Four sorts of meson

An extensive spectrum of light non-strange qq states up to a mass of 2400 MeV has emerged from Crystal Barrel and PSI 72 data on PP --> Resonance --> A + B in 17 final states. These data are reviewed with detailed comments on the status of each resonance. For I = 0, C = +1, the spectrum is complete and very secure. Six 'extra' states are identified. Four of them have I = 0, C = + 1 and spin-parities predicted for glueballs. Their mass ratios agree closely with predictions from lattice QCD calculations. However, branching ratios for decays are not flavour blind and require mixing with neighbouring qq states. This mixing remains controversial. Two further states, eta(2)(1870) and pi(2)(1880), are candidates for a pair of hybrids. Their decay branching ratios BR[eta(2)(1870) --> f(2)(1270)eta/BR[eta(2)(1870) --> a(2)(1320)pi] and BR[pi(2)(1880) --> a2(1320)eta]/BR[pi(2)(1880) f(2)(1270)pi] are much larger than predicted by SU(3) for qq states. A fourth category of anomalously light 0(+) mesons is made of sigma(540), kappa(750), a(0)(980) and f(0)(980), forming a non-qq nonet. Their properties are reviewed using data from both Crystal Barrel and elsewhere. (C) 2004 Elsevier B.V. All rights reserved.