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Isospin Breaking and Fine Tuning in Top-Color Assisted Technicolor

Recently, Hill has proposed a model in which new, potentially low-energy, top-color interactions produce a top-condensate ({\em a la} Nambu---Jona-Lasinio) and accommodate a heavy top quark, while technicolor is responsible for producing the $W$ and $Z$ masses. Here we argue that isospin breaking gauge interactions, which are necessary in order to split the top and bottom quark masses, are likely to couple to technifermions. In this case they produce a significant shift in the $W$ and $Z$ masses (i.e.~contribute to $Δρ_* = αT$) if the scale of the new interactions is near 1 TeV. In order to satisfy experimental constraints on $Δρ_*$, we find that either the effective top quark coupling or the top-color coupling must be adjusted to 1\%. Independent of the couplings of the technifermions, we show that the isospin-splitting of the top and bottom quarks implies that the top-color gauge bosons must have masses larger than about 1.4 TeV. Our analysis can also be applied to strong extended technicolor (ETC) models that produce the top-bottom splitting via isospin breaking ETC interactions.