Large effects onBsBsmixing by vectorlike quarks

We calculate the contributions of the vectorlike quark model to $B_s{B}_s$ mixing, taking into account the constraints from the decay $\overrightarrow{B}{X}_s\gamma .$ In this model the neutral bosons mediate flavor-changing interactions at the tree level. However, $B_s{B}_s$ mixing is dominated by contributions from the box diagrams with the top quark and the extra up-type quark. In sizable ranges of the model parameters, the mixing parameter $x_s$ is much different from the standard model prediction.