A Consistent model of Electroweak data including $Z\to b\overline{b}$   and $Z\to c \overline{c}$
  We have performed an overall fit to the electroweak data with the generation blind $U(1)$ extension of the Standard Model. As input data for fitting we have included the asymmetry parameters, the particle decay widths of $Z$, neutrino scattering, and atomic parity violation. The QCD coupling $\alpha_s$ has been constrained to the world average obtained from all data except the $Z$ width. On the basis of our fit we have constructed a viable gauge model that not only explains $R_b$ and $R_c$ but also provides a much better overall fit to the data than the Standard Model. Despite its phenomenological viability, our model is unfortunately not simple from the theoretical viewpoint. Atomic parity violation experiments strongly disfavor more aesthetically appealing alternatives that can be grand unified. 
K. Agashe, M. Graesser, Ian Hinchliffe, M. Suzuki