The effects of large extra dimensions on associated $t\bar{t}{h}^0$ production at linear colliders

In the framework of the large extra dimensions (LED) model, the effects of LED on the processes \rrtth and \eetth at future linear colliders are investigated in both polarized and unpolarized collision modes. The results show that the virtual Kaluza-Klein (KK) graviton exchange can significantly modify the standard model expectations for these processes with certain polarizations of initial states. The process \rrtth with $\sqrt{s}=3.5TeV$ allows the effective scale ${\mathrm{\Lambda }}_T$ to be probed up to 7.8 and 8.6 TeV in the unpolarized and $P_{\gamma }=0.9$, J=2 polarized $\gamma \gamma$ collision modes, respectively. For the \eetth process with $\sqrt{s}=3.5TeV$, the upper limits of ${\mathrm{\Lambda }}_T$ to be observed can be 6.7 and 7.0 TeV in the unpolarized and $P_{e^{+}}=0.6$, $P_{e^{-}}=0.8$, $-+$ polarized $e^{+}{e}^{-}$ collision modes, respectively. We find the \rrtth channel in J=2 polarized photon collision mode provides a possibility to improve the sensitivity to the graviton tower exchange.