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.