Model-independent Constraints on Topcolor from $R_b$

Abstract

We identify and compute the main corrections to $R_b$ 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 cancelling terms. The resulting value of $R_b$ leads to severe constraints on Topcolor theories.

Comments: 12 pages; uses LaTeX and psfig. Version to be published in Physics Letters B. Expanded discussion on top-pion and b-pion contributions, abridged discussion on vector mesons, change of Fig.2, one reference added. The full postscript version is also available at http://www.physik.tu-muenchen.de/tumphy/d/T30d/PAPERS/ TUM-HEP-267-97.ps.gz