Constraints on a light leptophobic mediator from LEP data

A bstract We apply data taken at the e + e − collider LEP in the 1990’s at center-of-mass energy up to 209 GeV to constrain Dark Matter models with a light leptophobic spin−1 mediator R . We assume that the dark sector particle (DSP) is a spin−1/2 fermion χ . This scenario is well studied in the context of LHC searches for mediator mass from 100 GeV to several TeV. Emission of the mediator off a quark or antiquark at LEP gives rise to di-jet plus missing energy and 4−jet signatures, which we use to limit the relevant couplings. We focus on scenarios with 2 m χ > m R , which are poorly constrained by LHC data. We recast published searches by the ALEPH collaboration. For m χ ≲ 20 GeV the best bounds result from an analysis at s ≃ M Z of di-jet plus missing energy events. For heavier DSP but m R ≲ 70 GeV meaningful bounds can be derived from a four jet analysis at s = 183 GeV. Unfortunately published searches using four jet final states at s ≃ M Z use only a small fraction of the total data sample. Moreover, all published searches for di-jet plus missing energy final states at s ≥ 130 GeV have poor efficiency for our model; we therefore design new cuts that combine good background rejection with higher efficiency. Re-analyzing the higher energy data using our new cuts, and an analysis of the complete four jet data sample taken at s ≃ M Z , can explore new regions of parameter space.