Deciphering Strong-Interaction Phenomenology through Precision Hadron-Spectroscopy

7 - 31 October 2019

Stephan Paul, Nora Brambilla, Simon Eidelman, Christoph Hanhart, Luciano Maiani

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

  • The key questions of the proposed programme are related to the key questions in hadron physics, which in turn are vital in understanding of the subtle symmetry violation observed or sought for in heavy flavour decays.
  • What is the excitation spectrum of light and heavy hadrons?
  • Are all those states truly observed and what is the nature of seemingly superfluous states observed?
  • What is the best description of the exotic hadrons: colour-neutral components (hadron molecules) or coloured subconstituents (e.g. diquarks)?
  • What are the experimental signatures to tell one picture from another?
  • What can we learn from the light meson spectrum about the heavy meson spectrum and vice versa?
  • Can we build an excitation spectrum from observed heavy quarkonium states XYZ?
  • How can the limits of QCD lattice calculations be pushed to make it capable of describing states decaying into various multiparticle final states?
  • To what extent can dispersion theory be employed to allow for a proper description of heavy meson transition to a multipion final state?
  • How to derive subtle understanding of confinement from spectroscopic data?

More specifically, we will discuss several key issues:

  1. How to improve our analysis models and which analysis methods do we expect to be most promising?
  2. Which experiments (measurements) should be pursued with highest priority and which analysis tools should be used?
  3. How can experimental as well as theoretical tools be improved?
  4. How can we combine data from various experiments and how should we approach joint fits to published data?
  5. How to organize unbiased databases for the large number of spectroscopic analyses?