where ϵ ϵ μ μ κ κ ρ ρ ν ν is the Levi-Civita symbol, γ γ κ κ are Dirac matrices (with κ κ = 0 , 1 , 2 , 3 ) and γ γ 5 = i γ γ 0 γ γ 1 γ γ 2 γ γ 3 , m is the mass, σ σ μ μ ν ν ≡ ≡ i 2 [ γ γ μ μ , γ γ ν ν ] , and ψ ψ ν ν is a vector-valued spinor with additional components compared to the four component spinor in the Dirac equation. It corresponds to the (⁠ 1 / 2 ⁠, ⁠ 1 / 2 ⁠) ⊗ ((⁠ 1 / 2 ⁠, 0) ⊕ (0, ⁠ 1 / 2 ⁠)) representation of the Lorentz group, or rather, its (1, ⁠ 1 / 2 ⁠) ⊕ (⁠ 1 / 2 ⁠, 1) part.