  The phenomenology of dark sector is complicated if dark sector is charged
under a confined hidden gauge group. In such kind of model, a dark parton
produced at a high energy collider showers and hadronize to a cluster of dark
mesons. Dark mesons then decay to visible particles and produce a jet-like
signal, which is called "dark jet" in this work. Collider signal of dark jet
depends on the property of dark mesons. For example, a finite lifetime of dark
meson would provide displaced vertex or displaced track, thus one can use these
displaced objects to tag dark jet. However if the lifetime of dark meson is
collider-negligible (too short to manifest a displaced vertex), it would be
difficult to distinguish a dark jet from SM QCD jets. In this work we propose a
new tagging strategy to identify dark jets from QCD backgrounds. This strategy
is based on jet-substructure analysis. We study various jet-substructure
variables and find out variables with good discrimination ability. Our result
shows that by combining multiple jet-substructure variables, one could
distinguish dark jets from QCD background, and thus enhance the sensitivity of
dark sector search at collider.
