Internal pseudospin, lepton-number superselection, and neutrino--antineutrino coherence in massive neutral-fermion one-particle states

At fixed three-momentum, massive Dirac neutrino one-particle states span a 4D space of particle–antiparticle identity and helicity. We show that helicity flip, charge conjugation, and their product close an internal $SU(2)$ pseudospin subalgebra within $SU(4)$, distinct from the Wigner little group. Its helicity generator is the lepton-number-weighted spin rotation $U_1=2L{J}_2$. The lepton-number $L$ and helicity ${\mathcal{H}}_h$ grade the 16 generators ${\tau }_{\mu }\otimes {\sigma }_{\nu }$, organizing the $\mathrm{\Delta }L=2$ sector. Helicity-preserving directions ${\tau }_{1,2}\otimes {\sigma }_{0,3}$ carry the pseudo-Dirac mixing ${\kappa }_{\mathrm{p}\mathrm{D}}=\mathrm{\Delta }{m}^2\mathrm{/}4E$ (active–sterile), while helicity-flipping directions ${\tau }_{1,2}\otimes {\sigma }_{1,2}$ carry the neutrinoless double-beta decay mass factor (active–active). Furthermore, charge conjugation matches the $U_2$ generator. The Majorana condition is thus a projection onto the $U_2=+1$ eigenspace, where only the Wigner algebra survives. This framework algebraically classifies Majorana masses and pseudo-Dirac splittings without assuming neutrinos are Majorana particles.