Basing on the latest results of the PNPI (Gatchina) and QM&W College (London)
groups, I discuss systematics of the IJ^{PC} q anti-q states in terms of
trajectories on the (n,M^2) 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 q anti-q systematics, I discuss:
1) the results of the K-matrix analysis of the spectra pi-pi, pi-pi-pi-pi, K
anti-K, eta-eta, eta-eta', pi-eta and characteristics of the resonances in the
scalar sector, 2) q anti-q nonet classification of scalar bare states, 3)
accumulation of widths of the q anti-q states by the glueball due to the
overlapping of f_0 resonances at 1200-1700 MeV, 4) systematics of scalar q
anti-q states, both bare states and resonances, on the (n,M^2) plots, 5)
constraints on the quark-gluonium content of the resonances f_0(980),
f_0(1300), f_0(1500), f_0(1750), and the broad state f_0(1420^{+150}_{-70})
from hadronic decays, 6) radiative decays of the P-wave q anti-q resonances:
scalars f_0(980), a_0(980), and tensor mesons a_2(1320), f_2(1270), f_2(1525).
The analysis proves that in the scalar sector we face two exotic mesons: the
light sigma-meson, f_0(450), and the broad state f_0(1420^{+150}_{-70}), which
is the descendant of the glueball.Comment: 12 pages, Latex, epsfig.sty, 6 figures, Talk given on "HADRON 2001
