A survey of heavy-heavy hadronic molecules

TL;DR

This survey reviews the spectrum and potential of heavy-heavy hadronic molecules, predicts new states, and discusses recent experimental findings like the LHCb $T_{cc}$, highlighting the natural emergence of these molecules from resonance saturation.

Contribution

It provides a comprehensive overview of heavy-heavy hadronic molecules, extending previous work on heavy-antiheavy systems and predicting new molecular states with experimental relevance.

Findings

Heavy-heavy molecules are naturally expected from constant interactions.

Predicted molecules include $DD^*$ and $D^{(*)}\Sigma_c^{(*)}$ states.

Recent LHCb $T_{cc}$ observation aligns with predictions.

Abstract

The spectrum of hadronic molecules composed of heavy-antiheavy charmed hadrons has been obtained in our previous work. The potentials are constants at the leading order, which are estimated from resonance saturation. The experimental candidates of hadronic molecules, say $X(3872)$, $Y(4260)$, three $P_c$ states and $P_{cs}(4459)$, fit the spectrum well. The success in describing the pattern of heavy-antiheavy hadronic molecules stimulates us to give more predictions for the heavy-heavy cases, which are less discussed in literature than the heavy-antiheavy ones. Given that the heavy-antiheavy hadronic molecules, several of which have strong experimental evidence, emerge from the dominant constant interaction from resonance saturation, we find that the existence of many heavy-heavy hadronic molecules is natural. Among these predicted heavy-heavy states we highlight the $DD^*$ molecule and the $D^{(*)}\Sigma_c^{(*)}$ molecules, which are the partners of the famous $X(3872)$ and $P_c$ states. Quite recently, LHCb collaboration reported a doubly charmed tetraquark state, $T_{cc}$, which is in line with our results for the $DD^*$ molecule. With the first experimental signal of this new kind of exotic states, the upcoming 3pdate of the LHCb experiment as well as other experiments will provide more chances of observing the heavy-heavy hadronic molecules.