Doubly heavy tetraquarks in an extended chromomagnetic model

Using an extended chromomagnetic model, we perform a systematic study of the masses of the doubly heavy tetraquarks. We find that the ground states of the doubly heavy tetraquarks are dominated by color-triplet $ket{(qq)^{bar{3}_{c}}(bar{Q}bar{Q})^{3_{c}}}$ configuration, which is opposite to that of the fully heavy tetraquarks. The combined results suggest that the color-triplet configuration becomes more important when the mass difference between the quarks and antiquarks increases. We find three stable states which lie below the thresholds of two pseudoscalar mesons. They are the $IJ^{P}=01^{+}$ $nnbar{b}bar{b}$ tetraquark, the $IJ^{P}=00^{+}$ $nnbar{c}bar{b}$ tetraquark and the $J^{P}=1^{+}$ $nsbar{b}bar{b}$ tetraquark.