We calculate the relic density of stau at the beginning of the Big-Bang Nucleosynthesis ( BBN ) era in the coannihilation scenario of minimal supersymmetric standard model ( MSSM ) .
In this scenario , stau can be long-lived and form bound states with nuclei .
We put constraints on the parameter space of MSSM by connecting the calculation of the relic density of stau to the observation of the light elements abundance , which strongly depends on the relic density of stau .
Consistency between the theoretical prediction and the observational result , both of the dark matter abundance and the light elements abundance , requires the mass difference between the lighter stau and the lightest neutralino to be around 100MeV , the stau mass to be 300 – 400 GeV , and the mixing angle of the left and right-handed staus to be \sin \theta _ { \tau } = ( 0.65 \textrm { - - } 1 ) .