$Zb\bar{b}$ Loop Correction with Charge 2/3 Singlet Quarks
  We calculate the non-universal correction to the $Zb\bar{b}$ vertex in a simple extension of the Standard Model, where a charge $+2/3$ isosinglet quark is added to the standard spectrum. Comparison is made with other solutions to $R_b$ (and $R_c$) that demand particles lighter than $M_W$. 
Wei-Shu Hou and Hsien-chung Kao