Year: 2026
Type: article
Abstract: In a spontaneously broken dark U (1) X gauge sector with a dark photon Z ′ and a dark scalar ϕ , the rare cascade Z → Z ′ ϕ → 3 Z ′ yields six muons in three resonances whose masses lock together: each dimuon at the dark-photon mass 𝑚 𝑍 ′ , each dark-photon pair at the scalar mass m ϕ , and all six at the Z -boson mass m Z , so the final state tags itself against nonresonant backgrounds. We treat the dark-photon decay length as a continuous discriminant rather than a prompt-or-displaced regime, in a single analysis spanning decay lengths from microns to meters. From a parametric fast-detector projection of the Tera- Z run of a Circular Electron–Positron Collider (CEPC) with 𝑁 𝑍 = 4 . 4 6 × 1 0 1 2 Z bosons, backgrounds from a simulation-calibrated control-region factorization, we set median expected 95% confidence-level upper limits on the visible six-muon cascade branching product ℬ 6 𝜇 over the dark-photon mass–decay-length plane ( 𝑚 𝑍 ′ , 𝑐 𝜏 ) , reaching ℬ 6 𝜇 ≃ 6 . 7 × 1 0 − 1 3 near the most sensitive lifetime. Recast as a production coupling, the reach on g X ζ , the dark gauge coupling times Z – Z ′ mixing, varies by only a factor of four across an eighty-fold range in 𝑚 𝑍 ′ . The sensitivity is bounded not by lifetime from below but by selection geometry: above 𝑚 𝑍 ′ ≃ 1 0 GeV a sensitivity trough at the 4 mm inner edge of the displaced-vertex fiducial region would split a fixed-rate exclusion contour into two disconnected islands, a feature of the adopted prompt definition and inner radial cut, normally hidden when the two regimes are searched separately.
Source: Physics Letters B
Author: Renjie Wang
Institutions: Chinese Academy of Sciences, Institute of High Energy Physics
Language: English
Subfield: Nuclear and High Energy Physics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: diamond
Funder: Institute of High Energy Physics
OpenAlex ID: W7223931308