Collider Stress-Test of a Low-Mass Scalar Prediction at m_S ≃ 58.1 GeV:windows in Atlas/CMS, Analytic Recast, and Areproducible Pantheon+ Pipeline

We provide a falsifiable stress test for a light, narrow scalar statewith a fixed target mass mS ≃58.1 GeV. The collider component is for-mulated exclusively in terms of experimentally meaningful observables,upper limits on σ(pp→S) ×BR(S →X), and an analytic recast intoportal parameters such as sin2 θ×BR. We document why a genuinenon-observation can persist at low masses (bounded analysis windows,trigger and bandwidth bottlenecks) and we spell out minimal, concretearchive-analysis requests to ATLAS/CMS. In parallel, we provide afully executable Pantheon+ likelihood pipeline reporting χ2min togetherwith AIC/BIC for ΛCDM (pipeline validation) and for a simple exten-sion. The derivations used throughout are included in the manuscript appendices.