Gluons are confined inside protons and this makes the study of their properties difficult. Only in high energy scattering processes those properties become apparent. The momentum distribution of gluons inside protons is quite well known in the region where the interactions among the gluons is small and where the gluon density is relatively small. What happens in extreme conditions, where the interactions are very large and/or the gluon density is very large is much less understood. This is a main topic of my current research, where I focus not only on the momentum distribution, but also on the polarization state(s) of the gluons. Different polarization states can give different contributions to scattering cross sections. One objective of my research is to find the simplest observables sensitive to the gluonic polarization states and to predict their dependence on energy and momentum. Also I try to construct expressions that can be used in numerical studies on the lattice in order to test our expectations and to relate them to the outcomes of experiments at hadron colliders such as RHIC and LHC and at a future Electron-Ion Collider. At present the observables under investigation typically involve angular correlations between pairs of particles produced in the final state.
Theory (QCD) predicts that at high gluon densities a saturation phenomenon will occur leading to what is called the Color Glass Condensate (CGC). The observable consequences of the CGC need to be worked out in detail. The CGC has its own characteristic momentum scale, called the saturation scale, but how to extract that scale unambiguously from experiment? The change of the CGC with energy is predicted by theory using various approximations and limits, and this points to the conclusion that at asymptotically high energies there will be a scaling behavior, geometric scaling. It implies that asymptotically it does not matter whether one scatters off a proton or a heavy ion, because both will have reached the maximal gluon density. This naturally leads to the question whether scaling or the onset of scaling already occurs in experiments at presently accessible energies? One main result of my research on this subject has been that the CGC is actually highly polarized. Also this property is still awaiting experimental testing.
Part of my research on this topic has been financially supported by the Dutch foundation FOM.