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Data on the beam asymmetry Σ in the photoproduction of η mesons off protons are reported for tagged photon energies from 1130 to 1790 MeV (mass range from W = 1748 MeV to W = 2045 MeV). The data cover the full solid angle that allows for a precise moment analysis. For the first time, a strong cusp effect in a polarization observable has been observed that is an effect of a branch-point singularity at the pη threshold [Eγ = 1447 MeV (W = 1896 MeV)]. The latest BnGa partial wave analysis includes the new beam asymmetry data and yields a strong indication for the N (1895) 1 2 − nucleon resonance, demonstrating the importance of including all singularities for a correct determination of partial waves and resonance parameters.

The reaction γp → pπ 0 η has been studied with the CBELSA/TAPS detector at the electron stretcher accelerator ELSA in Bonn for incident photon energies from threshold up to 3.1 GeV. This paper has been motivated by the recently claimed observation of a narrow structure in the MNη invariant mass distribution at a mass of 1678 MeV/c 2 . The existence of this structure cannot be confirmed in the present work. Instead, for Eγ = 1400 -1500 MeV and the cut M pπ 0 ≤ 1190 MeV/c 2 a statistically significant structure in the Mpη invariant mass distribution near 1700 MeV/c 2 is observed with a width of Γ ≈ 35 MeV/c 2 while the mass resolution is σres = 5 MeV/c 2 . Increasing the incident photon energy from 1420 to 1540 MeV this structure shifts in mass from ≈ 1700 MeV/c 2 to ≈ 1725 MeV/c 2 ; the width increases to about 50 MeV/c 2 and decreases thereafter. The cross section associated with this structure reaches a maximum of ≈ 100 nb around Eγ ≈ 1490 MeV (W ≈ 1920 MeV), which coincides with the pa0 threshold. Three scenarios are discussed which might be the origin of this structure in the Mpη invariant mass distribution. The most likely interpretation is that it is due to a triangular singularity in the γp → pa0 → pπ 0 η reaction.