Demystifying Freeze-In Dark Matter at the LHC

Freeze-in mechanism provides robust dark matter production in the early universe. Due to its feeble interactions, freeze-in dark matter leaves signals at colliders which are often involved with long lived particle decays and consequent displaced vertices (DV). In this paper, we develop a method to read off mass spectrum of particles being involved in the DV events at the LHC. We demonstrate that our method neatly works under a limited statistics, detector resolution and smearing effects. The signature of DV at the LHC can come from either highly suppressed phase-space or a feeble coupling of particle decay processes. By measuring invisible particle mass spectrum, one can discriminate these two cases and thus extract information of dominant freeze-in processes in the early universe at the LHC.