$B \to K \etap$ decays in the SM with fourth generation fermions

By employing the perturbative QCD (pQCD) factorization approach, we calculate the new physics contributions to the four $B \to K \etap$ decays in the Standard Model (SM) with a fourth generation of fermions (SM4), induced by the loop diagrams involving $\tp$ quark. Within the considered parameter space of the SM4 we find that (a) the next-to-leading order (NLO) pQCD predictions for the branching ratios and CP-violating asymmetries in both the SM and SM4 generally agree with the data within one standard deviation; (b) for $Br(B \to K \eta)$, the inclusion of the fourth generation contributions can improve the agreement between the theoretical predictions and the data effectively; (c) for $Br(B \to K \etar)$, however, the decrease due to $\tp$ loops is disfavored by the data; and (d) the new physics corrections to the CP-violating asymmetries of the considered decays are about 10% only.