  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.
