A few years ago the Memorandum of Understanding was signed between Joint Institute for Nuclear Research (JINR) and European Centre for Theoretical Studies in Nuclear Physics and Related Areas (ECT*, Italy). ECT* provides dedicated and structured combination of scientific activities for a large international scientific community. It promotes coordination of research efforts in nuclear and hadron physics and the related areas. Following the memorandum, the two institutions will seek further opportunities to cooperate in scientific research.

In 2019, it was decided by ECT* Scientific Board and ECT* director, Prof. Jochen Wambach to hold an international workshop on this important and relevant topic entitled “Theoretical and experimental challenges in flavour hadrons, quarkonia and multiquark physics” which was scheduled for the dates from 30th August to 3rd September 2021.

The Scientific Board approved the international organizing committee. To introduce and promote the topics of the incoming events to a wider audience, the ECT* has proposed the series of “colloquia style” presentations. The recorded presentation of the topic related to the workshop is available below. During the meeting treated the following issues will be addressed, balancing more theoretical with experimental ones, such as hadronization, effective theories of heavy quarkonium, the spectrum of heavy hadrons, multiquarks and exotics, the interaction of charmed mesons with nuclear matter, heavy mesons with lattice QCD, heavy hadron production and decays, hadron and heavy ion collisions.

The main goal of the event is to bring together well-known theorists and experimentalists for direct communication and join their efforts to further solving the problem. It serves as an opportunity to encounter new ideas and discuss the most popular topics with experts, which is especially valuable for young scientists and junior specialists.

One of the primary efforts in hadron physics is to understand the nature of hadrons. Therefore, a great deal of research activity revolves around two fundamental questions: what constituents are the hadrons made of and how does QCD, the strong interaction component of the Standard Model, produce them? To understand the measurable content of QCD, hadron spectroscopy is a valuable and time-tested tool. Experimental investigations of hadron structure and spectrum via hadron-hadron scattering processes, photon-, meson- and electro-production from nucleons at facilities world-wide have produced an enormous growth of available data that vastly improved our knowledge of baryon and meson spectrum, establishing the existence of new states of matter. Even so, many observed states remain puzzling and can not be uniquely explained in the framework of the existing theoretical approaches. This stimulates and motivates for new searches and ideas to understand their nature. The existing facilities and experiments: BES III, Belle II, and LHCb, and the planned ones: FAIR, NICA, and the J-PARC upgrade, represent excellent tools to this end.

The physics of heavy-flavour hadrons and charmonium-like mesons is one of the most relevant and promising areas in modern particle physics. The charm quark, in particular, sits in an uncomfortable mass region as it is neither a light quark for which a flavour symmetry extension is justified nor is it heavy enough to allow for a reliable heavy-quark expansion and associated factorization theorems as for the beauty quark. Various complementary theoretical tools have emerged to determine the heavy hadron spectrum, their decays, oscillations and form factors, among them are relativistic quark models, QCD sum rules, heavy quark effective theory, lattice QCD, non-perturbative continuum approaches such as Dyson-Schwinger and Bethe-Salpeter equations and Nambu-Jona-Lasinio models, as well as effective Lagrangians and coupled-channel models.

On the other hand, spectroscopy experiments have been at the heart of physics in the past century. Their results led, for example, to the development of quantum mechanics and atomic physics, but also to the quark model. However, if we really understood the strong interaction, hadron spectroscopy would nowadays be a dull rather than a challenging enterprise. In fact, the contrary seems to be the case: while the experimental and theoretical studies become more refined and complex, the open problems are emphasized. Even the heavy-quark states that were thought to be well understood, have continued to produce many surprises. This indicates that our understanding of long-distance dynamics is still at a somewhat primitive stage and we still have a great deal to learn from future spectroscopy experiments.

In particular, new forms of hadronic matter like multiquark states, glueballs or hybrids will deepen our understanding of the strong interaction and hadronic matter. In this context, the future experimental facilities such as FAIR, J-PARC upgraded will not only be a XYZ-factory producing copious amounts of elusive states like the X(3872) or Zc(3900)± and many others with unprecedented statistics. D-meson physics will also be pursued, as the quantum numbers of the D*(2300), D₀*(2317), D₁*(2420) and D_s1*(2536) mesons, for instance, still await experimental confirmation and are of strong interest for the BES III and PANDA experiments. The high resolution of future facilities will also allow making precision measurements of line shapes of these particles, eventually revealing their true nature, namely whether they are mesons, tetraquarks or hadronic molecules. On the other hand, the structure of hadrons and their formation implies the even deeper question of how confinement is realized in QCD. From a different viewpoint, one may likewise ask under which conditions can hadrons be deconfined using temperature and density as parameters. To this end, the existing heavy-ion colliders, as well as forthcoming ones such as NICA, represent the primary source to shed light on the phase diagram and deconfinement.

M.Yu. Barabanov, A.S. Vodopyanov, A. Kisiel