Exotic Meson Spectroscopy (Project XYZ)

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Over the past two decades a plethora of new states containing heavy quarks (c- or b-quarks) have been discovered near or above open heavy-flavor thresholds. These states are exotic, in the sense that they cannot be interpreted as quark-antiquark states but rather as tetraquark states based on QCD diquarks, QCD hybrids, hadronic molecules, or hadro-charmonium states. The project XYZ aims to extract the properties of existing and new exotic states, which will lead to a more detailed understanding of their nature. The project is split into three parts that encompass different aspects of XYZ states. Firstly, through a collaborative effort between the BESIII Collaboration and theorists within the RU, a partial-wave analysis will be performed of the hidden-flavor decay modes of exotic vector XYZ states in the charmonium sector into two-meson final states. These processes have a direct connection to the light-by-light fusion processes in light meson systems, for which the dispersive formalism has already been developed by the theorists within the RU. As an additional tool, model-independent light-by-light sum rules will be used to investigate the nature of exotic states through the radiative transitions of vector charmonium and bottomonium states. Finally, the proposed experimental program on measuring non-vector charmonium states in the electron-positron annihilation via two-photon production at BESIII has the potential to be a discovery channel of charmonium states and will allow one to distinguish between the various models describing their intrinsic nature.