Searching for Long-Lived Dark Scalars with MAPP-1 and MAPP-2 at the HL-LHC

Abstract

Feebly interacting particles (FIPs) arise generically in dark-sector extensions of the Standard Model (SM). Among the simplest and most theoretically motivated such candidates is a dark scalar that mixes with the SM Higgs boson, acting as a mediator between the dark sector and the SM. For sufficiently small mixing, such a dark scalar manifests at collider experiments as a long-lived particle with a macroscopic decay length---a signature to which conventional general-purpose detectors have limited sensitivity. The MoEDAL-MAPP experiment is a dedicated search facility at the Large Hadron Collider (LHC) targeting long-lived FIPs as avatars of physics beyond the SM. We present $95\%$ confidence-level background-free sensitivity projections for the MAPP-1 and MAPP-2 detectors at the High-Luminosity LHC to Higgs-portal dark scalars produced via rare $B$-meson decays and exotic Higgs boson decays. Our results establish the proposed MAPP-2 detector as a complementary probe of feebly-coupled dark scalars, with sensitivity to lifetimes beyond the reach of existing collider searches.