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, (h) over tilde, Higgs, W and Z particles appear at the displaced decays, (h) over tilde -> h (a) over tilde, Z (a) over tilde and (h) over tilde (+/-) W-+/-(a) over tilde. The scale of axion physics, f, is predicted to be in the range (3 x 10(8) - 10(12)) GeV and, over much of this range, can be extracted from the decay length.