Chirality of the Seeley-DeWitt coefficients and quartic Weyl structure in the spectral action

We prove that the traced Seeley-DeWitt coefficient tr (a₈) for the Dirac operator on a Ricci-flat four-manifold has the form c (p² + q²) with zero cross-term pq, where p = |C⁺|² and q = |C⁻|² are the norms of the self-dual and anti-self-dual Weyl tensors. The proof rests on a chirality theorem: the generators σ^rs = (1/4) γʳ, γˢ commute with γ₅ in d = 4, rendering the spin connection curvature Ω⏛⏜ block-diagonal in the chiral basis. Consequently the heat kernel e^−tD² splits into left- and right-handed sectors, each coupled to only one half of the Weyl tensor (crossed chirality assignment). This result applies to the full Standard Model Dirac operator D on the product geometry M × F and holds at all orders in the Seeley-DeWitt expansion. The spectral action at mass dimension 8 therefore generates a single effective quartic Weyl structure, reducing the three-loop finiteness problem from an overdetermined system to a single ratio condition.