## Search for exotic Charmonia

Since the beginnings of the quark model in the 1960s, it has been an open question whether other forms of hadronic matter exist apart from ordinary baryons ($$qqq$$) and mesons $$(q\bar{q})$$.
In particular, the discovery of a variety of charmonium-like structures ($$X$$, $$Y$$, $$Z$$ states) has led to speculation if some of these structures are genuine exotic forms of hadronic matter; such as tetraquarks, hybrids or meson-meson molecules. The $$X$$, $$Y$$, $$Z$$ states show a number of unusual properties.
For example, more charmonium-like states are known than are expected for a naive $$c\bar{c}$$ potential model, so some of them must be supernumerary. Some of the states (e.g., $$X(3872)$$) are unusually narrow, although they are above the open-charm threshold.

The $$Y$$ states, on the other hand, can be produced as vector particles ($$J^{PC} = 1^{--}$$) directly in $$e^{+}e^{-}$$ interactions and due to their decay to $$\pi^{+}\pi^{-} J/\psi$$ it is known, that they have a large $$c\bar{c}$$ contribution in their wavefunction. However, the $$Y$$ states show only a weak coupling to open-charm states ($$c\bar{c}\to(c\bar{q}) (\bar{c}q)$$). Of particular interest are the $$Z$$ states. If these are real resonances, their charge ($$Z^{+-}_{c}$$) excludes identification as conventional Charmonia.
A combined analysis of the data sets of the BESIII experiment allows to search for isospin singlet partners of the isospin triplet state ($$Z^{+}_{c}$$, $$Z^{0}_{c}$$, $$Z^{-}_{c}$$) as expected in some tetraquark models. In addition, the verification or falsification of previously unobserved decay modes of known $$X$$, $$Y$$, $$Z$$ states may contribute to a better understanding of their internal structure.

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