Deciphering the recently discovered tetraquark candidates around 6.9 GeV
Jacob Sonnenschein, Dorin Weissman
Abstract
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 $$\Xi _{cc}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi>Ξ</mml:mi> <mml:mrow> <mml:mi>cc</mml:mi> </mml:mrow> </mml:msub> </mml:math> 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 , 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 $${\bar{c}} {\bar{c}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mover> <mml:mrow> <mml:mi>c</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>¯</mml:mo> </mml:mrow> </mml:mover> <mml:mover> <mml:mrow> <mml:mi>c</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>¯</mml:mo> </mml:mrow> </mml:mover> </mml:mrow> </mml:math> anti-diquark. We examine these states as the analogs of the V-baryonium states $$\Psi (4360)$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>Ψ</mml:mi> <mml:mo>(</mml:mo> <mml:mn>4360</mml:mn> <mml:mo>)</mml:mo> </mml:mrow> </mml:math> and Y (4630)/ $$\Psi (4660)$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>Ψ</mml:mi> <mml:mo>(</mml:mo> <mml:mn>4660</mml:mn> <mml:mo>)</mml:mo> </mml:mrow> </mml:math> 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 $$\Xi _{cc}{\overline{\Xi }}_{cc}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msub> <mml:mi>Ξ</mml:mi> <mml:mrow> <mml:mi>cc</mml:mi> </mml:mrow> </mml:msub> <mml:msub> <mml:mover> <mml:mi>Ξ</mml:mi> <mml:mo>¯</mml:mo> </mml:mover> <mml:mrow> <mml:mi>cc</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> . Such a decay would be the analog of the decay of the state Y (4630) into to $$\Lambda _c{\overline{\Lambda }}_c$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msub> <mml:mi>Λ</mml:mi> <mml:mi>c</mml:mi> </mml:msub> <mml:msub> <mml:mover> <mml:mi>Λ</mml:mi> <mml:mo>¯</mml:mo> </mml:mover> <mml:mi>c</mml:mi> </mml:msub> </mml:mrow> </mml:math> . 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.