Measurement of cross sections and polarisation observables in η photoproduction from neutrons and protons bound in light nuclei

The structure of matter, i.e. the binding of nucleons to nuclei and the formation of quarks to nucleons or other hadrons, is governed by the strong interaction. The underlying Gauge theory, Quantum Chromodynamics (QCD), is well established and has a characteristic property: the coupling constant is decreasing as a function of the momentum transfer (energy). In high-energy reactions, quarks and gluons behave as free particles and the coupling constant is small. This regime of QCD, where quarks and gluons interact only weakly, is called asymptotic freedom and pertubative calculations can be used to predict interactions. However, at small energies, the quarks interact strongly and virtual gluons can produce gluon-gluon pairs and confine quarks in colorless hadrons. Due to the large coupling constant, pertubative calculations of QCD are unreliable at low energies and cannot explain the confinement. In this low-energy range, only phenomenological models such as quark models or numerical calculations (lattice QCD) can be used to solve QCD. To verify QCD models at low energies, the excitation spectrum of the nucleon is of particular interest. Comparison of the model predictions and the experimentally observed states have shown a large discrepancy in number and ordering of the levels. Many more states are predicted than have been experimentally observed, which is known as the problem of missing resonances. This mismatch may either originate from the effective degrees of freedom of the models or from experimental bias.
In the beginning of hadron spectroscopy, most results have been obtained from pion-nucleon scattering experiments. However, since the intermediate nucleon resonance depends on the production mechanism, only resonances that couple to π N have been observed. In the last decades, these results have been supplemented with data on unpolarised cross sections obtained from meson photoproduction at various acceleration facilities. These results could clarify the situation to some extent. Nevertheless, the problem of missing resonances persists, which is mainly caused by the fact that many resonances are broad and overlapping. Thus, current experiments focus on the measurement of single and double polarisation observables, which may improve the situation since observables are sensitive to interference terms and thus can enhance weak contributions from resonances.
In this work, η photoproduction from quasi-free protons and neutrons has been studied. Photoproduction of η mesons is of particular interest since former results of different collaborations have shown an unusual narrow structure in the cross section on the neutron, which is not visible on the proton. Various theoretical models exist that try to explain this effect, but no conclusive solution has been found yet. Thus, to get a final interpretation of this effect, unpolarised cross sections, the double polarisation observable E and the helicity dependent cross sections σ1/2 and σ3/2 have been extracted in this work.
Unpolarised total and differential cross sections have been determined for protons and neutrons bound in light nuclei, i.e. deuterium and 3He. Data have been measured with the CBELSA/TAPS experiment at the Electron Stretcher Accelerator (ELSA) in Bonn (deuterium, December 2008) and with the A2 experiment at the Mainzer Microtron (MAMI) in Mainz (3He, November 2008). Both setups used energy-tagged photon beams to produce η mesons from cryogenic liquid targets. The target was surrounded by an almost 4π covering detector setup. At CBELSA/TAPS the combined setup of Crystal Barrel (CBB) and MiniTAPS was used, at A2 the main detectors were Crystal Ball (CB), TAPS.
Furthermore, experiments aiming at the extraction of the double polarisation observable E, have been run at both acceleration facilities. A circularly polarised photon beam and a longitudinally polarised deuterated butanol (dButanol) target have been used.
The results obtained in this work give input to new partial wave analysis and help to straighten out the situation of η photoproduction from the neutron.