2-LOOP CORRECTIONS TO THE HEAVY TOP LIMIT IN THE RHO-PARAMETER

The top quark contribution to two-loop, purely electroweak radiative c orrections is investigated in the case of the rho parameter, extracted from the ratio of neutral to charged current amplitudes in low-energy neutrino-lepton scattering. It is shown that the order O(G(mu)(2)m(t) (2)m(Z)(2) ) contribution is numerically comparable and shares the sam e sign with the leading O(G(mu)(2)m(t)(4)) term, for realistic values of the top and Higgs masses. The result raises doubts on the reliabili ty of the heavy top approximation in the two-loop contributions to ele ctroweak observables. A naive extrapolation of this result to LEP obse rvables would suggest an overall two-loop electroweak contribution clo se to the ultimate experimental sensitivity.