_{R}$. The theorem on the absence of a Majorana-Weyl fermion in $d=4$ states $\tilde{C}\gamma_{5}\tilde{C}^{-1}= -\gamma_{5}$ with $\tilde{C}=C\gamma_{4}^{T}$, and thus the charge conjugation of the equivalent Majorana $\psi_{+}=(\frac{1+\gamma_{5}}{2})\nu_{R} + (\frac{1-\gamma_{5}}{2})C\overline{\nu_{R}}^{T}$ vanishes without subsidiary $\gamma_{5}\rightarrow - \gamma_{5}$, namely, not defined in field theory. To be consistent with the theorem, it is common to use a doublet representation of chirality preserving charge conjugation $\hat{C}:\nu_{R,L}\rightarrow C\overline{\nu_{L,R}}^{T}$ and parity $\hat{P}: \nu_{R,L}\rightarrow i\gamma^{0}\nu_{L,R}$ in theory containing both $\nu_{R,L}$. In the type I seesaw model, the latter formulation is applicable but $\psi_{+}=\nu_{R} + C\overline{\nu_{R}}^{T}$ is not a Majorana fermion. An analogue of the Bogoliubov transformation converts $\psi_{\pm}=\nu_{R, L} \pm C\overline{\nu_{R, L}}^{T}$, which are obtained by a precise diagonalization of the seesaw model, to Majorana fermions $\psi_{M_{1,2}}=(\psi \pm C\overline{\psi}^{T})/\sqrt{2}$ with a Dirac-type fermion $\psi$, as originally defined by Majorana. A chiral projection $[(1+\gamma_{5})/2] \psi_{M_{1}}$ of a Majorana fermion is not a chiral fermion, which ensures the presence of the neutrino-less double beta decay.