Description
Quantum Chromodynamics (QCD) is the theory of the hadronic interactions. It is an elegant theory whose full non perturbative solution has escaped our knowledge since its formulation more than 40 years ago. The theory is asymptotically free and confining. A particularly good test of our understanding of the nonperturbative aspects of QCD is to study particles where the gauge field plays a more important dynamical role than in the standard hadrons. In particular glueballs, bound states of gluons, represent such a scenario.
The glueball spectrum has attracted much attention since the early days of QCD. The interest in this subject is related to the significant progress in the understanding of the properties of such states within QCD, as well as, in the new possibilities for their identification in modern experiments. Glueballs have not been an easy subject to study due to the lack of phenomenological support and therefore much debate has been associated with their properties. The main achievement of the theoretical approaches is the understanding of the deep relation between the properties of the glueball states and the structure of the QCD vacuum.
From the phenomenological point of view, it has become clear by now that it is difficult to single out which states of the hadronic spectrum are glueballs because we lack the necessary knowledge to determine their decay properties. Moreover, the strong expected mixing between glueballs and quark states leads to a broadening of the possible glueball states which does not simplify their isolation. The wishful sharp resonances which would confer the glueball spectra the beauty and richness of the baryonic and mesonic spectra are lacking. This confusing picture has led to a loss of theoretical and experimental interest in these hadronic states. However, it is important to stress, that if they were to exist, they would be a beautiful and unique consequence of QCD.
I consider that any plan for future research in nuclear physics has to focus on the theoretical and experimental study of glueballs, their spectrum, their decays, their interactions and as a consequences in a better understanding of QCD.