Extending LHC Coverage to Light Pseudoscalar Mediators and Coy Dark
  Sectors
  Many dark matter models involving weakly interacting massive particles
(WIMPs) feature new, relatively light pseudoscalars that mediate dark matter
pair annihilation into Standard Model fermions. In particular, simple models of
this type can explain the gamma ray excess originating in the Galactic Center
as observed by the Fermi Large Area Telescope. In many cases the pseudoscalar's
branching ratio into WIMPs is suppressed, making these states challenging to
detect at colliders through standard dark matter searches. Here, we study the
prospects for observing these light mediator states at the LHC without
exploiting missing energy techniques. While existing searches effectively probe
pseudoscalars with masses between 5 - 14 GeV and above 90 GeV, the LHC reach
can be extended to cover much of the interesting parameter space in the
intermediate 20 - 80 GeV mass range in which the mediator can have appreciable
Yukawa-like couplings to Standard Model fermions but would have escaped
detection by LEP and other experiments. Models explaining the Galactic Center
excess via a light pseudoscalar mediator can give rise to a promising signal in
this regime through the associated production of the mediator with bottom
quarks while satisfying all other existing constraints. We perform an analysis
of the backgrounds and trigger efficiencies, detailing the cuts that can be
used to extract the signal. A significant portion of the otherwise
unconstrained parameter space of these models can be conclusively tested at the
13 TeV LHC with 100 fb$^{-1}$, and we encourage the ATLAS and CMS
collaborations to extend their existing searches to this mass range.
