Probing Msugra Models at Linear Colliders A

A feasibility study of a 500-GeV linear collider is presented for mSUGRA models in co-annihilation region. We find an active mask is critical to suppress e + e − → e + e − τ + τ − events to probe the models. The grand unification of the three gauge coupling constants arise naturally in minimal SUGRA (mSUGRA) models with universal soft breaking parameters (m 1/2 , m 0 , A 0 , tan β, sign of µ). 1,2 The experimental data to provide the most significant constraints are the Higgs mass bound, the b → sγ branching ratio, and (possibly) the muon magnetic moment anomaly (a µ) along with the amount of relic density of the lightest neutralino (˜ χ 0 1) as cold dark matter. These constraints produce a lower bound of m 1/2 > ∼ 300 GeV (Higgs mass and b → sγ) and an upper bound of m 1/2 < ∼ 900 GeV (a µ), making the SUSY spectrum well accessible to the LHC. 3 The relic density constraints limit the most part of the allowed parameter space to narrow band where˜τ 1-˜ χ 0 1 co-annihilation occurs. 4,5 See, for example, Fig. 1. 6 Further, at large tan β (e.g., 40), M ˜ τ 1 becomes the next to lightest SUSY particle and all other SUSY particles would be too heavy to produce in pair at e + e − linear colliders (LCs). The only accessible production at 500-GeV LCs could be˜χ 0 2 ˜ χ 0 1 , ˜ τ + 1 ˜ τ − 1 and˜χ 0 1 ˜ χ 0 1 , where M ˜ χ ± 1 > 250 GeV and ∆M ≡ M ˜ τ 1 − M ˜ χ 0 1 = 5-30 GeV. Furthermore, the˜χ 0 2 → ˜ τ 1 τ decay is dominant. This guides us to the conclusion that one of the most critical experimental sigantures is τ + τ − plus missing energy. We present a feasibility study of a 500-GeV LC for the signature of two hadronically-decaying τ lepton (τ h) plus missing energy.