Model-independent constraints on Topcolor from Rb

We identify and compute the main corrections to Rb in Topcolor theories. They are due to the top-pions, the pseudo-Goldstone bosons associated with the chiral symmetry breaking responsible for the top quark mass, and amount to −(1–4)%, independent of the details of the Topcolor model. We show that the cancellation of this effect against the contributions from Topcolor vector and scalar resonances is unlikely, given the inherently distinct character of the two scales governing the potentially canceling terms. The resulting value of Rb leads to severe constraints on Topcolor theories.