Z -> b b-bar Excess from R-Parity Violation
  The $\Zbb$ excess, $\Zcc$ deficit, and low left-right asymmetry $A_b$ may be explained by a single term $\lam C^c B^c B^{\prime c}$ in the superpotential. This operator violates R-parity and requires a sequential 4th generation. 1-loop diagrams involving squark exchange interfere with the tree-level processes to give an excess of right-handed $b$ quarks, and a deficit of right-handed $c$ quarks. Though the coupling must be large ($\lam\approx 2$ or $3$), the model is phenomenologically and cosmologically acceptable. 
David E. Brahm (CMU)