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

Abstract

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 $\tildeχ^0_1$ 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 ($h \to X X$) at the center-of-mass energy $\sqrt{s} =$ 240 GeV, while the other three types of LLPs are produced either from $Z$-boson decays (viz. $Z \to ν\, N, ~\tildeχ^0_1\, \tildeχ^0_1 $) or direct scattering process ($ e^- e^+ \to ~γ\, a$) at $\sqrt{s} =$ 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. We also provide a dedicated estimate of a potentially important SM background from long-lived neutral kaons in hadronic $Z$ decays, and show that it is strongly suppressed by the combined requirements of the main detector and LAYCAST. 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.

Figures (24)

• Figure 1: The front and side cross-section views of the proposed experiment.
• Figure 2: The ALP production process, $e^- e^+ \to \gamma \, a$, at electron-positron colliders Tian:2022rsi.
• Figure 3: The average decay probability of the light scalar $X$ as functions of $c\tau_X$, for LAYCAST and the CEPC/FCC-ee's main detector and for $m_X=0.5$ GeV and 10 GeV.
• Figure 4: Sensitivity reaches of LAYCAST compared to those of FD1, FD3, and FD6 investigated in Ref. Wang:2019xvx, shown in the Br($h \rightarrow XX$) vs. $c\tau_X$ plane for $m_X = 0.5$ GeV (upper panel) and 10 GeV (lower panel). Together shown is also the current limit on the Higgs invisible decay branching ratio at 13% ATLAS:2022vkf, as a horizontal gray dashed line.
• Figure 5: Sensitivity reaches of LAYCAST compared to the CEPC/FCC-ee's main detector and the LHC's far detectors CODEX-b and MATHUSLA Wang:2019xvx for $m_X = 0.5$ GeV (upper panel) and 10 GeV (lower panel). The present bound on the SM Higgs invisible decay branching ratio 13% ATLAS:2022vkf is plotted as a horizontal gray dashed curve.