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) <= 1190 MeV/c(2) a statistically significant structure in the M-p eta invariant mass distribution near 1700 MeV/c(2) is observed with a width of Gamma approximate to 35 MeV/c(2) while the mass resolution is sigma(res) = 5 MeV/c(2). Increasing the incident photon energy from 1420 to 1540 MeV this structure shifts in mass from approximate to 1700 MeV/c(2) to approximate 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 approximate to 100 nb around E gamma approximate to 1490 MeV (W approximate to 1920 MeV), which coincides with the pa(0) threshold. Three scenarios are discussed which might be the origin of this structure in the M-p eta invariant mass distribution. The most likely interpretation is that it is due to a triangular singularity in the gamma p -> pa(0) -> p pi(0)eta reaction.