The field of charmonium spectroscopy is one of the most exciting areas of research in particle physics. In the past years a whole series of mysterious discoveries were made. These totally unexpected states are called XYZ particles as the quark content of these states might be exotic. An exotic hadron is classified as a bound object of quarks with a quark content different from the well known meson (consisting of a quark and an anti-quark) or baryon (consisting of three quarks) states. XYZ particles are indeed candidates for exotic hadrons.

The situation has dramatically changed with the discovery of the Zc(3900) particle at the BESIII experiment in China. The fact that the Zc(3900) particle has an electric charge different from zero is a clear proof of its exotic nature. This discovery, together with a series of additional findings in the recent past, proofs that BESIII is the world leading facility for XYZ physics. There are very good chances that the high statistics and quality of data at BESIII will lead to a final explanation of one of the most urgent puzzles in particle physics: Are there particles in the Universe with an exotic quark content and can we quantitatively understand their quark binding?

The applicant of this proposal is the leading scientists at BESIII in the field of charmonium spectroscopy and will bring to Europe the expertise regarding XYZ physics. To keep track of these fascinating new results is of utmost for European research. Europe in near future will invest in major projects itself in this field. For instance in Darmstadt/Germany the PANDA experiment will investigate XYZ particles at the FAIR facility. Dr. Liu will cooperate with the PANDA team in order to shape its physics program. Secondly, theoreticians in Europe will have a chance to cooperate with Dr. Liu and together with his experimental results, and the nature of XYZ particles can be clarified. Mainz is ideally suited for this project thanks to various possible synergies.