Tcc+ and its partners

Abstract

Inspired by the ${T}_{cc}^{+}$ signal discovered by the LHCb Collaboration, we systematically investigate the doubly heavy tetraquark states with the molecule configuration $[{Q}_{1}{\overline{q}}_{2}{]}_{{\mathbf{1}}_{c}}[{Q}_{3}{\overline{q}}_{4}{]}_{{\mathbf{1}}_{c}}$ ($Q=c$ and $b$, $q=u$, $d$, and $s$) in a nonrelativistic quark model. The model involves a color screening confinement potential, meson-exchange interactions, and one-gluon-exchange interactions. The state ${T}_{cc}^{+}$ with $I{J}^{P}=0{1}^{+}$ is a very loosely bound deuteronlike state with a binding energy around 0.34 MeV and a huge size of 4.32 fm. Both the meson exchange force and the coupled channel effect play a pivotal role. Without the meson exchange force, there does not exist the ${T}_{cc}^{+}$ molecular state. In strong contrast, the QCD valence bond forms clearly in the ${T}_{bb}^{\ensuremath{-}}$ system when we turn off the meson-exchange force, which is very similar to the hydrogen molecule in QED. Moreover, the ${T}_{bb}^{\ensuremath{-}}$ becomes a heliumlike QCD-atom if we increase the bottom quark mass by a factor of three. Especially, the states ${T}_{bb}^{\ensuremath{-}}$ with ${01}^{+}$, ${T}_{bc}^{0}$ with ${00}^{+}$ and ${01}^{+}$, and the $V$-spin antisymmetric states ${T}_{bbs}^{\ensuremath{-}}$ with $\frac{1}{2}{1}^{+}$, ${T}_{bcs}^{0}$ with $\frac{1}{2}{0}^{+}$ and $\frac{1}{2}{1}^{+}$ can form a compact, hydrogen moleculelike or deuteronlike bound state with different binding dynamics. The high-spin states ${T}_{bc}^{0}$ with ${02}^{+}$ and ${T}_{bcs}^{0}$ with $\frac{1}{2}{2}^{+}$ can decay into $D$-wave $\overline{B}D$ and ${\overline{B}}_{s}D$ although they are below the thresholds ${\overline{B}}^{*}{D}^{*}$ and ${\overline{B}}_{s}^{*}{D}^{*}$, respectively. The isospin and $V$-spin symmetric states are unbound. We also calculate their magnetic moments and axial charges.