Break-up and Coherent Photoproduction ofηMesons on the Deuteron

We present new break-up and coherent data for $\eta$ meson photoproduction on the deuteron, using a deuterium target and tagged bremsstrahlung photons up to 1 GeV. The differential cross sections for the coherent process were measured from threshold to 800 MeV. They are much smaller than those previously reported. The break-up channel provides a direct measurement of the neutron to proton differential cross section ratios. At the $S_{11}\left(1535\right)$ resonance peak, ${\sigma }_n/{\sigma }_p=0.68\mathrm{}\pm 0.06$ leading to an isoscalar to isovector amplitude ratio of $A_s{/A}_{\upsilon }=0.096\mathrm{}\pm 0.02$.