A new approach to long-lived particle detection at hadron colliders: the $\textsf{DELIGHT-SHIELD}$ concept

Abstract: We propose a fundamental shift in the search for beyond the Standard Model long-lived particles (LLPs) at high-luminosity hadron colliders by prioritizing physical background suppression over traditional inner tracking. We introduce $\textsf{DELIGHT-SHIELD}$, a dedicated detector design for a 100 TeV Future Circular Collider at a dedicated interaction point for LLP searches. By replacing the inner parts of the detector with a multi-layered composite shield, followed by tracking volumes, we estimate a suppression of Standard Model hadronic and electromagnetic backgrounds by up to seven orders of magnitude analytically. Full Geant4 simulations validate the effectiveness of this design. Although the achieved suppression is somewhat lower than the analytical estimate, primarily due to secondary particle production within the shield, the residual background remains at a level that is manageable for LLP analyses. It can be further mitigated by applying energy thresholds, as well as vertexing and timing cuts in the downstream detector. Benchmarking against dark scalar model, we show that this shielding based detector concept achieves sensitivity to branching ratios as low as $\mathcal{O}(10^{-9})$ for $h\rightarrowφφ$ process under zero background condition $-$ outperforming general-purpose detector baselines. This strategy not only expands the discovery reach for neutral LLPs but also provides a rigorous experimental handle to distinguish new physics from Standard Model punch-through backgrounds. We further discuss a phased implementation at the High-Luminosity LHC as a critical testbed for this novel detection concept.