Implications of [formula omitted] for supersymmetry searches and model-building

Abstract

Assuming that the actual values of M t at FNAL and of Γ(Z → b b ̄ )/Γ(Z → hadrons) at LEP are within their current 1σ reported ranges, we present a No-Lose Theorem for superpartner searches at LEP II and an upgraded Tevatron. We impose only two theoretical assumptions: the Lagrangian is that of the Minimal Supersymmetric Standard Model (MSSM) with arbitrary soft-breaking terms, and all couplings remain perturbative up to scales ∼ 10 16 GeV; there are no assumptions about the soft supersymmetry breaking parameters, proton decay, cosmology, etc. In particular, if the LEP and FNAL values hold up and supersymmetry is responsible for the discrepancy with the Standard Model prediction of Γ(Z → b b ̄ ) , then we must have charginos and/or top squarks observable at the upgraded machines (for LEP the superpartner threshold is below √ s = 140 GeV). Furthermore, little deviation from the Standard Model is predicted within “super-unified” supersymmetry, so these models predict that the discrepancy between experiment and the Standard Model prediction for Γ(Z → b b ̄ ) will fade with time. Finally, it appears to be extremely difficult to find any unified MSSM model, regardless of the form of soft supersymmetry breaking, that can explain Γ(Z → b b ̄ ) for large tan β; in particular, no model with t b τ Yukawa coupling unification appears to be consistent with the experiments.