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Mesons beyond the quark-antiquark picture

The vast majority of mesons can be understood as quark-antiquark states. Yet, various other possibilities exists: glueballs (bound-state of gluons), hybrids (quark-antiquark plus gluon), and four-quark states (either as diquark-antidiquark or molecular objects) are expected. In particular, the existence of glueballs represents one of the first predictions of QCD, which relies on the nonabelian feature of its structure; this is why the search for glueballs and their firm discovery would be so important, both for theoretical and experimental developments. At the same time, many new resonances ($X,Y,$ and $Z$ states) were discovered experimentally, some of which can be well understood as four-quark objects. In this lecture, we review some basic aspects of QCD and show in a pedagogical way how to construct an effective hadronic model of QCD. We then present the results for the decays of the scalar and the pseudoscalar glueballs within this approach and discuss the future applications to other glueball states. In conclusion, we briefly discuss the status of four-quark states, both in the low-energy domain (light scalar mesons) as well as in the high-energy domain (in the charmonia region).