  The ultraviolet structure of $s$-channel mediator dark matter simplified
models at hadron colliders is considered. In terms of commonly studied
$s$-channel mediator simplified models it is argued that at arbitrarily high
energies the perturbative description of dark matter production in high energy
scattering at hadron colliders will break down in a number of cases. This is
analogous to the well documented breakdown of an EFT description of dark matter
collider production. With this in mind, to diagnose whether or not the use of
simplified models at the LHC is valid, perturbative unitarity of the scattering
amplitude in the processes relevant to LHC dark matter searches is studied. The
results are as one would expect: at the LHC and future proton colliders the
simplified model descriptions of dark matter production are in general valid.
As a result of the general discussion, a simple new class of previously
unconsidered `Fermiophobic Scalar' simplified models is proposed, in which a
scalar mediator couples to electroweak vector bosons. The Fermiophobic
simplified model is well motivated and exhibits interesting collider and direct
detection phenomenology.
