GAUGE-INVARIANT TOP-QUARK FORM-FACTORS FROM E(-) EXPERIMENTS()E()

Motivated by the possibility of experimental determination of top quar k form factors in upcoming e(+)e(-) experiments, as recently discussed by Atwood and Soni, we show at the one loop level that the convention ally defined magnetic dipole moment (MDM) form factor of the top quark is gauge dependent, in the class of renormalizable (R(xi)) gauges. We explicitly calculate its gauge dependence, which, due to the massiven ess of the top quark, turns out to be numerically very sizable. We sho w how to use the S-matrix pinch technique in order to define gauge-ind ependent form factors. The form factors so obtained coincide with thos e conventionally defined only if the latter are calculated in the Feyn man guage (xi = 1). The real and imaginary parts of the gauge-independ ent MDM form factor are calculated and their dependence on the top qua rk and Higgs boson masses is discussed. Most noticeably, the dependenc e on the Higgs boson mass turns out to be stronger than that of the cr oss section as a whole.