Uncovering tau leptons-enriched semi-visible jets at the LHC.

This Letter proposes a new signature for confining dark sectors at the Large Hadron Collider. Under the assumption of a QCD-like hidden sector, hadronic jets containing stable dark bound states could manifest in proton-proton collisions. We present a simplified model with a Z' boson yielding the production of jets made up of dark bound states and subsequently leading to the decays of those that are unstable to τ leptons and Standard Model quarks. The resulting signature is characterised by non-isolated τ lepton pairs inside semi-visible jets. We estimate the constraints on our model from existing CMS and ATLAS analyses. We propose a set of variables that leverage the leptonic content of the jet and exploit them in a supervised jet tagger to enhance the signal-to-background separation. Furthermore, we discuss the performance and limitations of current triggers for accessing sub-TeV Z' masses, as well as possible strategies that can be adopted by experiments to access such low mass regions. We estimate that with the currently available triggers, a high mass search can claim a 5σ discovery (exclusion) of the Z' boson with a mass up to 4.5 TeV (5.5 TeV) with the full Run 2 data of the LHC when the fraction of unstable dark hadrons decaying to τ lepton pairs is around 50%, and with a coupling of the Z' to right-handed up-type quarks of 0.25. Furthermore, we show that, with new trigger strategies for Run 3, it may be possible to access Z' masses down to 700 GeV, for which the event topology is still composed of two resolved semi-visible jets.