GGI Lectures on Exotic Hadrons

It is well known that M. Gell-Mann, introducing quarks in 1964 to describe the known mesons and baryons, hinted at the existence of further $qq\bar q\bar q$ mesons (tetraquarks) and $qqqq\bar q$ baryons (pentaquarks). In 1977, R. Jaffe proposed a model of the lightest scalar mesons as diquark-antidiquark pairs and A. de Rujula, H. Georgi and S. Glashow coined the term hadron molecules, to describe possible hadrons made by meson-antimeson pairs bound by the familiar nuclear forces, also an overall tetraquark system. The two alternative pictures have been employed to interpret the unexpected hadron discovered by Belle in 2003, the $X(3872)$, confirmed by BaBar and seen in many other High Energy experiments. Since then, a wealth of Exotic Hadrons have been discovered, mesons and baryons that cannot be described by the classical Gell-Mann, $q\bar q$ and $qqq$, configurations, opening a new chapter of Hadron Spectroscopy.