Experimental study of nucleon resonance contributions to η-photoproduction on the neutron

Nucleon resonances play an important role in the process of understanding the low energy regime of the strong force. Quantum Chromodynamics (QCD), being the established theory for the description of this interaction, cannot be applied directly using perturbation theory, as the coupling constant is large at typical energies around 1 GeV. Therefore, effective models are used, which introduce higher lying degrees of freedom compared to the quarks and gluons of QCD. Validation of these models needs experimental input that was obtained until twenty years ago mainly via pion-nucleon scattering, in which a large number of nucleon resonances were identified. Unfortunately, most models predict an even higher number of states, leading to the problem of 'missing resonances'. Complementary data were obtained in the last years from meson photoproduction reactions using the electromagnetic formation channel.
This thesis aims at contributing to the experimental base of photoproduction data by measuring unpolarized differential cross sections of the reaction g+n->eta+n with high precision. Several previous experiments observed an unusual structure in the total cross section around W = 1680 MeV. One of the various and heavily debated interpretations of this phenomenon is the existence of an exotic antidecuplet with J^P = 1/2^+ containing five quark states. As statistics of the previous measurements is moderate, especially for the angular distributions, a new precision measurement was urgently needed to shed more light on the issue.
The experiment for this work was performed at MAMI (Mainz, Germany). A real photon beam was produced via tagged bremsstrahlung technique from the 1.5 GeV electron beam of MAMI-C. A liquid deuterium target was surrounded by an almost 4pi-covering combined detector setup consisting of the Crystal Ball and the TAPS calorimeters. Discrimination of charged and neutral particles was performed by dedicated Veto detectors in both calorimeters. The eta-mesons were identified using the eta->2g and the eta->3pi^0 decays. The reactions g+p->eta+p and g+n->eta+n were measured exclusively in quasi-free kinematics along with an inclusive measurement of g+N->eta+(N).
Differential cross sections were calculated as a function of the center-of-mass energy W = sqrt(s) and cos(theta_eta^cm) determined from the initial state. The resulting cross sections are affected by a loss of resolution due to the Fermi motion of the initial state nucleons. In addition, the center-of-mass energy was reconstructed from the final state using both a kinematic reconstruction and a time-of-flight measurement of the nucleons in forward direction. The corresponding cross sections are not affected by Fermi motion but only by the resolution of the applied W-reconstruction.
The consistency of the neutron measurement was verified by a comparison with the proton and the inclusive measurement. From the comparison of the quasi-free proton results to free proton measurements, it was found that nuclear effects play a minor role or are sufficiently under control in the quasi-free analysis. This justifies the interpretation of the extracted observables from the quasi-free neutron measurement as approximated observables of the free neutron.
The results of this work confirm the presence of a structure around a center-of-mass energy W = 1670 MeV in the total cross section of g+n->eta+n with unprecedented statistical evidence. The best overall estimate for the position is W_R = (1670 +/- 5) MeV and an upper limit for the width of Gamma_R <= (51 +/- 10) MeV was extracted. Taking into account the resolution of the kinematic W-reconstruction leads to an estimation of the intrinsic width of approximately Gamma_R = (30 +/- 5) MeV. Assuming that the structure is caused by a single J = 1/2 state, the coupling strength is determined to be sqrt(b_eta)A^n_1/2 = (12.4 +/- 0.8) 10^-3 GeV^-1/2.
The differential cross sections obtained in this work show for the first time the angular dependence of the structure with high precision. This should allow more detailed studies in terms of partial-wave analyses that will hopefully yield in a better understanding of the phenomenon.