$R_b$ -- $R_c$ Crisis and New Physics

Abstract

The experimental values of $R_b$ and $R_c$ are the only data which do not seem to agree with Standard Model predictions. Although it is still premature to draw any definite conclusions, it is timely to look for new physics which could explain the excess in $R_b$ and deficit in $R_c$. We investigate this problem in a simple extension of the Standard Model, where a charge $+2/3$ isosinglet quark is added to the standard spectrum. Upon the further introduction of an extra scalar doublet, one finds a solution with interesting consequences.