A New Probe for Long-Lived Particles at Higgs Factories: Displaced Photons in the Hadronic Calorimeter

Abstract: The search for dark matter and other photon-portal long-lived particles (LLPs) at electron-positron colliders often relies on the mono-photon signature. At future Higgs factories operating at the $Z$-pole, this approach faces a critical challenge: the irreducible background from $e^+e^- \to \nu\bar{\nu}\gamma$ becomes overwhelming. We propose a novel strategy that overcomes this limitation by searching for displaced photons from LLP decays within the barrel of the hadronic calorimeter. This signature exploits the architectural shielding of the detector to create a nearly background-free environment. Our analysis demonstrates exceptional sensitivity to LLPs with decay lengths from $\sim$1 to $10^6$ meters, improving upon conventional searches by up to one order of magnitude for benchmark photon-portal models.