Quantum ChromoDynamics (QCD) governs the forces between quarks and gluons. While their short-distance (large momentum transfer) interactions are well understood from QCD, the long-distance (low momentum transfer) interactions cannot easily be inferred from QCD, although it is believed to contain all features of the strong interaction, including confinement, which reflects the non-observation of free quarks and gluons – these partons appear only as composite systems, the so called hadrons. Examples of this shortcoming are that we do not understand the excitation spectrum of hadrons, the composition of hadrons in terms of valence quarks is sometimes obscure and the interaction of hadrons among themselves can only be described with good accuracy under certain conditions. In light of these shortcomings effective field theories have been developed which still allow one to exploit certain features of QCD also in the low energy regime. Moreover, lattice QCD in principle allows one to study full QCD via numerical simulations on a discretized space-time, but at present also this tool can be used for selected systems only. Finally QCD-inspired models still play an important role on the path to unmask the mysteries of the strong interaction.
Large theoretical progress in hadron spectroscopy is now confronted with precise data and often surprising findings of new hadronic states. While detailed spectroscopy has for long been confined to light quarks accompanied with the frustration of enhanced theoretical difficulties and often contradicting experimental results, this situation drastically altered when undisputed but unexplainable new hadrons were observed, which contain heavy quarks. This field has since seen an unexpected revival, with many publications in particular in theory, but also experimental results with unprecedented citation records.
The identification and interpretation of new states is in many cases very indirect since demanding partial wave analyses need to be employed. For some of the yet unexplained states it is even questioned if they exist at all or if the observed structure is simply from an underlying kinematic singularity. Whether in the complex sector of light quark spectroscopy or the seemingly cleaner area of heavy quark hadrons, progress can only be gained by a very close collaboration between different theory communities (like phenomenologists and lattice practitioners) as well as experimentalists.
We aim at bringing together theorists and experimentalists actively working in the field, who will summarize the status of experimental findings of the last decade as well as new theoretical ideas and pin down the most challenging pending problems. The discussions should lead to a compelling priority list of theoretical as well as experimental activities in the near and midterm future.
Key scientific questions addressed by the MIAPP programme