Abstract
We propose a class of once-subtracted dispersion relations for the vacuum-polarization functions of massive fermions, in which the subtraction constants are determined explicitly from Ward identities. We show that in perturbation theory to O(α) and O( αα s ) this approach, the alternative dispersion relations proposed by Chang, Gaemers and van Neerven, and dimensional regularization all give the same contributions to electroweak observables such as Δϱ and Δr. The threshold behaviours of the subtraction integrands are, however, very different and the two dispersion methods are expected to lead to significantly variant estimates of contributionsarising from non-perturbative tt threshold effects.
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