It is shown that the bosonic contributions to the S, T, and U parameters, defined in terms of conventional self-energies, are gauge dependent in the standard model (SM). Moreover, T and U are divergent unless a constraint is imposed among the gauge parameters. Implications of this result for renormalization schemes of the SM are discussed. A gauge-invariant formulation of S, T, and U is proposed in the pinch-technique framework. The modified S, T, and U parameters provide a gauge-invariant parametrization of leading electroweak radiative corrections in the SM and some of its extensions.
Degrassi, G., Kniehl, B.a., Sirlin, A. (1993). GAUGE-INVARIANT FORMULATION OF THE S, T, AND U PARAMETERS. PHYSICAL REVIEW D, 48(9), R3963-R3966.