Analysis of B_d→ψ K_S CP Asymmetry in a Flavour Changing Z' Model

According to the recent experimental data, the time-dependent CP asymmetry S_ψK_S for B_d→K_S decay might still be inconsistent with the standard model (SM) expectations. We try to understand the anomaly with a Z' model associated with flavour changing neutral currents (FCNCs) at the tree level. We find that (i) if neglecting the contributions from the right-handed flavour changing coupling B^R_sb and setting ξ^LL=ξ^LR=ξ and ξ^LL=ξ, ξ^LR=0, we may obtain 0.01< ξ <0.014, 0.0175< ξ <0.0205, 0.021< ξ <0.0255 and 0.004< ξ <0.008, 0.018< ξ <0.028, 0.042< ξ <0.048 for the B_d→ψ K_S decay, respectively; (ii) if considering the contributions coming from B^R_sb and setting
ξ^LL=ξ^RL=ξ^LR=ξ^RR=ξ and ξ^LL=ξ^RL=ξ, ξ^LR=ξ^RR=0, we can obtain 0.005<
ξ<0.075, 0.0085< ξ<0.0105, 0.011<ξ<0.013 and 0.0125<ξ<0.0177, 0.0224< ξ<0.025. These results are consistent with the constraints and the assumptions in some literature.