Top-quark spin correlation at linear colliders with anomalous couplings

We investigate the feasibility of probing anomalous top-quark couplings of $Wtb,$ $\mathrm{Zt}\overline{t},$ and $\gamma t\overline{t}$ in terms of an effective Lagrangian with dimension-six operators at future $e^{+}{e}^{-}$ linear colliders with a c.m. energy $\sqrt{s}\sim 500-800\mathrm{GeV}.$ We first examine the constraints on these anomalous couplings from the $Z\to b\overline{b}$ data at CERN LEP I and from unitarity considerations. We then consider in detail the effects of anomalous couplings on $t\overline{t}$ spin correlations in the top-pair production and decay with three spin bases: the helicity, beamline, and off-diagonal bases. Our results show that the polarized beams are more suitable for exploring the effects of different new operators. For polarized beams, the helicity basis yields the best sensitivity.