The search for an unknown low-lying nucleon resonance with a real photon beam

Full text: Results obtained by different collaborations of eta-photoproduction off the neutron led to speculations about a yet unknown nucleon resonance suggesting surprising properties such as a low mass of about 1685 MeV and a narrow width of 30 MeV. A possible candidate for such a state would be the non-strange member of the antidecuplet of pentaquarks. The existence of this kind of states is however highly doubted. In addition various models succeed to interpret the experimental results of eta-photoproduction off the neutron without the introduction of a new nucleon resonance. On the other hand recent findings in Compton scattering on the neutron give further evidence for the scenario of an unknown resonance. Preliminary results on this question from the Crystal Ball/TAPS collaboration are presented. The experiment was performed using a liquid deuterium target. The beam of tagged photons was produced via Bremsstrahlung of the 1.5 GeV electron beam provided by the MAMI accelerator facility in Mainz (Germany). Particles were detected in the two calorimeters Crystal Ball and TAPS covering around 94% of full solid angle. (author)