Isospin splitting of transition amplitudes from photoproduction of eta mesons on deuterium

Abstract

Measurements of the total and differential cross sections for η-meson photoproduction on 1 H, 2 D, and 14 N liquid targets from threshold to 1.2 GeV, have been taken using the tagged Bremsstrahlung photon beam produced by the electrons extracted from the ELSA storage ring at Bonn. The reaction was identified by detecting the η decay products in the neutral meson spectrometer SPES0-2π, while the recoil baryons (proton, neutron or deuteron) were detected by a variety of large angle scintillator detectors. We present here our experimental results on the Deuterium target, both the comparison of the quasi-free proton and neutron as well as results on the coherent production. Our preliminary results for the counting rates integrated on the S 11 excitation show significantly smaller figures for the neutrons than for the protons. This is consistent with an Isoscalar part of the Amplitude roughly ten times smaller than the Isovector one and also explains the very low rate of coherent deuterons observed.