Constraints on a Light Leptophobic Mediator from LEP Data

TL;DR

This paper uses LEP collider data to set new constraints on a light leptophobic mediator in dark matter models, especially in regions poorly tested by LHC, by recasting existing searches and designing improved analysis cuts.

Contribution

It provides novel bounds on light leptophobic mediators by reanalyzing LEP data with optimized cuts, extending the parameter space coverage beyond previous studies.

Findings

Bounds on mediator couplings for m_χ < 20 GeV from Z-pole data.

Constraints on heavier DSP with m_R < 70 GeV from 183 GeV data.

Potential to explore new parameter regions with reanalyzed LEP data.

Abstract

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 $\chi$. 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 $2 m_\chi > m_R$, which are poorly constrained by LHC data. We recast published searches by the ALEPH collaboration. For $m_\chi < 20$ GeV the best bounds result from an analysis at $\sqrt{s} \simeq M_Z$ 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 $\sqrt{s} = 183$ GeV. Unfortunately published searches using four jet final states at $\sqrt{s} \simeq M_Z$ use only a small fraction of the total data sample. Moreover, all published searches for di-jet plus missing energy final states at $\sqrt{s} \geq 130$ 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 $\sqrt{s}\simeq M_Z$, can explore new regions of parameter space.