$B \to K \nu \nu$ as a search for the QCD axion

by Diego Guadagnoli (LAPTh Annecy)

Wednesday 10 Dec 2025, 12:30 → 13:30 Europe/Zurich

4/2-037 - TH meeting room (CERN)

We present a model-independent method to reinterpret Belle II results using only public data, by analytically reconstructing the mapping between true and reconstructed kinematics in the Inclusive Tagging Analysis. This enables rare-decay measurements to probe light invisible particles -- such as the QCD axion -- without relying on internal simulations. Applying the method to B⁺ → K⁺ νν̄ yields the strongest limit on B⁺ → K⁺ a, improving previous bounds by about a factor of nine and constraining the axion’s flavour-changing b–s coupling. The approach also establishes B⁺ → K⁺ νν̄ as a dual probe --of short-distance new physics in B⁺ → K⁺ νν̄, and of light invisible final states -- with the two effects working independently to an excellent approximation.