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We discuss the theoretical expectations and phenomenological evidence for the lightest glueballs and the members of the meson nonet with quantum numbers J P C = 0 ++ . We reconsider the recent evidence for candidate states with masses below ∼1700 MeV, but include also the results from earlier phase-shift analyses. Arguments are presented to classify the scalars f 0 (980) and f 0 (1500) as members of the 0 ++ nonet, with a mixing rather similar to that of the pseudoscalars η and η. The S-wave states called f 0 (400 − 1200) and f 0 (1370) are considered as different signals from a single broad resonance, which we take to be the lowest-lying 0 ++ glueball. This state together with η(1440) and f J (1710) with spin J = 2 form the basic triplet of binary gluonic bound states. We argue that these hypotheses are consistent with what can be expected theoretically.

We study theories with the exceptional gauge group G(2). The 14 adjoint "gluons" of a G(2) gauge theory transform as {3}, {3} and {8} under the subgroup SU(3), and hence have the color quantum numbers of ordinary quarks, anti-quarks and gluons in QCD. Since G(2) has a trivial center, a "quark" in the {7} representation of G(2) can be screened by "gluons". As a result, in G(2) Yang-Mills theory the string between a pair of static "quarks" can break. In G(2) QCD there is a hybrid consisting of one "quark" and three "gluons". In supersymmetric G(2) Yang-Mills theory with a {14} Majorana "gluino" the chiral symmetry is Z Z(4) χ . Chiral symmetry breaking gives rise to distinct confined phases separated by confined-confined domain walls. A scalar Higgs field in the {7} representation breaks G(2) to SU(3) and allows us to interpolate between theories with exceptional and ordinary confinement. We also present strong coupling lattice calculations that reveal basic features of G(2) confinement. Just as in QCD, where dynamical quarks break the Z Z(3) symmetry explicitly, G(2) gauge theories confine even without a center. However, there is not necessarily a deconfinement phase transition at finite temperature.

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