Searching for long-lived dark scalars at the FCC-ee
This paper investigates the search for long-lived dark scalars from exotic Higgs boson decays at the Future Circular Collider in its $e^+e^-$ stage, FCC-ee, considering an integrated luminosity of 10.8 $\text{ab}^{-1}$ collected during the ZH run at a center-of-mass energy $\sqrt{s}=240$ GeV. The work considers $Zh$ events where the $Z$ boson decays leptonically and the Higgs boson $h$ decays into two long-lived dark scalars $s$ which further decay into bottom anti-bottom quark pairs. The analysis is performed using a parametrized simulation of the IDEA detector concept and targets dark scalar decays in the tracking volume, resulting in multiple displaced vertices in the final state. The sensitivity towards long-lived dark scalars at FCC-ee is estimated using an event selection requiring two opposite-charge, same-flavor leptons compatible with the $Z$ boson, and at least two displaced vertices in the final state. The selection is seen to efficiently remove the Standard Model background, while retaining sensitivity for dark scalar masses between $m_s=20$ GeV and $m_s=60$ GeV and mean proper lifetimes $c\tau$ between approximately 10 mm and 10 m The results show that the search strategy has potential to probe Higgs to dark scalar branching ratios as low as $10^{-4}$ for a mean proper lifetime $c\tau\approx 1$ m. The results provide the first sensitivity estimate for exotic Higgs decays at FCC-ee with the IDEA detector concept, using the common FCC framework.