Systematics of qq states, scalar mesons and glueball

Basing on the latest results of the PNPI (Gatchina) and QM&W College (London) groups, I discuss systematics of the IJPC qq̄ states in terms of trajectories on the (n,M2) plane, where n is the radial quantum number and M is its mass. In the scalar sector, which is the most interesting because of the presence of extra states with respect to the qq̄ systematics, I discuss: 1) the results of the K‐matrix analysis of the spectra ππ, ππππ, KK̄, ηη, ηη′, πη and characteristics of the resonances in the scalar sector, 2) qq̄‐nonet classification of scalar bare states, 3) accumulation of widths of the qq̄ states by the glueball due to the overlapping of f0‐resonances at 1200–1700 MeV, 4) systematics of scalar qq̄ states, both bare states and resonances, on the (n,M2)‐plots, 5) constraints on the quark‐gluonium content of the resonances f0(980), f0(1300), f0(1500), f0(1750), and the broad state f0(1420−70+150) from hadronic decays, 6) radiative decays of the P‐wave qq̄‐resonances: scalars f0(980), a0(980), and tensor mesons a2(1320), f2(1270), f2(1525). The analysis proves that in the scalar sector we face two exotic mesons: the light σ‐meson, f0(450), and the broad state f0(1420−70+150), which is the descendant of the glueball.