Seminario INFN di Gruppo III

The quark-antiquark picture delivers a well-established and useful classification of mesons. Nevertheless, many issues are still open, some of which are discussed in this talk. (i) The resonance a1(1230) is often interpreted as the chiral partner of the ρ meson, yet in some approaches it is a molecular state. How to settle thie nature of this state? (ii) Ground-state vector mesons form an ideal nonet, but the nonet of orbitally excited vectors is still not complete: the strange-antistrange member φ(1930) was not yet measured. Can it be found in the near future? (iii) It is well known that the mesons η and η’ meson have a large strange-nonstrange admixture due to the chiral anomaly, while ω and φ(1024) are nonstrange and strange, respectively. How to understand this difference and what about other nonets? The pseudotensors η2(1645) and η2(1870) are promising objects for future studies, since they could also be largely mixed . (iv) Last but not least, where are the glueballs?