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Upper limits at 90% CL for the elements of the extended neutrino mixing matrix |Ue4|^2 and |Umu4|^2 depending on the assumed heavy neutral lepton mass. Upper limits on |U_mu4|^2 vs assumed neutrino mass [MeV].

A2 Collaboration at MAMI. High statistics angular distributions and total cross sections for the photoproduction of $\pi^0$ 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 Microtron (MAMI) for photon energies between 0.45 and 1.4 GeV, using an almost 4$\pi$ electromagnetic calorimeter composed of the Crystal Ball and TAPS detectors. A complete kinematic reconstruction of the final state removed the effects of Fermi motion. 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. The total overall systematic uncertainty was estimated at 7%. Excitation function at cos(Theta_pi0)cm = -0.25