Deciphering the recently discovered tetraquark candidates around 6.9 GeV

Abstract

Recently a novel hadronic state of mass 6.9 GeV, that decays mainly to a pair of charmonia, was observed in LHCb. The data also reveals a broader structure centered around 6490 MeV and suggests another unconfirmed resonance centered at around 7240 MeV, very near to the threshold of two doubly charmed ${\Xi }_{cc}$ baryons. We argue in this note that these exotic hadrons are genuine tetraquarks and not molecules of charmonia. It is conjectured that they are V-baryonium tetraquarks, namely, have an inner structure of a baryonic vertex with a $cc$ diquark attached to it, which is connected by a string to an anti-baryonic vertex with a $\bar{c}\bar{c}$ anti-diquark. We examine these states as the analogs of the states $\Psi (4360)$ and $Y(4630)$/$\Psi (4660)$ which are charmonium-like tetraquarks. One way to test these claims is by searching for a significant decay of the state at 7.2 GeV into ${\Xi }_{cc}{\overline{\Xi }}_{cc}$. Such a decay would be the analog of the decay of the state $Y(4630)$ into to ${\Lambda }_c{\overline{\Lambda }}_c$. We further argue that there should be trajectories of both orbital and radial excited states of the $X(6900)$. We predict their masses. It is possible that a few of these states have already been seen by LHCb.

Comments: v1: 22 pages, 4 figures v2: typos corrected, minor additions and references added. 22 pages, v3: references added. 22 pages