Coherent π0ηd photoproduction at forward deuteron angles measured at BGOOD

The coherent reaction, γd → π0ηd was studied with the BGOOD experiment at ELSA from threshold to a centre-of-mass energy of 3200 MeV for forward deuteron angles. A full kinematic reconstruction was made, with final state deuterons identified in the Forward Spectrometer and π0 and η decays in the central BGO Rugby Ball. The strength of the differential cross section exceeds what can be described by phenomenological models of coherent photoproduction due to the large momentum transfer to the deuteron. The origin of this enhancement is unresolved, whether this is due, for example, to intermediate dibaryon formation or pion re-scattering terms. A toy model of quasi-free Δ and N(1535) photoproduction with final state interactions is discussed, which gives a qualitative description of the differential cross section.