Minimal SUGRA Model and Collider Signals

The SUSY signals in the dominant stau-neutralino coannihilation region at a 500(800) GeV linear collider are investigated. The region is consistent with the WMAP measurement of the cold dark matter relic density as well as all other current experimental bounds within the mSUGRA framework. The signals are characterized by an existence of very low-energy tau leptons in the final state due to small mass difference between stau_1 and chi_1 (5-15 GeV). We study the accuracy of the mass difference measurement with a 1^deg active mask to reduce a huge SM two-photon background.