2-LOOP ELECTROWEAK CORRECTIONS TO THE RHO-PARAMETER BEYOND THE LEADING APPROXIMATION

We show that in the framework of the pinch technique the universal par t of the rho parameter can be meaningfully defined. beyond one loop. T he universal part so obtained satisfies the crucial requirements of ga uge independence, finiteness, and process independence, even when subl eading contributions of the top quark are included. The mechanism whic h enforces the aforementioned properties is explained in detail. and s everal subtle field theoretical issues are discussed. Explicit calcula tions of the subleading two-loop corrections of order O(G(mu)(2)m(t)(2 )M(z)(2)) an carried out in the context of an SU(2) model, with M(W) = M(Z), and various intermediate and final results are reported.