Speaker
Description
The two-Higgs-doublet model with a light pseudoscalar singlet (2HDM + a) provides a wellmotivated dark matter (DM) framework, but hadron colliders face severe background ceilings
in the light-scalar (mh1) and light-mediator (ma) windows, while non-relativistic directdetection bounds are momentum-suppressed. To address these experimental bottlenecks, this study investigates the potential of future circular e+e− Higgs factories—specifically the CEPC and FCC-ee operating at √s = 240 GeV—to chart this elusive parameter space using the model-independent recoil mass method. Automating Monte Carlo simulation, we map the multi-body signal e+e− → Zh1 → µ+µ− + 4χ over a comprehensive two-dimensional grid spanning mh1 =125 GeV and ma ∈ [20, 80] GeV. Discrete DM mass targets (mχ = 5, 10 GeV) demonstrate threshold-driven phase-space enhancements immediately above ma > 2mχ. We show that future lepton colliders can achieve definitive discovery metrics well above the 5σ threshold that remain highly resilient under a conservative 10% background systematic scaling.
