Constraints on Light Top Squark from $B^0$-$\bar{B}^0$ mixing

Abstract

We discuss the constraints on the mass of the lighter top squark from \bbbar\ mixing in the minimal supersymmetric standard model. A light top squark whose mass is less than half of the $Z^0$-boson mass has not yet been excluded from direct search experiments at LEP. However, the existence of the light top squark may exceedingly enhance \bbbar\ mixing, owing to the box diagrams exchanging the charginos and the up-type squarks. We show that for a sizable region of parameter space the light top squark contribution to \bbbar\ mixing becomes the same order of magnitude as the standard $W$-boson contribution. Taking into account the experimental results for \bbbar\ and \kkbar\ mixings, the existence of the light top squark is excluded in an appreciable region of the parameter space which LEP experiments have not ruled out.