Non-universal gauge boson $Z'$ and the spin correlation of top quark   pair production at $e^{-}e^{+}$ colliders
  In the off-diagonal basis, we discuss the contributions of the non-universal gauge boson $Z'$ predicted by the topcolor-assisted technicolor ($TC2$) model to the spin configurations and the spin correlation observable of the top quark pair production via the process $e^{-}e^{+}\to t\bar{t}$. Our numerical results show that the production cross sections for the like-spin states, which vanish in the standard model, can be significantly large as $M_{Z'}\approx \sqrt{S}$. With reasonable values of the $Z'$ mass $M_{Z'}$ and the coupling parameter $k_{1}$,   $Z'$ exchange can generate large corrections to the spin correlation observable. 
Chong-Xing Yue and Li-Na Wang