Searching for long-lived dark scalars at the FCC-ee

TL;DR

This study explores the potential of the FCC-ee collider to detect long-lived dark scalars from exotic Higgs decays, demonstrating sensitivity to very rare decay modes with displaced vertices in a simulated detector environment.

Contribution

First sensitivity estimate for detecting long-lived dark scalars from Higgs decays at FCC-ee using the IDEA detector simulation.

Findings

Can probe Higgs to dark scalar branching ratios down to 10^-4.

Effective background suppression with displaced vertex selection.

Sensitive to dark scalar masses between 20 and 60 GeV with lifetimes from 10 mm to 10 m.

Abstract

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.