Tagging a jet from a dark sector with jet substructures at colliders

The phenomenology of the dark sector is complicated if the dark sector is charged under a confined hidden gauge group. In such a kind of model, a dark parton produced at a high-energy collider showers and hadronizes to a cluster of dark mesons. Dark mesons then decay to visible particles and produce a jetlike signal, which is called a "dark jet" in this work. Collider signal of a dark jet depend on the property of dark mesons. For example, a finite lifetime of a dark meson would provide a displaced vertex or displaced track; thus, one can use these displaced objects to tag a dark jet. However, if the lifetime of a dark meson is collider negligible (too short to manifest a displaced vertex), it would be difficult to distinguish a dark jet from standard model 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 the dark sector search at a collider.