LAYCAST: LAYered CAvern Surface Tracker at future electron-positron colliders

We propose a detector concept, LAYered CAvern Surface Tracker (LAYCAST), to be installed on the ceiling and the wall of the cavern hosting the main experiment of future electron-positron colliders such as CEPC and FCC-ee. With detailed and realistic considerations of the design of such a new experiment, the proposed detector is dedicated to extending the sensitivity reach of the main detector to various theoretical scenarios of long-lived particles (LLPs). We study carefully four such scenarios involving a light scalar boson X, the heavy neutral lepton N, the lightest neutralino χ̃⁰₁ in the R-parity-violating supersymmetry, and the axion-like particle a. Long-lived light scalar bosons are considered to be produced from the Standard-Model (SM) Higgs boson's decay (h → X X) at the center-of-mass energy √s = 240 GeV, while the other three types of LLPs are produced either from Z-boson decays (viz. Z → ν\, N, ~χ̃⁰₁\, χ̃⁰₁) or direct scattering process (e⁻ e⁺ → ~γ\, a) at √s = 91.2 GeV, where γ and ν denote the SM photon and neutrino, respectively. With Monte-Carlo simulations, we derive the sensitivities of the proposed experiment to these LLPs and the corresponding signal-event numbers. Our findings show that LAYCAST can probe large new parameter space beyond both current bounds and the expected reach of the main experiments at CEPC and FCC-ee. Comparison with existing works in similar directions is also made.