Freeze-In dark matter with displaced signatures at colliders

Dark matter, X, may be generated by new physics at the TeV scale during an early matter-dominated (MD) era that ends at temperature T_R ≤ TeV. Compared to the conventional radiation-dominated (RD) results, yields from both Freeze-Out and Freeze-In processes are greatly suppressed by dilution from entropy production, making Freeze-Out less plausible while allowing successful Freeze-In with a much larger coupling strength. Freeze-In is typically dominated by the decay of a particle B of the thermal bath, B → X. For a large fraction of the relevant cosmological parameter space, the decay rate required to produce the observed dark matter abundance leads to displaced signals at LHC and future colliders, for any m_X in the range keV < m_X < m_B and for values of m_B accessible to these colliders. This result applies whether the early MD era arises after conventional inflation, when T_R is the usual reheat temperature, or is a generic MD era with an alternative origin. In the former case, if m_X is sufficiently large to be measured from kinematics, the reheat temperature T_R can be extracted. Our result is independent of the particular particle physics implementation of B → X, and can occur via any operator of dimension less than 8 (4) for a post-inflation (general MD) cosmology. An interesting example is provided by DFS axion theories with TeV-scale supersymmetry and axino dark matter of mass GeV to TeV, which is typically overproduced in a conventional RD cosmology. If B is the higgsino, , Higgs, W and Z particles appear at the displaced decays, → a, Z ã and ^± → W^± ã. The scale of axion physics, f, is predicted to be in the range (3×10⁸ - 10¹²) GeV and, over much of this range, can be extracted from the decay length.