A major goal in strong-interaction physics is to understand the nature of hadrons, which make up visible matter, and much research activity revolves around two fundamental questions: what are hadrons made up of and how does Quantum Chromo-dynamics (QCD), the strong-interaction component of the Standard Model, produce them? Although these questions are simple, the answers may not be. To address these questions, spectroscopy is a valuable and time-honored tool, as it enables us to understand the structure of mesons, baryons and exotics and how they are produced. In this context, the recent discovery of many new hadronic states, in particular the plethora of observed X, Y, Z states, is exciting, as these objects challenge the commonplace view of hadrons as either quark-antiquark or three-quark color-singlet states.
Topics: Experiment-HEP, Phenomenology-HEP, Theory-HEP