Gauge-invariant formulation of theS,T, andUparameters

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.