Speaker
Description
In this talk, we discuss triple $Z'$ boson signals via the decay chain of $Z \to Z' \phi \to Z' Z' Z'$, with a new light scalar $\phi$, at future Z factories such as CEPC and FCC-ee. These new bosons $\phi$ 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 $Z \to Z' \phi \to Z' Z' Z'$ can be larger than $10^{-12}$, which gives $O(1)$ events at Tera-Z experiments, when a product of $g_X^{}$ (new gauge coupling) and $\zeta$ ($Z$-$Z'$ mixing) is larger than around $10^{-6}$. We find that the search for $Z \to Z'Z'Z'$ can significantly improve the current bound on a kinetic mixing parameter $\epsilon$ in the dark photon case, where $e \epsilon$ larger than $\mathcal{O}(10^{-5})$ with $g_X^{}={\cal O}(1)$ can be explored at Tera-Z experiments.
