feynlag.dirac.majorana_symmetry_sign

feynlag.dirac.majorana_symmetry_sign(gamma)

Sign s with ψ₁ᵀ (C Γ) ψ₂ = s · ψ₂ᵀ (C Γ) ψ₁ for Grassmann ψ.

Anticommutation gives ψ₁ᵀ M ψ₂ = −ψ₂ᵀ Mᵀ ψ₁, so a symmetric matrix M yields s = −1 and an antisymmetric one s = +1. The Majorana mass structure C P_L is antisymmetric as a matrix, hence s = +1 — the neutrino mass matrix comes out symmetric in flavour, as it must. Raises if M is neither (an unsupported structure).