In this paper we survey all radial and orbital excitations of the Iϭ0 and Iϭ1 nn system anticipated up to 2.1 GeV. We give detailed predictions of their quasi-two-body branching fractions and identify characteristic decay modes that can isolate quarkonia; this should be useful in distinguishing quarkonia from glueballs and hybrids. Several of the ''missing mesons'' with L q q ϭ2 and L q q ϭ3 are predicted to decay dominantly into certain Sϩ P and SϩD modes, and should appear in experimental searches for hybrids in the same mass region. We also consider the topical issues of whether some of the recently discovered or controversial meson resonances, including glueball and hybrid candidates, can be accommodated as quarkonia.

We show that two nonets and a glueball provide a consistent description of data on scalar mesons below 1.7 GeV. Above 1 GeV the states form a conventional qq nonet mixed with the glueball of lattice QCD. Below 1 GeV the states also form a nonet, as implied by the attractive forces of QCD, but of more complicated nature. Near the center they are (qq)3(qq) 3 in S-wave, with some qq in P-wave, but further out they rearrange as (qq) 1 (qq) 1 and finally as meson-meson states. A simple effective chiral model for such a system with two scalar nonets can be made involving two coupled linear sigma models. One of these could be looked upon as the Higgs sector of nonpertubative QCD. 1

Following the discovery of two new scalar mesons, fo(1370) and fo(1500) at the Low Energy Antiproton Ring at CERN, we argue that the observed properties of this pair are incompatible with them both being QQ mesons. We show instead that 10(1500) is compatible with the ground state glueball expected around 1500 MeV mixed with the nearby states of the 0++ QQ nonet.

We show that the newly discovered scalar meson f 0 (1500) at LEAR has properties compatible with the lightest scalar glueball predicted by lattice QCD and incompatible with a QQ state. We suggest that decays of glueballs are into pairs of glueballs (including η, η ′ or (ππ) S ) or by mixing with nearby QQ states. The partial widths of f 0 (1500) are in accord with this hypothesis, tests of which include characteristic radiative decays to γφ, γω, γρ and the prediction of a further scalar state, f ′ 0 (1500 − 1800) which couples strongly to KK, ηη and ηη ′ .

Recent signals for narrow hadrons containing heavy and light flavours are compared with quark model predictions for spectroscopy, strong decays, and radiative transitions. In particular, the production and identification of excited charmed and cs states are examined with emphasis on elucidating the nature of 0 + and 1 + states. Roughly 200 strong decay amplitudes of D and Ds states up to 3.3 GeV are presented. Applications include determining flavour content in η mesons and the mixing angle in P and D wave states and probes of putative molecular states. We advocate searching for radially excited D * s states in B decays.

We discuss the implications of a possible quasinuclear DK bound state at 2.32 GeV. Evidence for such a state was recently reported in D s ϩ 0 by the BaBar Collaboration. We first note that a conventional quark model cs assignment is implausible, and then consider other options involving multiquark systems. An Iϭ0 csnn baryonium assignment is one possibility. We instead favor a DK meson molecule assignment, which can account for the mass and quantum numbers of this state. The higher-mass scalar cs state expected at 2.48 GeV is predicted to have a very large DK coupling, which would encourage formation of an Iϭ0 DK molecule. Isospin mixing is expected in hadron molecules, and a dominantly Iϭ0 DK state with some Iϭ1 admixture could explain both the narrow total width of the 2.32 GeV state as well as the observed decay to D s ϩ 0 .Additional measurements that can be used to test this and related scenarios are discussed.

Tests are discussed to distinguish cc, hybrid charmonium and molecular interpretations of the narrow Belle resonance at 3872 MeV.

Existing predictions for the branching ratio for φ → KKγ via φ → Sγ (where S denotes one of the scalar mesons f 0 (975) and a 0 (980)) vary by several orders of magnitude. Given the importance of these processes for both hadron spectroscopy and CP-violation studies at φ factories (where φ → K 0K 0 γ poses a possible background problem), this state of affairs is very undesirable. We show that the variety of predictions is due in part to errors and in part to differences in modelling. The latter variation leads us to argue that the radiative decays of these scalar states are interesting in their own right and may offer unique insights into the nature of the scalar mesons. As a byproduct we find that the branching ratio for φ → K 0K 0 γ is < ∼ 0(10 −7 ) and will pose no significant background to proposed studies of CP-violation. * For an historical perspective see Ref. [8]; for a more recent study see Ref. [9].