Investigating tetraquarks composed of us(d)over-bar(b)over-bar and ud(s)over-bar(b)over-bar

In the framework of the quark delocalization color screening model, we investigate tetraquarks composed of us (d) over bar(b) over bar and ud (s) over bar(b) over bar in two structures: meson-meson structure and diquark-antidiquark structure. Neither bound state nor resonance state is found in the system composed of us (d) over bar(b) over bar. The reported X(5568) cannot be explained as a molecular state or a diquark-antidiquark resonance of us (d) over bar(b) over bar in present calculation. However, two bound states of the diquark-antidiquark structure are obtained in the tetraquarks system composed of ud (s) over bar(b) over bar: an IJ=00 state with the mass of 5701 MeV, and an IJ=01 state with the mass of 5756 MeV, which maybe the better tetraquark states with four different flavors. Our results indicate that the diquark-antidiquark configuration would be a good choice for the tetraquarks ud (s) over bar(b) over bar with quantum numbers IJ=00 and IJ=01. The tetraquarks composed of ud (s) over bar(b) over bar is more possible to form bound states than the one composed of us (d) over bar(b) over bar. These bound states are worth investigating in future experiments.