Search for Gauginos and Gauge Mediated SUSY Breaking Scenarios at LEP

Abstract

Data collected by the OPAL detector at LEP up to the highest energies of 209 GeV are analyzed to search for evidence of supersymmetric (SUSY) particle production. Chargino pair-production (e+e−→ χ 1 χ − 1 ) and neutralino associated production (e +e−→ χ0 1 χ 0 2 ) are considered in models where supersymmetry breaking is mediated by gravitational interactions from the hidden sector to the visible sector of the Standard Model and SUSY particles. R-parity conservation is assumed. The focus of the searches is on topologies arising when scalar fermions are heavy and the lightest neutralino is the lightest SUSY particle (LSP). Searches for topologies predicted by gauge-mediated SUSY breaking (GMSB) models are also discussed. In these models the LSP is the gravitino and the phenomenology is driven by the nature of the next-to-lightest SUSY particle (NLSP) which is either the lightest neutralino, stau or mass degenerate sleptons. As the NLSP decay length is effectively unconstrained, all possible values are considered and results independent of the NLSP lifetime are presented for all relevant final states including direct NLSP pair-production and NLSP production via cascade decays of heavier SUSY particles. None of the searches shows evidence for SUSY particle production. Cross-section limits are presented at the 95% confidence level both for direct NLSP production and, for the first time, also for cascade decays, providing the most general, almost model independent results. These are then interpreted in the framework of minimal theoretical models. Large areas of the accessible parameter space are excluded. In the super-gravity inspired Constrained MSSM (CMSSM) we derive the following limits for m0 > 500 GeV, A0 = 0 and ∆M > 5 GeV on the lightest gaugino masses: mχ± 1 > 101 GeV, mχ0 1 > 40 GeV and mχ0 2 > 78 GeV, independent of tanβ . In the minimal GMSB model, the NLSP masses are constrained to be mχ0 1 > 53.5 GeV, mτ1 > 87.4 GeV and m > 91.9 GeV in the neutralino, stau and slepton co-NLSP scenarios, respectively.