Observation of a structure in the M-p eta invariant mass distribution near 1700 MeV/c(2) in the gamma p -> p pi(0)eta reaction

The reaction gamma p -> p pi(0)eta 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 MN n 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 E-gamma = 1400-1500 MeV and the cut M-P pi(0) pa(0) -> p pi(0)eta reaction

Observation of the p η ′ Cusp in the New Precise Beam Asymmetry Σ Data for γ p → p η

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)12− nucleon resonance, demonstrating the importance of including all singularities for a correct determination of partial waves and resonance parameters