The Cosmological Constant as a Zero-Parameter Prediction of Asymptotic Safety with Standard Model Matter

In the asymptotic safety approach to quantum gravity, the ultraviolet fixed point of the gravitational renormalisation group constrains all low-energy parameters, potentially making the cosmological constant Λ a calculable prediction rather than an input. We compute this fixed point in the f(R) truncation with full Standard Model matter content (Nₛ = 4, N_D = 45/2, N_V = 12), extending the polynomial expansion from order n = 3 to n = 6. The UV critical surface has dimension three, stable across all orders. Three UV constraints combined with two measured IR quantities — Newton's constant and the inflationary amplitude Aₛ = 2.1 × 10⁻⁹ — leave zero free parameters: Λ is uniquely determined. We identify and correct a factor-of-two error in the matter trace coefficients (confirmed by R. Percacci, private communication), and discover multiple non-Gaussian fixed points in the coupling space, selecting the physical branch by its eigenvalue structure. The polynomial truncation at n = 6 produces Aₛ ∼ O(1), confirming that the quantitative extraction of Λ requires the full non-perturbative f(R) flow equation. We note the potential complementarity with the topological protection of Λ via gravitational θ-vacua (Alexander, Bernardo, and Hui 2026).