Decoding the $X(5568)$ as a fully open-flavor $su\bar b\bar d$   tetraquark state
  We investigate the recent evidence for a charged $X(5568)$ meson as an exotic open-flavor tetraquark state $su\bar b\bar d$ with $J^P=0^+/1^+$ in the framework of QCD sum rules. We use the color antisymmetric $[\mathbf{\bar 3_c}]_{su} \otimes [\mathbf{3_c}]_{\bar{b}\bar d}$ tetraquark currents in both scalar and axial-vector channels to perform evaluations and numerical analyses. Our results imply that the $X(5568)$ can be interpreted as both the scalar $su\bar b\bar d$ tetraquark state and the axial-vector one, which are in good agreement with the experimental measurement. We also discuss the possible decay patterns of the $X(5568)$ and suggest to search for its neutral partner in the radiative decay into $B_s^0 \gamma$ and $B_s^* \gamma$, which can be used to determine its spin-parity quantum numbers. Moreover, we predict its charmed partner state around $2.55$ GeV with the quark content $su\bar c\bar d$ and $J^P=0^+/1^+$. 
Wei Chen, Hua-Xing Chen, Xiang Liu, T. G. Steele and Shi-Lin Zhu
hep-ph hep-ex hep-lat nucl-ex