Search for new long-lived hadronically-decaying particles produced in association with a Z boson with the ATLAS detector at the LHC

The standard model fails to fully explain several observations about the universe, one of the prominent ones is the existence of dark matter. Many different models have been proposed for dark matter candidates, but it is possible that the dark matter itself will remain elusive. Dark sector models contain both dark matter and a new force which acts upon it. The force-carrying boson of this dark sector may be more easily detected than dark matter itself. These models contain a number of free parameters, some combinations of which result in the dark sector particle being long-lived but unstable, resulting in a unique experimental signature. A search is presented for the production of an uncharged long-lived particle in association with a standard model Z boson. The search is based on 36.01 fb −1 of proton-proton collision data at √s = 13 TeV recorded in 2015-2016 with the ATLAS detector at the Large Hadron Collider. Data are analyzed in a dilepton + jets region which is characterized by two high transverse momentum electrons or muons and at least one jet with a very large fraction of its energy deposited in the hadronic calorimeter. Background rates for such jets are estimated using a single lepton + jets region consistent with a lepton from the decay of a standard model W boson. No significant excess is observed and 95% confidence level upper limits are derived on the cross section times the branching ratio as a function of the particle’s proper lifetime.