Excited Charmonia and Charmonium Resonances from Lattice QCD

Figure 1: Examples of the results: left is the spectrum in the isospin-1 hidden-charm channel with JP=1+ (J is spin, P is parity), right is the isospin-0 doubly-charmed channel with JP=1+. Boxes are the computed masses and one-sigma statistical uncertainties; horizontal lines show the meson-meson energy levels expected if there were no meson-meson interactions with an adjacent number indicating the degeneracy if it is larger than one (lattice QCD calculations are performed in a finite volume and so the spectrum of meson-meson energy levels is discrete). The number of energy levels in the computed spectrum is equal to the number of expected non-interacting meson-meson levels and they all lie close to the non-interacting levels. This suggests that there are only weak meson-meson interactions and no strong indications of a bound state or narrow resonance in these channels.

Experiments have observed a number of puzzling charmonium-like states in recent years. Of particular interest are states with ‘exotic flavour’, i.e. those with flavour quantum numbers which cannot arise from solely a quark-antiquark pair and so signal physics beyond a conventional meson. One proposed explanation for such states is that they are tetraquarks, compact states containing two quarks and two antiquarks. In Cheung et al [JHEP 1711 (2017) 033, arXiv:1709.01417] we developed methods to investigate the existence of tetraquarks using first-principles lattice QCD computations. The spectrum of states was calculated in a selection of exotic-flavour channels (isospin-1 hidden-charm and doubly-charmed channels), including a number where tetraquarks are expected in phenomenological models. There were no strong indications for any bound states or narrow resonances in the channels studied and so no sign of tetraquark candidates.