  This paper proposes a new search program for dark sector parton showers at
the Large Hadron Collider (LHC). These signatures arise in theories
characterized by strong dynamics in a hidden sector, such as Hidden Valley
models. A dark parton shower can be composed of both invisible dark matter
particles as well as dark sector states that decay to Standard Model particles
via a portal. The focus here is on the specific case of 'semi-visible jets,'
jet-like collider objects where the visible states in the shower are Standard
Model hadrons. We present a Simplified Model-like parametrization for the LHC
observables and propose targeted search strategies for regions of parameter
space that are not covered by existing analyses. Following the 'mono-$X$'
literature, the portal is modeled using either an effective field theoretic
contact operator approach or with one of two ultraviolet completions;
sensitivity projections are provided for all three cases. We additionally
highlight that the LHC has a unique advantage over direct detection experiments
in the search for this class of dark matter theories.
