Constraints on light top squark from B0-B0 mixing

We discuss the constraints on the mass of the lighter top squark from B0-B0 mixing in the minimal supersymmetric standard model. A light top squark whose mass is less than half of the Z0-boson mass has not yet been excluded from direct search experiments at LEP. However, the existence of the light top squark may exceedingly enhance B0-B0 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 B0-B0 mixing becomes the same order of magnitude as the standard W-boson contribution. Taking into account the experimental results for B0-B0 and K0-K0 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.