Inspired by Ξcc\documentclass[12pt]{minimal}
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\begin{document}$$\varXi _{cc}$$\end{document} reported by LHCb Collaboration and X(5568) reported by D0 Collaboration, the QQq¯q¯\documentclass[12pt]{minimal}
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\begin{document}$$QQ{\bar{q}}{\bar{q}}$$\end{document} (Q=c,b,s,q=u,d\documentclass[12pt]{minimal}
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\begin{document}$$Q=c,b,s, q=u,d$$\end{document}) tetraquark states, are studied in the present work. With the help of Gaussian expansion method, two structures, diquark–antidiquark and meson–meson, with all possible color configurations are investigated systematically in a chiral quark model to search for the possible stable states. The results show that there is no bound state in the iso-vector QQq¯q¯\documentclass[12pt]{minimal}
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\begin{document}$$QQ{\bar{q}}{\bar{q}}$$\end{document} system, while there are rather deep bound states in the iso-scalar bbq¯q¯\documentclass[12pt]{minimal}
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\begin{document}$$bb{\bar{q}}{\bar{q}}$$\end{document}, ccq¯q¯\documentclass[12pt]{minimal}
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\begin{document}$$cc{\bar{q}}{\bar{q}}$$\end{document} and bcq¯q¯\documentclass[12pt]{minimal}
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\begin{document}$$bc{\bar{q}}{\bar{q}}$$\end{document} systems. There are also several shallow bound states in QQ′\documentclass[12pt]{minimal}
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\begin{document}$$QQ^{\prime }$$\end{document} system. Mixing two structures of diquark–antidiquark and meson–meson can introduce more attractions and convert some unbound iso-scalar states into shallow bound states. The large mass of the heavy quark is beneficial to the formation of the bound state. The separations between quarks are calculated to unravel the spacial structure of the system. We also compare our result with that of other approaches.
