Is the exotic X(5568) a bound state?

Stimulated by the recent observation of the exotic X(5568) state by the D0 Collaboration, we study the four-quark system us anti b anti d with quantum numbers J^P = 0⁺ in the framework of the chiral quark model. Two structures, diquark-antidiquark and meson-meson, with all possible color configurations are investigated by using the Gaussian expansion method. The results show that the energies of the tetraquark states with diquark-antiquark structure are too high to be candidates of X(5568), and no molecular structure can be formed in our calculations. The calculation is also extended to the four-quark system us anti c anti d and the same results as that of us anti b anti d are obtained. (orig.)