Long-lived particles from exotic Higgs decays at the FCC-ee - detector comparison and additional Z decay modes

This note presents work expanding upon the first sensitivity analysis for long-lived scalar particles from exotic Higgs boson decays at the electron-positron stage of the Future Circular Collider (FCC-ee) within the FCCAnalyses framework. The original analysis studies the production of a Higgs boson in association with a Z boson in electron-positron collisions at a center-of-mass energy of 240 GeV, where the Higgs boson decays into two long-lived scalars, each decaying into a bottom anti-bottom quark pair, and the Z boson decays leptonically. Signal points with scalar masses of ms = 20 and 60 GeV are considered, with mixing angles of sin(θ) = 1e−5, 1e−6, and 1e−7. The original analysis uses a displaced vertex-based event selection, with displaced vertices reconstructed using the FCCAnalyses version of the secondary vertex finding algorithm, LCFIPlus.

We expand upon the original analysis by comparing results obtained using the IDEA and CLD detector designs, providing an assessment of displaced tracking performance with each tracker design. It is found that, particularly for the ≈ 0.3 and 8 m lifetime signal points, sensitivity is lost using the CLD detector, as tracks from the scalar particle decay products are not formed due to a lack of hits or acceptance within the CLD tracker. Separately, analysis strategies incorporating hadronic and invisible Z decays are presented, however, not yet in a final state. To identify the Z boson in each signal, a missing energy cut is used for invisible decays, and a jet energy cut is used for hadronic decays. The original displaced vertex selection is still applied to identify the long-lived scalars. The preliminary results indicate that incorporating additional Z decay modes could greatly expand the signal sensitivity, however, the strategy particularly incorporating hadronic Z decays needs to still be refined to improve background rejection.