Bounds on Scalar Leptoquarks from Z Physics

We analyse the constraints on scalar leptoquarks coming from radiative corrections to $Z$ physics. We perform a global fitting to the LEP data including the contributions of the most general effective Lagrangian for scalar leptoquarks, which exhibits the $SU(2{)}_L\times U(1{)}_Y$ gauge invariance. We show that the bounds on leptoquarks that couple to the top quark are much stronger than the ones obtained from low energy experiments.