In this thesis, hadron structure was explored by studying three problems. In each case some underlying hard process, or a characteristic hard momentum, yielded important physical information such as structure and fragmentation functions describing hadrons. This provided a test of QCD predictions.
In the first problem, spin-dependent quark structure functions were estimated for nuclei. The multipole L=2 structure function, measurable in deeply inelastic scattering of unpolarized leptons off a polarized j â‰¥ 1 nuclear target, is a good indicator of exotic quark-gluon components in the nucleus. I estimated this structure function for two different classes of nuclei-light nuclei describable in an independent-particle model approach, as well as for heavy nuclei described by slowly rotating collective variables. An estimate of the exotic effects was made within the context of a model wherein gluons and quarks from separate nucleons fuse together to alter the parton densities relative to that in isolated nucleons.
In the second problem, spin-dependent gluonic structure functions in a transversely polarized proton were identified and the classification according to twist was discussed. I found that there were two twist-three transverse spin gluonic structure functions, called herein H1(x, Q2) and H2(x, Q2). These are potentially measurable in X2(3555) production in hard polarised p-p collisions. Crossection formulae were calculated for a variety of polarization states, assuming a simple effective interaction for X2 production from gluon fusion.
In the third, and final problem, the emphasis shifted from spin-dependent structure functions of polarised hadrons to the formulation of an effective, low energy, field theory of s-wave quarkonia, constituent heavy quarks,and gluons. I constructed an interaction Lagrangian which has the form of a twist expansion, as typically encountered in hard processes, and involves derivatives of arbitrary order. The parameters in the interaction were related with the non-relativistic wavefunction, and the standard results for QQ inclusive decays and radiative transitions were shown to be easily recovered. The theory is manifestly gauge invariant. The light-cone gluon momentum distribution at very small x was calculated and shown to be uniquely determined by the non-relativistic wave function. The distribution has a part which goes as x-1logx, i.e. is more singular than the usually assumed 1/x behavior. The fragmentation function for a virtual gluon to inclusively decay into an 1Å‹c or 1Å‹6 was also calculated. I found that the emission of low momentum gluons made this process quite sensitive to assumptions about the binding energy of heavy quarks in quarkonia. This gauge invariant theory is extendable to p-wave quarkonia where the non-locality of the meson state is enhanced by the centrifugal barrier, thereby making a gauge invariant description still more important. A framework to study quarkonia with spin is provided in this part of the thesis.