Di-decay signature of new physics particles at intensity frontier experiments

We present a discovery-era strategy for Intensity Frontier experiments based on di-decay'' events, in which feebly interacting particles (FIPs) are produced in pairs and then both decay inside the detector. This signature may exist in broad classes of models with FIPs decaying in the same way as minimal portal’’ particles. The two displaced vertices allow reconstruction of the pair invariant mass and hence reveal the production channel, information normally inaccessible at these facilities. This, in turn, lets one discriminate between FIP models that share identical decay signatures. As a case study, we consider the model of Higgs-like scalars and show that SHiP, Belle II, and the Downstream@LHCb can each cover wide domains of viable parameter space using di-decays.