Triple Z′ signatures at Z factories

We discuss triple Z′ boson signatures via the decay chain of Z→Z′ϕ→Z′Z′Z′, with a new light scalar ϕ, at future Z factories such as the Circular Electron-Positron Collider and Future Circular Collider. These new bosons ϕ and Z′ naturally appear in models with a new U(1) gauge symmetry which is spontaneously broken and introduced in various new physics scenarios. The branching ratio of Z→Z′ϕ→Z′Z′Z′ can be larger than 10−12, which gives O(1) events at Tera-Z experiments, when a product of gX (new gauge coupling) and ζ (Z−Z′ mixing) is larger than around 10−6. We find that the search for Z→Z′Z′Z′ can significantly improve the current bound on a kinetic mixing parameter ε in the dark photon case, where eε≳O(10−5) with gX=O(1) can be explored at Tera-Z experiments. We also show that a sufficiently large number of events with multilepton plus hadronic jets can be obtained in benchmark points, which cannot be realized by the usual decay of Z in the standard model. Published by the American Physical Society 2025