Hidden in Plain Sight: Long-Lived Scalars at the FCC-ee

Long-lived particles (LLPs) provide a powerful probe of new physics scenarios characterized by very weak couplings and macroscopic decay lengths. In this talk, I explore the prospects for discovering a light, long-lived scalar produced in rare B-meson decays at FCC-ee. Taking advantage of the enormous sample of Z bosons expected at the Z-pole run, I evaluate the sensitivity of the proposed IDEA (Innovative Detector for an Electron-Positron Accelerator) detector for several representative benchmark scenarios. Different LLP decay channels are analyzed, with particular attention to long-lived Standard Model backgrounds and their impact on the search. I also discuss how dedicated LLP detector concepts could extend the reach of FCC-ee toward even longer lifetimes. The results demonstrate the unique potential of a future high-luminosity lepton collider to uncover light, weakly coupled scalars that may otherwise remain hidden in plain sight.