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Background: Photoproduction of mesons off quasi-free nucleons bound in the deuteron allows to study the electromagnetic excitation spectrum of the neutron and the isospin structure of the excitation of nucleon resonances. The database for such reactions is much more sparse than for free proton targets.Purpose: Study experimentally single π 0 photoproduction off quasi-free nucleons from the deuteron. Investigate nuclear effects by a comparison of the results for free protons and quasi-free protons. Use the quasi-free neutron data (corrected for nuclear effects) to test the predictions of reaction models and partial wave analysis (PWA) for γn → nπ 0 derived from the analysis of the other isospin channels.Methods: High statistics angular distributions and total cross sections for the photoproduction of π 0 mesons off the deuteron with coincident detection of recoil nucleons have been measured for the first time. The experiment was performed at the tagged photon beam of the Mainz MAMI accelerator for photon energies between 0.45 GeV and 1.4 GeV, using an almost 4π electromagnetic calorimeter composed of the Crystal Ball and TAPS detectors. A complete kinematic reconstruction of the final state removed the effects of Fermi motion.Results: Significant effects from final state interactions (FSI) were observed for participant protons in comparison to free proton targets (between 30% and almost 40%). The data in coincidence with recoil neutrons were corrected for such effects under the assumption that they are identical for participant protons and neutrons. Reaction model predictions and PWA for γn → nπ 0 , based on fits to data for the other isospin channels, disagreed between themselves and no model provided a good description of the new data. Conclusions:The results demonstrate clearly the importance of a mesurement of the fully neutral final state for the isospin decmposition of the cross section. Model refits, for example from the Bonn-Gatchina analysis, show that the new and the previous data for the other three isospin channels can be simultaneously described when the contributions of several partial waves are modified. The results are also relevant for the suppression of the higher resonance bumps in total photoabsorption on nuclei, which are not well understood.The photoproduction of mesons is a prime tool for the study of the excitation spectrum of the nucleon, which is a major testing ground for the properties of the strong interaction in the non-perturbative regime. The pion is the lightest meson and has a strong coupling to many nucleon excited states. Although recent years have provided new photoproduction data for many different final states, pion scattering and photoproduction of pions are still central to most analyses which aim to identify and characterize the excited states of nucleons. Many theoretical frameworks are employed to extract this information. They include the SAID multipole analysis [1,2], the MAID unitary isobar model [3,4], the Dubna-Mainz-Taipei (DMT) dynamical model [5], the Bonn-Gatchi...

We report the first large-acceptance measurement of the beam-spin asymmetry for deuteron photodisintegration ( γd → pn) in the photon energy range 420 < E γ < 620 MeV. The measurement provides important new constraints on the mechanisms of photodisintegration above the ∆ resonance and on the photocoupling of the recently discovered d * (2380) hexaquark.

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