Rethinking search signatures: Hadronic decays of GeV-scale feebly coupled particles

Abstract

Experimental searches for GeV-scale feebly interacting particles commonly target the simplest decay signatures with two charged particles, including hadrons. Hadronic modes often dominate the inclusive decay rate, and their simulation relies extensively on \textsc{Pythia}. Its fragmentation model, calibrated mainly on LEP data, is unreliable in this mass range: it populates symmetry-forbidden states and misidentifies the dominant allowed channels. We construct a simple hadronization model tuned to electromagnetic scattering data and constrained by conservation laws and available exclusive decay calculations, while retaining a common prescription applicable to different particle models. Considering the SHiP experiment as an example, we show that our model shifts the dominant search signature after event selection from two-particle to multiparticle decays, including mixed charged-neutral states. We provide the model in a form that can be readily integrated into \textsc{Pythia}-based experimental simulation frameworks.