Doubly heavy tetraquarks in an extended chromomagnetic model *

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