A study of eta photoproduction on the proton at MAMI

[摘要] In recent years, there has been much interest in the possible existence of the N*(1685) narrow nucleon resonance, as predicted by the chiral soliton model.Several eta photoproduction experiments have been performed that rely on extracting neutron observables from deuteron target data. These have shown some evidence of narrow structure, however, no structure was observed in cross section measurements on the proton channel.Within the A2 collaboration at the MAMI accelerator in Mainz, Germany, a more detailed study has been undertaken using eta photoproduction on a liquid hydrogen target, and is the subject for this thesis. The study utilised the high resolution and high precision possible with the A2 detector setup, in an attempt to overcome the predicted low photocoupling between the N*(1685) and the proton.The MAMI electron beam was used to produce bremsstrahlung photons, the energies of which are determined by detecting the recoil electrons in the Glasgow-Mainz Tagged Photon Spectrometer.The reaction products from the interaction of the photon beam and liquid hydrogen were detected by the Crystal Ball and TAPS calorimeters.The gamma proton to eta proton reaction channel was identified via the 3 neutral pion decay branch.The reaction cross section was measured between E = 707 and 1477 MeV.Both differential and total cross sections have been presented with a particular emphasis on the centre of mass energy range around 1685 MeV. The peak structure seen on the neutron channel is not observed in the case of the proton. However, a dip in the cross section is observed, which is more prominent at forward polar angles.The leading models are not fully able to reproduce the data in this energy region.As such, given the high energy resolution of the measurement the results can be used to constrain future models and in doing so aid the understanding of the nucleon's resonance spectrum.

[发布机构] University:University of Glasgow;Department:School of Physics and Astronomy

[关键词] meson photoproduction, baryon resonances, N*(1685)