Triple $Z'$ signatures at $Z$ factories

TL;DR

This paper explores the potential to detect triple $Z'$ boson signatures at future Z factories, proposing new decay channels involving a light scalar and assessing their observability and impact on dark photon parameter bounds.

Contribution

It introduces a novel decay chain involving $Z'$ and a light scalar at future Z factories, providing estimates of branching ratios and experimental sensitivities.

Findings

Branching ratio of $Z o Z' o Z' Z' Z'$ can exceed $10^{-12}$.

Future experiments can probe kinetic mixing parameter $oldsymbol{ extepsilon extgreater{} 10^{-6}}$.

Multi-lepton plus jet events from these decays can surpass Standard Model expectations.

Abstract

We discuss triple $Z'$ boson signatures 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 $\epsilon \gtrsim 10^{-6}$ with $g_X^{}={\cal O}(1)$ can be explored at Tera-Z experiments. We also show that a sufficiently large number of events with multi-lepton plus hadronic jets can be obtained in benchmark points, which cannot be realized by the usual decay of Z in the standard model.