CERN Accelerating science

Article
Report number	arXiv:2009.07795
Title	Coupled-channel interpretation of the LHCb double-$J/\psi$ spectrum and hints of a new state near the $J/\psi J/\psi$ threshold
Related title	Coupled-Channel Interpretation of the LHCb Double- $J/\psi$ Spectrum and Hints of a New State Near the $J/\psi J/\psi$ Threshold
Author(s)	Dong, Xiang-Kun (Beijing, Inst. Theor. Phys. ; Beijing, GUCAS) ; Baru, Vadim (Bonn U., HISKP ; U. Bonn, Phys. Inst., BCTP ; Kurchatov Inst., Moscow ; Lebedev Inst.) ; Guo, Feng-Kun (Beijing, Inst. Theor. Phys. ; Beijing, GUCAS) ; Hanhart, Christoph (IAS, Julich ; Julich, Forschungszentrum ; JCHP, Julich) ; Nefediev, Alexey (LPI, Moscow (main) ; Moscow, MIPT)
Publication	2021-09-07
Imprint	2020-09-16
Number of pages	6
Note	7 pages, 7 figures; typos corrected, results not affected
In:	Phys. Rev. Lett. 126 (2021) pp.132001
DOI	10.1103/PhysRevLett.126.132001 (publication) 10.1103/PhysRevLett.127.119901 (erratum)
Subject category	nucl-th ; Nuclear Physics - Theory ; hep-lat ; Particle Physics - Lattice ; hep-ex ; Particle Physics - Experiment ; hep-ph ; Particle Physics - Phenomenology
Accelerator/Facility, Experiment	CERN LHC ; LHCb
Abstract	Recently, the LHCb Collaboration reported pronounced structures in the invariant mass spectrum of $J/\psi$-pairs produced in proton-proton collisions at the Large Hadron Collider. In this Letter, we argue that the data can be very well described within two variants of a coupled-channel approach employing $T$-matrices consistent with unitarity: (i) with just two channels, $J/\psi J/\psi$ and $\psi(2S)J/\psi$, as long as energy-dependent interactions in these channels are allowed, or (ii) with three channels $J/\psi J/\psi$, $\psi(2S)J/\psi$ and $\psi(3770)J/\psi$ with just constant contact interactions. Both formulations hint at the existence of a near-threshold state in the $J/\psi J/\psi$ system with the quantum numbers $J^{PC}=0^{++}$ or $2^{++}$, which we refer to as $X(6200)$. We suggest experimental tests to check the existence of this state and discuss what additional channels need to be studied experimentally to allow for distinctive tests between the two mechanisms proposed. If the molecular nature of the $X(6200)$, as hinted by the three-channel approach, is confirmed, many other double-quarkonium states should exist driven by the same binding mechanism. In particular, there should be an $\eta_c\eta_c$ molecule with a similar binding energy.
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