Hadron spectroscopy is key to understanding of the enigmatic properties of QCD, such as confinement, dynamical chiral symmetry breaking, emergence of topological matter, etc. Recent discoveries, of novel hadronic phenomena appearing in the spectrum of hadrons, the so called XYZ states, challenge predictions of the quark model and indicate possible existence of new forms of QCD matter that may include hybrid, multi-quark or hadronic bound states. Gluons are expected to play a key role in these emergent phenomena. They are the most perplexing of all the elementary particles in the Standard Model being the mediators of the confining force and simultaneously subject to confinement. Spectroscopy of gluon-rich mesons and baryons is expected to be particularly important in studies of QCD phenomenology.
The Jefferson Lab CLAS12 and GlueX experiments and in the future the EIC, using photon and electron beams are unique in their capability for producing new hadronic states. They are part of the worldwide network of experiments aiming to address challenges in QCD phenomenology.
Discoveries of new particles require identification and control of many effects, and it is a challenging theoretical problem to determine the underlying, analytical reaction amplitudes. There are possible overlapping resonances, threshold cusps, background and particle exchanges, and precision data require analysis that is based on principles of relativistic scattering such as unitarity, analyticity, crossing symmetry and considers QCD specific features, e.g., chiral symmetry.
Description of the photo-production data from the CLAS experiment. It illustrates the dominance of natural over unnatural Regge exchanges (solid vs. dashed lines). (From )
Within the Joint Physics Analysis Center (JPAC), we develop theoretical and phenomenological understanding of production and decays of hadron resonances. JPAC does not rely on a single model or approach, but it is inclusive to various methodologies. JPAC products not only provide a better understanding of hadron phenomena but are of direct use in analysis and interpretation of experimental data and lattice gauge simulations. This is achieved thanks to close collaborations with theorists, experimentalists and lattice QCD practitioners.
 V. Mathieu, A. Pilloni, M. Albaladejo, Ł. Bibrzycki, A. Celentano, C. Fernández-Ramírez and A. P. Szczepaniak, Phys. Rev. D 102, 014003 (2020).