Two-loop electroweak top corrections: are they under control?

Abstract

The assumption that two-loop top corrections are well approximated by the $O(G_m{u}^{2}m{t}^4)$ contribution is investigated. It is shown that in the case of the ratio neutral-to-charged current amplitudes at zero momentum transfer the $O(G_m{u}^{2}m{t}^2{M}_Z^2)$ terms are numerically comparable to the $m_t^4$ contribution for realistic values of the top mass. An estimate of the theoretical error due to unknown two-loop top effect is presented for a few observables of LEP interest.

Comments: 13 pages, LaTeX using equations, doublespace, cite macros. Hard copies of the paper including one figure are available from degrassi@mvxpd5.pd.infn.it