Vectorial axion-down-strange coupling from K + → π + ν ν ¯ data

We reinterpret publicly available K + → π + ν ν ¯ data collected by NA62 from 2016 to 2024 to constrain the fundamental vectorial coupling of the QCD axion to down and strange quarks. Using a fully reproducible likelihood analysis and a complete renormalization-group evolution of the axion couplings from the Peccei-Quinn (PQ) scale to the kaon scale, we translate the experimental limit into bounds on both the low-energy flavor-violating coupling and the fundamental UV parameters. In the generic regime where strong contributions dominate the decay amplitude, we obtain | ( F V ) sd ( μ K ) | > 1.6 × 10 12 GeV . The coexistence of parametrically suppressed weak contributions implies a second, conceptually distinct constraint: The fact that weak-amplitude dominance arises from highly tuned UV coupling configurations translates into a conservative general lower limit on the PQ scale, f a > 4.9 × 10 4 GeV . These results provide the strongest accelerator-based constraints on axion-induced d ↔ s transitions and establish a robust lower bound on f a , complementary to astrophysical limits.