No abstract

In this review we discuss the analytical perturbative approach, based on perturbative QCD and local parton hadron duality (LPHD), and its application to multiparticle production in jets in the semisoft region. Analytical formulae are presented for various observables within the accuracy of the modified leading logarithmic approximation (MLLA), i.e. with terms of relative order [Formula: see text] taken into account systematically, and in some cases with even higher accuracy. These predictions are confronted with existing experimental data. Many details of the perturbative approach to multiple hadroproduction have been consolidated in recent years, and the overall picture is remarkably healthy. The prospects of future studies of the semisoft jet physics are also discussed.

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.

Calculations within QCD (lattice and sum rules) find the lightest glueball to be a scalar with mass in the range of about 1000-1700 MeV. Several phenomenological investigations are discussed which aim at the identification of the scalar meson nonets of lowest mass and the super-numerous states if any. Results on the flavour structure of the light scalars f 0 (500), f 0 (980) and f 0 (1500) are presented; the evidence for f 0 (1370) is scrutinized. A significant surplus of leading clusters of neutral charge in gluon jets is found at LEP in comparison with MCs, possibly a direct signal for glueball production; further studies with more energetic jets at LHC are suggested. As a powerful tool in the identification of the scalar nonets or other multiplets, along with signals from glueballs we propose the exploration of symmetry relations for decay rates of C = +1 heavy quark states like χ c or χ b . Results from χ c decays are discussed; they are not in support of a tetra-quark substructure of f 0 (980). A minimal scenario for scalar quarkonium-glueball spectroscopy is presented.