Authors: Dominik Werthmueller for the Crystal Ball/TAPS collaboration
Title: Investigation of the anomaly in eta-photoproduction off the neutron
Keywords: neutron anomaly eta photoproduction quasi-free
Abstract:
Quasi-free photoproduction of eta-meson off the neutron and off the proton has been measured using a liquid deuterium target and bremsstrahlung photons produced by MAMI-C with incident energies up to 1.5 GeV. The eta-mesons were detected in coincidence with the recoil nucleons in a combined setup of the Crystal Ball calorimeter and a TAPS forward wall which results in an almost 4pi acceptance. Previous measurements performed by the GRAAL, CBELSA/TAPS and the LNS-Sendai collaborations revealed a bump-like structure of still unknown nature in the quasi-free neutron excitation function around W = 1.68 GeV that is not found for the proton. If the true center-of-mass energy is calculated by taking into account the Fermi momentum of the participant nucleon the result of GRAAL and CBELSA/TAPS show that the width of this structure is decreasing which could be explained by a new narrow nucleon resonance at 1680 MeV. The aim of this work is to reproduce this result which much higher statistics, especially at backwards angles. The correction of the Fermi momentum is performed event-by-event by calculating the kinetic energy of the recoil nucleon from kinematics and by time-of-flight when the nucleon is detected in TAPS. Preliminary results for the exclusive reactions on the neutron and the proton show a similar behavior as seen in the previous experiments.
|