Since the beginning of this century our understanding of how QCD forms hadrons is challenged by the discovery of the XYZ states. These quarkonium like mesons show properties at odds with most simple quark models, but so far there is no consensus in the community about their structure. In the talk I will present the most popular theoretical proposals for the XYZ states with special emphasis on how to identify two-meson states, the so called hadronic molecules. The method will then be applied to an analysis of the structures recently found in the J/psi J/psi system by LHCb.
IFIC seminars