Higgs Boson Spookiness: Probing Quantum Nonlocality with Spacetime-Resolved H→τ^+τ^- Decays

We demonstrate that a future precision ee Higgs factory would be able to perform a spacetime-resolved test of quantum nonlocality in Higgs boson decays. In simulated ee→ ZH → (μμ)(ττ) events at sqrt{s}=240 GeV, we reconstruct τ lepton decay vertices and measure spin correlations as a function of the spacetime interval between the two τ decays. Such a measurement would be able to test Bell-inequality-violating correlations for spacelike-separated decays, enabling direct exclusion of superluminal, finite-speed entanglement signaling theories. With 0.75 ab^-1 of integrated luminosity, entanglement signal propagation speeds below approx2c can be excluded at 95\% CL. Signals establishing any spin correlation can be excluded for speeds below approx9c. This constitutes the first proposed spacetime-resolved measurement of electroweak quantum entanglement at a particle collider and demonstrates a unique capability of future Higgs factories.